text: /*-------------------------------------------------------------------------
   *
   * bufmgr.c
   *	  buffer manager interface routines
   *
   * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   *
   * IDENTIFICATION
   *	  src/backend/storage/buffer/bufmgr.c
   *
   *-------------------------------------------------------------------------
   */
  /*
   * Principal entry points:
   *
   * ReadBuffer() -- find or create a buffer holding the requested page,
   *		and pin it so that no one can destroy it while this process
   *		is using it.
   *
   * ReleaseBuffer() -- unpin a buffer
   *
   * MarkBufferDirty() -- mark a pinned buffer's contents as "dirty".
   *		The disk write is delayed until buffer replacement or checkpoint.
   *
   * See also these files:
   *		freelist.c -- chooses victim for buffer replacement
   *		buf_table.c -- manages the buffer lookup table
   */
  #include "postgres.h"
  
  #include <sys/file.h>
  #include <unistd.h>
  
  #include "access/xlog.h"
  #include "catalog/catalog.h"
  #include "catalog/storage.h"
  #include "executor/instrument.h"
  #include "lib/binaryheap.h"
  #include "miscadmin.h"
  #include "pg_trace.h"
  #include "pgstat.h"
  #include "postmaster/bgwriter.h"
  #include "storage/buf_internals.h"
  #include "storage/bufmgr.h"
  #include "storage/ipc.h"
  #include "storage/proc.h"
  #include "storage/smgr.h"
  #include "storage/standby.h"
  #include "utils/rel.h"
  #include "utils/resowner_private.h"
  #include "utils/timestamp.h"
  
  #include "pg_yb_utils.h"
  
  /* Note: these two macros only work on shared buffers, not local ones! */
  #define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
  #define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
  
  /* Note: this macro only works on local buffers, not shared ones! */
  #define LocalBufHdrGetBlock(bufHdr) \
  	LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]
  
  /* Bits in SyncOneBuffer's return value */
  #define BUF_WRITTEN				0x01
  #define BUF_REUSABLE			0x02
  
  #define DROP_RELS_BSEARCH_THRESHOLD		20
  
  typedef struct PrivateRefCountEntry
  {
  	Buffer		buffer;
  	int32		refcount;
  } PrivateRefCountEntry;
  
  /* 64 bytes, about the size of a cache line on common systems */
  #define REFCOUNT_ARRAY_ENTRIES 8
  
  /*
   * Status of buffers to checkpoint for a particular tablespace, used
   * internally in BufferSync.
   */
  typedef struct CkptTsStatus
  {
  	/* oid of the tablespace */
  	Oid			tsId;
  
  	/*
  	 * Checkpoint progress for this tablespace. To make progress comparable
  	 * between tablespaces the progress is, for each tablespace, measured as a
  	 * number between 0 and the total number of to-be-checkpointed pages. Each
  	 * page checkpointed in this tablespace increments this space's progress
  	 * by progress_slice.
  	 */
  	float8		progress;
  	float8		progress_slice;
  
  	/* number of to-be checkpointed pages in this tablespace */
  	int			num_to_scan;
  	/* already processed pages in this tablespace */
  	int			num_scanned;
  
  	/* current offset in CkptBufferIds for this tablespace */
  	int			index;
  } CkptTsStatus;
  
  /* GUC variables */
  bool		zero_damaged_pages = false;
  int			bgwriter_lru_maxpages = 100;
  double		bgwriter_lru_multiplier = 2.0;
  bool		track_io_timing = false;
  int			effective_io_concurrency = 0;
  
  /*
   * GUC variables about triggering kernel writeback for buffers written; OS
   * dependent defaults are set via the GUC mechanism.
   */
  int			checkpoint_flush_after = 0;
  int			bgwriter_flush_after = 0;
  int			backend_flush_after = 0;
  
  /*
   * How many buffers PrefetchBuffer callers should try to stay ahead of their
   * ReadBuffer calls by.  This is maintained by the assign hook for
   * effective_io_concurrency.  Zero means "never prefetch".  This value is
   * only used for buffers not belonging to tablespaces that have their
   * effective_io_concurrency parameter set.
   */
  int			target_prefetch_pages = 0;
  
  /* local state for StartBufferIO and related functions */
  static BufferDesc *InProgressBuf = NULL;
  static bool IsForInput;
  
  /* local state for LockBufferForCleanup */
  static BufferDesc *PinCountWaitBuf = NULL;
  
  /*
   * Backend-Private refcount management:
   *
   * Each buffer also has a private refcount that keeps track of the number of
   * times the buffer is pinned in the current process.  This is so that the
   * shared refcount needs to be modified only once if a buffer is pinned more
   * than once by an individual backend.  It's also used to check that no buffers
   * are still pinned at the end of transactions and when exiting.
   *
   *
   * To avoid - as we used to - requiring an array with NBuffers entries to keep
   * track of local buffers, we use a small sequentially searched array
   * (PrivateRefCountArray) and an overflow hash table (PrivateRefCountHash) to
   * keep track of backend local pins.
   *
   * Until no more than REFCOUNT_ARRAY_ENTRIES buffers are pinned at once, all
   * refcounts are kept track of in the array; after that, new array entries
   * displace old ones into the hash table. That way a frequently used entry
   * can't get "stuck" in the hashtable while infrequent ones clog the array.
   *
   * Note that in most scenarios the number of pinned buffers will not exceed
   * REFCOUNT_ARRAY_ENTRIES.
   *
   *
   * To enter a buffer into the refcount tracking mechanism first reserve a free
   * entry using ReservePrivateRefCountEntry() and then later, if necessary,
   * fill it with NewPrivateRefCountEntry(). That split lets us avoid doing
   * memory allocations in NewPrivateRefCountEntry() which can be important
   * because in some scenarios it's called with a spinlock held...
   */
  static struct PrivateRefCountEntry PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES];
  static HTAB *PrivateRefCountHash = NULL;
  static int32 PrivateRefCountOverflowed = 0;
  static uint32 PrivateRefCountClock = 0;
  static PrivateRefCountEntry *ReservedRefCountEntry = NULL;
  
  static void ReservePrivateRefCountEntry(void);
  static PrivateRefCountEntry *NewPrivateRefCountEntry(Buffer buffer);
  static PrivateRefCountEntry *GetPrivateRefCountEntry(Buffer buffer, bool do_move);
  static inline int32 GetPrivateRefCount(Buffer buffer);
  static void ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref);
  
  /*
   * Ensure that the PrivateRefCountArray has sufficient space to store one more
   * entry. This has to be called before using NewPrivateRefCountEntry() to fill
   * a new entry - but it's perfectly fine to not use a reserved entry.
   */
  static void
  ReservePrivateRefCountEntry(void)
  {
  	/* Already reserved (or freed), nothing to do */
  	if (ReservedRefCountEntry != NULL)
  		return;
  
  	/*
  	 * First search for a free entry the array, that'll be sufficient in the
  	 * majority of cases.
  	 */
  	{
  		int			i;
  
  		for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
  		{
  			PrivateRefCountEntry *res;
  
  			res = &PrivateRefCountArray[i];
  
  			if (res->buffer == InvalidBuffer)
  			{
  				ReservedRefCountEntry = res;
  				return;
  			}
  		}
  	}
  
  	/*
  	 * No luck. All array entries are full. Move one array entry into the hash
  	 * table.
  	 */
  	{
  		/*
  		 * Move entry from the current clock position in the array into the
  		 * hashtable. Use that slot.
  		 */
  		PrivateRefCountEntry *hashent;
  		bool		found;
  
  		/* select victim slot */
  		ReservedRefCountEntry =
  			&PrivateRefCountArray[PrivateRefCountClock++ % REFCOUNT_ARRAY_ENTRIES];
  
  		/* Better be used, otherwise we shouldn't get here. */
  		Assert(ReservedRefCountEntry->buffer != InvalidBuffer);
  
  		/* enter victim array entry into hashtable */
  		hashent = hash_search(PrivateRefCountHash,
  							  (void *) &(ReservedRefCountEntry->buffer),
  							  HASH_ENTER,
  							  &found);
  		Assert(!found);
  		hashent->refcount = ReservedRefCountEntry->refcount;
  
  		/* clear the now free array slot */
  		ReservedRefCountEntry->buffer = InvalidBuffer;
  		ReservedRefCountEntry->refcount = 0;
  
  		PrivateRefCountOverflowed++;
  	}
  }
  
  /*
   * Fill a previously reserved refcount entry.
   */
  static PrivateRefCountEntry *
  NewPrivateRefCountEntry(Buffer buffer)
  {
  	PrivateRefCountEntry *res;
  
  	/* only allowed to be called when a reservation has been made */
  	Assert(ReservedRefCountEntry != NULL);
  
  	/* use up the reserved entry */
  	res = ReservedRefCountEntry;
  	ReservedRefCountEntry = NULL;
  
  	/* and fill it */
  	res->buffer = buffer;
  	res->refcount = 0;
  
  	return res;
  }
  
  /*
   * Return the PrivateRefCount entry for the passed buffer.
   *
   * Returns NULL if a buffer doesn't have a refcount entry. Otherwise, if
   * do_move is true, and the entry resides in the hashtable the entry is
   * optimized for frequent access by moving it to the array.
   */
  static PrivateRefCountEntry *
  GetPrivateRefCountEntry(Buffer buffer, bool do_move)
  {
  	PrivateRefCountEntry *res;
  	int			i;
  
  	Assert(BufferIsValid(buffer));
  	Assert(!BufferIsLocal(buffer));
  
  	/*
  	 * First search for references in the array, that'll be sufficient in the
  	 * majority of cases.
  	 */
  	for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
  	{
  		res = &PrivateRefCountArray[i];
  
  		if (res->buffer == buffer)
  			return res;
  	}
  
  	/*
  	 * By here we know that the buffer, if already pinned, isn't residing in
  	 * the array.
  	 *
  	 * Only look up the buffer in the hashtable if we've previously overflowed
  	 * into it.
  	 */
  	if (PrivateRefCountOverflowed == 0)
  		return NULL;
  
  	res = hash_search(PrivateRefCountHash,
  					  (void *) &buffer,
  					  HASH_FIND,
  					  NULL);
  
  	if (res == NULL)
  		return NULL;
  	else if (!do_move)
  	{
  		/* caller doesn't want us to move the hash entry into the array */
  		return res;
  	}
  	else
  	{
  		/* move buffer from hashtable into the free array slot */
  		bool		found;
  		PrivateRefCountEntry *free;
  
  		/* Ensure there's a free array slot */
  		ReservePrivateRefCountEntry();
  
  		/* Use up the reserved slot */
  		Assert(ReservedRefCountEntry != NULL);
  		free = ReservedRefCountEntry;
  		ReservedRefCountEntry = NULL;
  		Assert(free->buffer == InvalidBuffer);
  
  		/* and fill it */
  		free->buffer = buffer;
  		free->refcount = res->refcount;
  
  		/* delete from hashtable */
  		hash_search(PrivateRefCountHash,
  					(void *) &buffer,
  					HASH_REMOVE,
  					&found);
  		Assert(found);
  		Assert(PrivateRefCountOverflowed > 0);
  		PrivateRefCountOverflowed--;
  
  		return free;
  	}
  }
  
  /*
   * Returns how many times the passed buffer is pinned by this backend.
   *
   * Only works for shared memory buffers!
   */
  static inline int32
  GetPrivateRefCount(Buffer buffer)
  {
  	PrivateRefCountEntry *ref;
  
  	Assert(BufferIsValid(buffer));
  	Assert(!BufferIsLocal(buffer));
  
  	/*
  	 * Not moving the entry - that's ok for the current users, but we might
  	 * want to change this one day.
  	 */
  	ref = GetPrivateRefCountEntry(buffer, false);
  
  	if (ref == NULL)
  		return 0;
  	return ref->refcount;
  }
  
  /*
   * Release resources used to track the reference count of a buffer which we no
   * longer have pinned and don't want to pin again immediately.
   */
  static void
  ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref)
  {
  	Assert(ref->refcount == 0);
  
  	if (ref >= &PrivateRefCountArray[0] &&
  		ref < &PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES])
  	{
  		ref->buffer = InvalidBuffer;
  
  		/*
  		 * Mark the just used entry as reserved - in many scenarios that
  		 * allows us to avoid ever having to search the array/hash for free
  		 * entries.
  		 */
  		ReservedRefCountEntry = ref;
  	}
  	else
  	{
  		bool		found;
  		Buffer		buffer = ref->buffer;
  
  		hash_search(PrivateRefCountHash,
  					(void *) &buffer,
  					HASH_REMOVE,
  					&found);
  		Assert(found);
  		Assert(PrivateRefCountOverflowed > 0);
  		PrivateRefCountOverflowed--;
  	}
  }
  
  /*
   * BufferIsPinned
   *		True iff the buffer is pinned (also checks for valid buffer number).
   *
   *		NOTE: what we check here is that *this* backend holds a pin on
   *		the buffer.  We do not care whether some other backend does.
   */
  #define BufferIsPinned(bufnum) \
  ( \
  	!BufferIsValid(bufnum) ? \
  		false \
  	: \
  		BufferIsLocal(bufnum) ? \
  			(LocalRefCount[-(bufnum) - 1] > 0) \
  		: \
  	(GetPrivateRefCount(bufnum) > 0) \
  )
  
  
  static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
  				  ForkNumber forkNum, BlockNumber blockNum,
  				  ReadBufferMode mode, BufferAccessStrategy strategy,
  				  bool *hit);
  static bool PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy);
  static void PinBuffer_Locked(BufferDesc *buf);
  static void UnpinBuffer(BufferDesc *buf, bool fixOwner);
  static void BufferSync(int flags);
  static uint32 WaitBufHdrUnlocked(BufferDesc *buf);
  static int	SyncOneBuffer(int buf_id, bool skip_recently_used, WritebackContext *flush_context);
  static void WaitIO(BufferDesc *buf);
  static bool StartBufferIO(BufferDesc *buf, bool forInput);
  static void TerminateBufferIO(BufferDesc *buf, bool clear_dirty,
  				  uint32 set_flag_bits);
  static void shared_buffer_write_error_callback(void *arg);
  static void local_buffer_write_error_callback(void *arg);
  static BufferDesc *BufferAlloc(SMgrRelation smgr,
  			char relpersistence,
  			ForkNumber forkNum,
  			BlockNumber blockNum,
  			BufferAccessStrategy strategy,
  			bool *foundPtr);
  static void FlushBuffer(BufferDesc *buf, SMgrRelation reln);
  static void AtProcExit_Buffers(int code, Datum arg);
  static void CheckForBufferLeaks(void);
  static int	rnode_comparator(const void *p1, const void *p2);
  static int	buffertag_comparator(const void *p1, const void *p2);
  static int	ckpt_buforder_comparator(const void *pa, const void *pb);
  static int	ts_ckpt_progress_comparator(Datum a, Datum b, void *arg);
  
  
  /*
   * ComputeIoConcurrency -- get the number of pages to prefetch for a given
   *		number of spindles.
   */
  bool
  ComputeIoConcurrency(int io_concurrency, double *target)
  {
  	double		new_prefetch_pages = 0.0;
  	int			i;
  
  	/*
  	 * Make sure the io_concurrency value is within valid range; it may have
  	 * been forced with a manual pg_tablespace update.
  	 */
  	io_concurrency = Min(Max(io_concurrency, 0), MAX_IO_CONCURRENCY);
  
  	/*----------
  	 * The user-visible GUC parameter is the number of drives (spindles),
  	 * which we need to translate to a number-of-pages-to-prefetch target.
  	 * The target value is stashed in *extra and then assigned to the actual
  	 * variable by assign_effective_io_concurrency.
  	 *
  	 * The expected number of prefetch pages needed to keep N drives busy is:
  	 *
  	 * drives |   I/O requests
  	 * -------+----------------
  	 *		1 |   1
  	 *		2 |   2/1 + 2/2 = 3
  	 *		3 |   3/1 + 3/2 + 3/3 = 5 1/2
  	 *		4 |   4/1 + 4/2 + 4/3 + 4/4 = 8 1/3
  	 *		n |   n * H(n)
  	 *
  	 * This is called the "coupon collector problem" and H(n) is called the
  	 * harmonic series.  This could be approximated by n * ln(n), but for
  	 * reasonable numbers of drives we might as well just compute the series.
  	 *
  	 * Alternatively we could set the target to the number of pages necessary
  	 * so that the expected number of active spindles is some arbitrary
  	 * percentage of the total.  This sounds the same but is actually slightly
  	 * different.  The result ends up being ln(1-P)/ln((n-1)/n) where P is
  	 * that desired fraction.
  	 *
  	 * Experimental results show that both of these formulas aren't aggressive
  	 * enough, but we don't really have any better proposals.
  	 *
  	 * Note that if io_concurrency = 0 (disabled), we must set target = 0.
  	 *----------
  	 */
  
  	for (i = 1; i <= io_concurrency; i++)
  		new_prefetch_pages += (double) io_concurrency / (double) i;
  
  	*target = new_prefetch_pages;
  
  	/* This range check shouldn't fail, but let's be paranoid */
  	return (new_prefetch_pages >= 0.0 && new_prefetch_pages < (double) INT_MAX);
  }
  
  /*
   * PrefetchBuffer -- initiate asynchronous read of a block of a relation
   *
   * This is named by analogy to ReadBuffer but doesn't actually allocate a
   * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
   * block will not be delayed by the I/O.  Prefetching is optional.
   * No-op if prefetching isn't compiled in.
   */
  void
  PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
  {
  #ifdef USE_PREFETCH
  	Assert(RelationIsValid(reln));
  	Assert(BlockNumberIsValid(blockNum));
  
  	/* Open it at the smgr level if not already done */
  	RelationOpenSmgr(reln);
  
  	if (RelationUsesLocalBuffers(reln))
  	{
  		/* see comments in ReadBufferExtended */
  		if (RELATION_IS_OTHER_TEMP(reln))
  			ereport(ERROR,
  					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  					 errmsg("cannot access temporary tables of other sessions")));
  
  		/* pass it off to localbuf.c */
  		LocalPrefetchBuffer(reln->rd_smgr, forkNum, blockNum);
  	}
  	else
  	{
  		BufferTag	newTag;		/* identity of requested block */
  		uint32		newHash;	/* hash value for newTag */
  		LWLock	   *newPartitionLock;	/* buffer partition lock for it */
  		int			buf_id;
  
  		/* create a tag so we can lookup the buffer */
  		INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
  					   forkNum, blockNum);
  
  		/* determine its hash code and partition lock ID */
  		newHash = BufTableHashCode(&newTag);
  		newPartitionLock = BufMappingPartitionLock(newHash);
  
  		/* see if the block is in the buffer pool already */
  		LWLockAcquire(newPartitionLock, LW_SHARED);
  		buf_id = BufTableLookup(&newTag, newHash);
  		LWLockRelease(newPartitionLock);
  
  		/* If not in buffers, initiate prefetch */
  		if (buf_id < 0)
  			smgrprefetch(reln->rd_smgr, forkNum, blockNum);
  
  		/*
  		 * If the block *is* in buffers, we do nothing.  This is not really
  		 * ideal: the block might be just about to be evicted, which would be
  		 * stupid since we know we are going to need it soon.  But the only
  		 * easy answer is to bump the usage_count, which does not seem like a
  		 * great solution: when the caller does ultimately touch the block,
  		 * usage_count would get bumped again, resulting in too much
  		 * favoritism for blocks that are involved in a prefetch sequence. A
  		 * real fix would involve some additional per-buffer state, and it's
  		 * not clear that there's enough of a problem to justify that.
  		 */
  	}
  #endif							/* USE_PREFETCH */
  }
  
  
  /*
   * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
   *		fork with RBM_NORMAL mode and default strategy.
   */
  Buffer
  ReadBuffer(Relation reln, BlockNumber blockNum)
  {
  	return ReadBufferExtended(reln, MAIN_FORKNUM, blockNum, RBM_NORMAL, NULL);
  }
  
  /* Only here for sequence support */
  extern HeapTuple YBReadSequenceTuple(Relation seqrel);
  
  /*
   * ReadBufferExtended -- returns a buffer containing the requested
   *		block of the requested relation.  If the blknum
   *		requested is P_NEW, extend the relation file and
   *		allocate a new block.  (Caller is responsible for
   *		ensuring that only one backend tries to extend a
   *		relation at the same time!)
   *
   * Returns: the buffer number for the buffer containing
   *		the block read.  The returned buffer has been pinned.
   *		Does not return on error --- elog's instead.
   *
   * Assume when this function is called, that reln has been opened already.
   *
   * In RBM_NORMAL mode, the page is read from disk, and the page header is
   * validated.  An error is thrown if the page header is not valid.  (But
   * note that an all-zero page is considered "valid"; see PageIsVerified().)
   *
   * RBM_ZERO_ON_ERROR is like the normal mode, but if the page header is not
   * valid, the page is zeroed instead of throwing an error. This is intended
   * for non-critical data, where the caller is prepared to repair errors.
   *
   * In RBM_ZERO_AND_LOCK mode, if the page isn't in buffer cache already, it's
   * filled with zeros instead of reading it from disk.  Useful when the caller
   * is going to fill the page from scratch, since this saves I/O and avoids
   * unnecessary failure if the page-on-disk has corrupt page headers.
   * The page is returned locked to ensure that the caller has a chance to
   * initialize the page before it's made visible to others.
   * Caution: do not use this mode to read a page that is beyond the relation's
   * current physical EOF; that is likely to cause problems in md.c when
   * the page is modified and written out. P_NEW is OK, though.
   *
   * RBM_ZERO_AND_CLEANUP_LOCK is the same as RBM_ZERO_AND_LOCK, but acquires
   * a cleanup-strength lock on the page.
   *
   * RBM_NORMAL_NO_LOG mode is treated the same as RBM_NORMAL here.
   *
   * If strategy is not NULL, a nondefault buffer access strategy is used.
   * See buffer/README for details.
   */
  Buffer
  ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
  				   ReadBufferMode mode, BufferAccessStrategy strategy)
  {
  	bool		hit;
  	Buffer		buf;
  
  	/* Open it at the smgr level if not already done */
  	RelationOpenSmgr(reln);
  
  	/*
  	 * Reject attempts to read non-local temporary relations; we would be
  	 * likely to get wrong data since we have no visibility into the owning
  	 * session's local buffers.
  	 */
  	if (RELATION_IS_OTHER_TEMP(reln))
  		ereport(ERROR,
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("cannot access temporary tables of other sessions")));
  
  	/* Special handling for sequences */
    if (RelationGetForm(reln)->relkind == RELKIND_SEQUENCE)
    {
      /* Get a sequence tuple */
      HeapTuple seqtuple = YBReadSequenceTuple(reln);
  
      /* Create an empty buffer to initialize with the sequence data */
      buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
                              forkNum, blockNum, RBM_ZERO_AND_LOCK, strategy, &hit);
  
      /* Insert onto the page */
      Page dp = BufferGetPage(buf);
      PageInit(dp, BLCKSZ, sizeof(*seqtuple));
      PageSetAllVisible(dp);
      OffsetNumber off = PageAddItemExtended(dp, (Item)(seqtuple->t_data), seqtuple->t_len,
                                             InvalidOffsetNumber, PAI_IS_HEAP);
      if (off == InvalidOffsetNumber) {
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                    errmsg("Failed to add sequence tuple to page")));
      }
  
      /* Unlock the buffer */
      LockBuffer(buf, BUFFER_LOCK_UNLOCK);
  
      return buf;
    }
  
  	/*
  	 * Read the buffer, and update pgstat counters to reflect a cache hit or
  	 * miss.
  	 */
  	pgstat_count_buffer_read(reln);
  	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
  							forkNum, blockNum, mode, strategy, &hit);
  	if (hit)
  		pgstat_count_buffer_hit(reln);
  	return buf;
  }
  
  
  /*
   * ReadBufferWithoutRelcache -- like ReadBufferExtended, but doesn't require
   *		a relcache entry for the relation.
   *
   * NB: At present, this function may only be used on permanent relations, which
   * is OK, because we only use it during XLOG replay.  If in the future we
   * want to use it on temporary or unlogged relations, we could pass additional
   * parameters.
   */
  Buffer
  ReadBufferWithoutRelcache(RelFileNode rnode, ForkNumber forkNum,
  						  BlockNumber blockNum, ReadBufferMode mode,
  						  BufferAccessStrategy strategy)
  {
  	bool		hit;
  
  	SMgrRelation smgr = smgropen(rnode, InvalidBackendId);
  
  	Assert(InRecovery);
  
  	return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
  							 mode, strategy, &hit);
  }
  
  
  /*
   * ReadBuffer_common -- common logic for all ReadBuffer variants
   *
   * *hit is set to true if the request was satisfied from shared buffer cache.
   */
  static Buffer
  ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
  				  BlockNumber blockNum, ReadBufferMode mode,
  				  BufferAccessStrategy strategy, bool *hit)
  {
  	BufferDesc *bufHdr;
  	Block		bufBlock;
  	bool		found;
  	bool		isExtend;
  	bool		isLocalBuf = SmgrIsTemp(smgr);
  
  	*hit = false;
  
  	/* Make sure we will have room to remember the buffer pin */
  	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
  
  	isExtend = (blockNum == P_NEW);
  
  	TRACE_POSTGRESQL_BUFFER_READ_START(forkNum, blockNum,
  									   smgr->smgr_rnode.node.spcNode,
  									   smgr->smgr_rnode.node.dbNode,
  									   smgr->smgr_rnode.node.relNode,
  									   smgr->smgr_rnode.backend,
  									   isExtend);
  
  	/* Substitute proper block number if caller asked for P_NEW */
  	if (isExtend)
  		blockNum = smgrnblocks(smgr, forkNum);
  
  	if (isLocalBuf)
  	{
  		bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
  		if (found)
  			pgBufferUsage.local_blks_hit++;
  		else
  			pgBufferUsage.local_blks_read++;
  	}
  	else
  	{
  		/*
  		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
  		 * not currently in memory.
  		 */
  		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
  							 strategy, &found);
  		if (found)
  			pgBufferUsage.shared_blks_hit++;
  		else
  			pgBufferUsage.shared_blks_read++;
  	}
  
  	/* At this point we do NOT hold any locks. */
  
  	/* if it was already in the buffer pool, we're done */
  	if (found)
  	{
  		if (!isExtend)
  		{
  			/* Just need to update stats before we exit */
  			*hit = true;
  			VacuumPageHit++;
  
  			if (VacuumCostActive)
  				VacuumCostBalance += VacuumCostPageHit;
  
  			TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
  											  smgr->smgr_rnode.node.spcNode,
  											  smgr->smgr_rnode.node.dbNode,
  											  smgr->smgr_rnode.node.relNode,
  											  smgr->smgr_rnode.backend,
  											  isExtend,
  											  found);
  
  			/*
  			 * In RBM_ZERO_AND_LOCK mode the caller expects the page to be
  			 * locked on return.
  			 */
  			if (!isLocalBuf)
  			{
  				if (mode == RBM_ZERO_AND_LOCK)
  					LWLockAcquire(BufferDescriptorGetContentLock(bufHdr),
  								  LW_EXCLUSIVE);
  				else if (mode == RBM_ZERO_AND_CLEANUP_LOCK)
  					LockBufferForCleanup(BufferDescriptorGetBuffer(bufHdr));
  			}
  
  			return BufferDescriptorGetBuffer(bufHdr);
  		}
  
  		/*
  		 * We get here only in the corner case where we are trying to extend
  		 * the relation but we found a pre-existing buffer marked BM_VALID.
  		 * This can happen because mdread doesn't complain about reads beyond
  		 * EOF (when zero_damaged_pages is ON) and so a previous attempt to
  		 * read a block beyond EOF could have left a "valid" zero-filled
  		 * buffer.  Unfortunately, we have also seen this case occurring
  		 * because of buggy Linux kernels that sometimes return an
  		 * lseek(SEEK_END) result that doesn't account for a recent write. In
  		 * that situation, the pre-existing buffer would contain valid data
  		 * that we don't want to overwrite.  Since the legitimate case should
  		 * always have left a zero-filled buffer, complain if not PageIsNew.
  		 */
  		bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
  		if (!PageIsNew((Page) bufBlock))
  			ereport(ERROR,
  					(errmsg("unexpected data beyond EOF in block %u of relation %s",
  							blockNum, relpath(smgr->smgr_rnode, forkNum)),
  					 errhint("This has been seen to occur with buggy kernels; consider updating your system.")));
  
  		/*
  		 * We *must* do smgrextend before succeeding, else the page will not
  		 * be reserved by the kernel, and the next P_NEW call will decide to
  		 * return the same page.  Clear the BM_VALID bit, do the StartBufferIO
  		 * call that BufferAlloc didn't, and proceed.
  		 */
  		if (isLocalBuf)
  		{
  			/* Only need to adjust flags */
  			uint32		buf_state = pg_atomic_read_u32(&bufHdr->state);
  
  			Assert(buf_state & BM_VALID);
  			buf_state &= ~BM_VALID;
  			pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
  		}
  		else
  		{
  			/*
  			 * Loop to handle the very small possibility that someone re-sets
  			 * BM_VALID between our clearing it and StartBufferIO inspecting
  			 * it.
  			 */
  			do
  			{
  				uint32		buf_state = LockBufHdr(bufHdr);
  
  				Assert(buf_state & BM_VALID);
  				buf_state &= ~BM_VALID;
  				UnlockBufHdr(bufHdr, buf_state);
  			} while (!StartBufferIO(bufHdr, true));
  		}
  	}
  
  	/*
  	 * if we have gotten to this point, we have allocated a buffer for the
  	 * page but its contents are not yet valid.  IO_IN_PROGRESS is set for it,
  	 * if it's a shared buffer.
  	 *
  	 * Note: if smgrextend fails, we will end up with a buffer that is
  	 * allocated but not marked BM_VALID.  P_NEW will still select the same
  	 * block number (because the relation didn't get any longer on disk) and
  	 * so future attempts to extend the relation will find the same buffer (if
  	 * it's not been recycled) but come right back here to try smgrextend
  	 * again.
  	 */
  	Assert(!(pg_atomic_read_u32(&bufHdr->state) & BM_VALID));	/* spinlock not needed */
  
  	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
  
  	if (isExtend)
  	{
  		/* new buffers are zero-filled */
  		MemSet((char *) bufBlock, 0, BLCKSZ);
  		/* don't set checksum for all-zero page */
  		smgrextend(smgr, forkNum, blockNum, (char *) bufBlock, false);
  
  		/*
  		 * NB: we're *not* doing a ScheduleBufferTagForWriteback here;
  		 * although we're essentially performing a write. At least on linux
  		 * doing so defeats the 'delayed allocation' mechanism, leading to
  		 * increased file fragmentation.
  		 */
  	}
  	else
  	{
  		/*
  		 * Read in the page, unless the caller intends to overwrite it and
  		 * just wants us to allocate a buffer.
  		 */
  		if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
  			MemSet((char *) bufBlock, 0, BLCKSZ);
  		else
  		{
  			instr_time	io_start,
  						io_time;
  
  			if (track_io_timing)
  				INSTR_TIME_SET_CURRENT(io_start);
  
  			smgrread(smgr, forkNum, blockNum, (char *) bufBlock);
  
  			if (track_io_timing)
  			{
  				INSTR_TIME_SET_CURRENT(io_time);
  				INSTR_TIME_SUBTRACT(io_time, io_start);
  				pgstat_count_buffer_read_time(INSTR_TIME_GET_MICROSEC(io_time));
  				INSTR_TIME_ADD(pgBufferUsage.blk_read_time, io_time);
  			}
  
  			/* check for garbage data */
  			if (!PageIsVerified((Page) bufBlock, blockNum))
  			{
  				if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
  				{
  					ereport(WARNING,
  							(errcode(ERRCODE_DATA_CORRUPTED),
  							 errmsg("invalid page in block %u of relation %s; zeroing out page",
  									blockNum,
  									relpath(smgr->smgr_rnode, forkNum))));
  					MemSet((char *) bufBlock, 0, BLCKSZ);
  				}
  				else
  					ereport(ERROR,
  							(errcode(ERRCODE_DATA_CORRUPTED),
  							 errmsg("invalid page in block %u of relation %s",
  									blockNum,
  									relpath(smgr->smgr_rnode, forkNum))));
  			}
  		}
  	}
  
  	/*
  	 * In RBM_ZERO_AND_LOCK mode, grab the buffer content lock before marking
  	 * the page as valid, to make sure that no other backend sees the zeroed
  	 * page before the caller has had a chance to initialize it.
  	 *
  	 * Since no-one else can be looking at the page contents yet, there is no
  	 * difference between an exclusive lock and a cleanup-strength lock. (Note
  	 * that we cannot use LockBuffer() or LockBufferForCleanup() here, because
  	 * they assert that the buffer is already valid.)
  	 */
  	if ((mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK) &&
  		!isLocalBuf)
  	{
  		LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_EXCLUSIVE);
  	}
  
  	if (isLocalBuf)
  	{
  		/* Only need to adjust flags */
  		uint32		buf_state = pg_atomic_read_u32(&bufHdr->state);
  
  		buf_state |= BM_VALID;
  		pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
  	}
  	else
  	{
  		/* Set BM_VALID, terminate IO, and wake up any waiters */
  		TerminateBufferIO(bufHdr, false, BM_VALID);
  	}
  
  	VacuumPageMiss++;
  	if (VacuumCostActive)
  		VacuumCostBalance += VacuumCostPageMiss;
  
  	TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
  									  smgr->smgr_rnode.node.spcNode,
  									  smgr->smgr_rnode.node.dbNode,
  									  smgr->smgr_rnode.node.relNode,
  									  smgr->smgr_rnode.backend,
  									  isExtend,
  									  found);
  
  	return BufferDescriptorGetBuffer(bufHdr);
  }
  
  /*
   * BufferAlloc -- subroutine for ReadBuffer.  Handles lookup of a shared
   *		buffer.  If no buffer exists already, selects a replacement
   *		victim and evicts the old page, but does NOT read in new page.
   *
   * "strategy" can be a buffer replacement strategy object, or NULL for
   * the default strategy.  The selected buffer's usage_count is advanced when
   * using the default strategy, but otherwise possibly not (see PinBuffer).
   *
   * The returned buffer is pinned and is already marked as holding the
   * desired page.  If it already did have the desired page, *foundPtr is
   * set true.  Otherwise, *foundPtr is set false and the buffer is marked
   * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
   *
   * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
   * we keep it for simplicity in ReadBuffer.
   *
   * No locks are held either at entry or exit.
   */
  static BufferDesc *
  BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
  			BlockNumber blockNum,
  			BufferAccessStrategy strategy,
  			bool *foundPtr)
  {
  	BufferTag	newTag;			/* identity of requested block */
  	uint32		newHash;		/* hash value for newTag */
  	LWLock	   *newPartitionLock;	/* buffer partition lock for it */
  	BufferTag	oldTag;			/* previous identity of selected buffer */
  	uint32		oldHash;		/* hash value for oldTag */
  	LWLock	   *oldPartitionLock;	/* buffer partition lock for it */
  	uint32		oldFlags;
  	int			buf_id;
  	BufferDesc *buf;
  	bool		valid;
  	uint32		buf_state;
  
  	/* create a tag so we can lookup the buffer */
  	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
  
  	/* determine its hash code and partition lock ID */
  	newHash = BufTableHashCode(&newTag);
  	newPartitionLock = BufMappingPartitionLock(newHash);
  
  	/* see if the block is in the buffer pool already */
  	LWLockAcquire(newPartitionLock, LW_SHARED);
  	buf_id = BufTableLookup(&newTag, newHash);
  	if (buf_id >= 0)
  	{
  		/*
  		 * Found it.  Now, pin the buffer so no one can steal it from the
  		 * buffer pool, and check to see if the correct data has been loaded
  		 * into the buffer.
  		 */
  		buf = GetBufferDescriptor(buf_id);
  
  		valid = PinBuffer(buf, strategy);
  
  		/* Can release the mapping lock as soon as we've pinned it */
  		LWLockRelease(newPartitionLock);
  
  		*foundPtr = true;
  
  		if (!valid)
  		{
  			/*
  			 * We can only get here if (a) someone else is still reading in
  			 * the page, or (b) a previous read attempt failed.  We have to
  			 * wait for any active read attempt to finish, and then set up our
  			 * own read attempt if the page is still not BM_VALID.
  			 * StartBufferIO does it all.
  			 */
  			if (StartBufferIO(buf, true))
  			{
  				/*
  				 * If we get here, previous attempts to read the buffer must
  				 * have failed ... but we shall bravely try again.
  				 */
  				*foundPtr = false;
  			}
  		}
  
  		return buf;
  	}
  
  	/*
  	 * Didn't find it in the buffer pool.  We'll have to initialize a new
  	 * buffer.  Remember to unlock the mapping lock while doing the work.
  	 */
  	LWLockRelease(newPartitionLock);
  
  	/* Loop here in case we have to try another victim buffer */
  	for (;;)
  	{
  		/*
  		 * Ensure, while the spinlock's not yet held, that there's a free
  		 * refcount entry.
  		 */
  		ReservePrivateRefCountEntry();
  
  		/*
  		 * Select a victim buffer.  The buffer is returned with its header
  		 * spinlock still held!
  		 */
  		buf = StrategyGetBuffer(strategy, &buf_state);
  
  		Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 0);
  
  		/* Must copy buffer flags while we still hold the spinlock */
  		oldFlags = buf_state & BUF_FLAG_MASK;
  
  		/* Pin the buffer and then release the buffer spinlock */
  		PinBuffer_Locked(buf);
  
  		/*
  		 * If the buffer was dirty, try to write it out.  There is a race
  		 * condition here, in that someone might dirty it after we released it
  		 * above, or even while we are writing it out (since our share-lock
  		 * won't prevent hint-bit updates).  We will recheck the dirty bit
  		 * after re-locking the buffer header.
  		 */
  		if (oldFlags & BM_DIRTY)
  		{
  			/*
  			 * We need a share-lock on the buffer contents to write it out
  			 * (else we might write invalid data, eg because someone else is
  			 * compacting the page contents while we write).  We must use a
  			 * conditional lock acquisition here to avoid deadlock.  Even
  			 * though the buffer was not pinned (and therefore surely not
  			 * locked) when StrategyGetBuffer returned it, someone else could
  			 * have pinned and exclusive-locked it by the time we get here. If
  			 * we try to get the lock unconditionally, we'd block waiting for
  			 * them; if they later block waiting for us, deadlock ensues.
  			 * (This has been observed to happen when two backends are both
  			 * trying to split btree index pages, and the second one just
  			 * happens to be trying to split the page the first one got from
  			 * StrategyGetBuffer.)
  			 */
  			if (LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
  										 LW_SHARED))
  			{
  				/*
  				 * If using a nondefault strategy, and writing the buffer
  				 * would require a WAL flush, let the strategy decide whether
  				 * to go ahead and write/reuse the buffer or to choose another
  				 * victim.  We need lock to inspect the page LSN, so this
  				 * can't be done inside StrategyGetBuffer.
  				 */
  				if (strategy != NULL)
  				{
  					XLogRecPtr	lsn;
  
  					/* Read the LSN while holding buffer header lock */
  					buf_state = LockBufHdr(buf);
  					lsn = BufferGetLSN(buf);
  					UnlockBufHdr(buf, buf_state);
  
  					if (XLogNeedsFlush(lsn) &&
  						StrategyRejectBuffer(strategy, buf))
  					{
  						/* Drop lock/pin and loop around for another buffer */
  						LWLockRelease(BufferDescriptorGetContentLock(buf));
  						UnpinBuffer(buf, true);
  						continue;
  					}
  				}
  
  				/* OK, do the I/O */
  				TRACE_POSTGRESQL_BUFFER_WRITE_DIRTY_START(forkNum, blockNum,
  														  smgr->smgr_rnode.node.spcNode,
  														  smgr->smgr_rnode.node.dbNode,
  														  smgr->smgr_rnode.node.relNode);
  
  				FlushBuffer(buf, NULL);
  				LWLockRelease(BufferDescriptorGetContentLock(buf));
  
  				ScheduleBufferTagForWriteback(&BackendWritebackContext,
  											  &buf->tag);
  
  				TRACE_POSTGRESQL_BUFFER_WRITE_DIRTY_DONE(forkNum, blockNum,
  														 smgr->smgr_rnode.node.spcNode,
  														 smgr->smgr_rnode.node.dbNode,
  														 smgr->smgr_rnode.node.relNode);
  			}
  			else
  			{
  				/*
  				 * Someone else has locked the buffer, so give it up and loop
  				 * back to get another one.
  				 */
  				UnpinBuffer(buf, true);
  				continue;
  			}
  		}
  
  		/*
  		 * To change the association of a valid buffer, we'll need to have
  		 * exclusive lock on both the old and new mapping partitions.
  		 */
  		if (oldFlags & BM_TAG_VALID)
  		{
  			/*
  			 * Need to compute the old tag's hashcode and partition lock ID.
  			 * XXX is it worth storing the hashcode in BufferDesc so we need
  			 * not recompute it here?  Probably not.
  			 */
  			oldTag = buf->tag;
  			oldHash = BufTableHashCode(&oldTag);
  			oldPartitionLock = BufMappingPartitionLock(oldHash);
  
  			/*
  			 * Must lock the lower-numbered partition first to avoid
  			 * deadlocks.
  			 */
  			if (oldPartitionLock < newPartitionLock)
  			{
  				LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
  				LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
  			}
  			else if (oldPartitionLock > newPartitionLock)
  			{
  				LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
  				LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
  			}
  			else
  			{
  				/* only one partition, only one lock */
  				LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
  			}
  		}
  		else
  		{
  			/* if it wasn't valid, we need only the new partition */
  			LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
  			/* remember we have no old-partition lock or tag */
  			oldPartitionLock = NULL;
  			/* this just keeps the compiler quiet about uninit variables */
  			oldHash = 0;
  		}
  
  		/*
  		 * Try to make a hashtable entry for the buffer under its new tag.
  		 * This could fail because while we were writing someone else
  		 * allocated another buffer for the same block we want to read in.
  		 * Note that we have not yet removed the hashtable entry for the old
  		 * tag.
  		 */
  		buf_id = BufTableInsert(&newTag, newHash, buf->buf_id);
  
  		if (buf_id >= 0)
  		{
  			/*
  			 * Got a collision. Someone has already done what we were about to
  			 * do. We'll just handle this as if it were found in the buffer
  			 * pool in the first place.  First, give up the buffer we were
  			 * planning to use.
  			 */
  			UnpinBuffer(buf, true);
  
  			/* Can give up that buffer's mapping partition lock now */
  			if (oldPartitionLock != NULL &&
  				oldPartitionLock != newPartitionLock)
  				LWLockRelease(oldPartitionLock);
  
  			/* remaining code should match code at top of routine */
  
  			buf = GetBufferDescriptor(buf_id);
  
  			valid = PinBuffer(buf, strategy);
  
  			/* Can release the mapping lock as soon as we've pinned it */
  			LWLockRelease(newPartitionLock);
  
  			*foundPtr = true;
  
  			if (!valid)
  			{
  				/*
  				 * We can only get here if (a) someone else is still reading
  				 * in the page, or (b) a previous read attempt failed.  We
  				 * have to wait for any active read attempt to finish, and
  				 * then set up our own read attempt if the page is still not
  				 * BM_VALID.  StartBufferIO does it all.
  				 */
  				if (StartBufferIO(buf, true))
  				{
  					/*
  					 * If we get here, previous attempts to read the buffer
  					 * must have failed ... but we shall bravely try again.
  					 */
  					*foundPtr = false;
  				}
  			}
  
  			return buf;
  		}
  
  		/*
  		 * Need to lock the buffer header too in order to change its tag.
  		 */
  		buf_state = LockBufHdr(buf);
  
  		/*
  		 * Somebody could have pinned or re-dirtied the buffer while we were
  		 * doing the I/O and making the new hashtable entry.  If so, we can't
  		 * recycle this buffer; we must undo everything we've done and start
  		 * over with a new victim buffer.
  		 */
  		oldFlags = buf_state & BUF_FLAG_MASK;
  		if (BUF_STATE_GET_REFCOUNT(buf_state) == 1 && !(oldFlags & BM_DIRTY))
  			break;
  
  		UnlockBufHdr(buf, buf_state);
  		BufTableDelete(&newTag, newHash);
  		if (oldPartitionLock != NULL &&
  			oldPartitionLock != newPartitionLock)
  			LWLockRelease(oldPartitionLock);
  		LWLockRelease(newPartitionLock);
  		UnpinBuffer(buf, true);
  	}
  
  	/*
  	 * Okay, it's finally safe to rename the buffer.
  	 *
  	 * Clearing BM_VALID here is necessary, clearing the dirtybits is just
  	 * paranoia.  We also reset the usage_count since any recency of use of
  	 * the old content is no longer relevant.  (The usage_count starts out at
  	 * 1 so that the buffer can survive one clock-sweep pass.)
  	 *
  	 * Make sure BM_PERMANENT is set for buffers that must be written at every
  	 * checkpoint.  Unlogged buffers only need to be written at shutdown
  	 * checkpoints, except for their "init" forks, which need to be treated
  	 * just like permanent relations.
  	 */
  	buf->tag = newTag;
  	buf_state &= ~(BM_VALID | BM_DIRTY | BM_JUST_DIRTIED |
  				   BM_CHECKPOINT_NEEDED | BM_IO_ERROR | BM_PERMANENT |
  				   BUF_USAGECOUNT_MASK);
  	if (relpersistence == RELPERSISTENCE_PERMANENT || forkNum == INIT_FORKNUM)
  		buf_state |= BM_TAG_VALID | BM_PERMANENT | BUF_USAGECOUNT_ONE;
  	else
  		buf_state |= BM_TAG_VALID | BUF_USAGECOUNT_ONE;
  
  	UnlockBufHdr(buf, buf_state);
  
  	if (oldPartitionLock != NULL)
  	{
  		BufTableDelete(&oldTag, oldHash);
  		if (oldPartitionLock != newPartitionLock)
  			LWLockRelease(oldPartitionLock);
  	}
  
  	LWLockRelease(newPartitionLock);
  
  	/*
  	 * Buffer contents are currently invalid.  Try to get the io_in_progress
  	 * lock.  If StartBufferIO returns false, then someone else managed to
  	 * read it before we did, so there's nothing left for BufferAlloc() to do.
  	 */
  	if (StartBufferIO(buf, true))
  		*foundPtr = false;
  	else
  		*foundPtr = true;
  
  	return buf;
  }
  
  /*
   * InvalidateBuffer -- mark a shared buffer invalid and return it to the
   * freelist.
   *
   * The buffer header spinlock must be held at entry.  We drop it before
   * returning.  (This is sane because the caller must have locked the
   * buffer in order to be sure it should be dropped.)
   *
   * This is used only in contexts such as dropping a relation.  We assume
   * that no other backend could possibly be interested in using the page,
   * so the only reason the buffer might be pinned is if someone else is
   * trying to write it out.  We have to let them finish before we can
   * reclaim the buffer.
   *
   * The buffer could get reclaimed by someone else while we are waiting
   * to acquire the necessary locks; if so, don't mess it up.
   */
  static void
  InvalidateBuffer(BufferDesc *buf)
  {
  	BufferTag	oldTag;
  	uint32		oldHash;		/* hash value for oldTag */
  	LWLock	   *oldPartitionLock;	/* buffer partition lock for it */
  	uint32		oldFlags;
  	uint32		buf_state;
  
  	/* Save the original buffer tag before dropping the spinlock */
  	oldTag = buf->tag;
  
  	buf_state = pg_atomic_read_u32(&buf->state);
  	Assert(buf_state & BM_LOCKED);
  	UnlockBufHdr(buf, buf_state);
  
  	/*
  	 * Need to compute the old tag's hashcode and partition lock ID. XXX is it
  	 * worth storing the hashcode in BufferDesc so we need not recompute it
  	 * here?  Probably not.
  	 */
  	oldHash = BufTableHashCode(&oldTag);
  	oldPartitionLock = BufMappingPartitionLock(oldHash);
  
  retry:
  
  	/*
  	 * Acquire exclusive mapping lock in preparation for changing the buffer's
  	 * association.
  	 */
  	LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
  
  	/* Re-lock the buffer header */
  	buf_state = LockBufHdr(buf);
  
  	/* If it's changed while we were waiting for lock, do nothing */
  	if (!BUFFERTAGS_EQUAL(buf->tag, oldTag))
  	{
  		UnlockBufHdr(buf, buf_state);
  		LWLockRelease(oldPartitionLock);
  		return;
  	}
  
  	/*
  	 * We assume the only reason for it to be pinned is that someone else is
  	 * flushing the page out.  Wait for them to finish.  (This could be an
  	 * infinite loop if the refcount is messed up... it would be nice to time
  	 * out after awhile, but there seems no way to be sure how many loops may
  	 * be needed.  Note that if the other guy has pinned the buffer but not
  	 * yet done StartBufferIO, WaitIO will fall through and we'll effectively
  	 * be busy-looping here.)
  	 */
  	if (BUF_STATE_GET_REFCOUNT(buf_state) != 0)
  	{
  		UnlockBufHdr(buf, buf_state);
  		LWLockRelease(oldPartitionLock);
  		/* safety check: should definitely not be our *own* pin */
  		if (GetPrivateRefCount(BufferDescriptorGetBuffer(buf)) > 0)
  			elog(ERROR, "buffer is pinned in InvalidateBuffer");
  		WaitIO(buf);
  		goto retry;
  	}
  
  	/*
  	 * Clear out the buffer's tag and flags.  We must do this to ensure that
  	 * linear scans of the buffer array don't think the buffer is valid.
  	 */
  	oldFlags = buf_state & BUF_FLAG_MASK;
  	CLEAR_BUFFERTAG(buf->tag);
  	buf_state &= ~(BUF_FLAG_MASK | BUF_USAGECOUNT_MASK);
  	UnlockBufHdr(buf, buf_state);
  
  	/*
  	 * Remove the buffer from the lookup hashtable, if it was in there.
  	 */
  	if (oldFlags & BM_TAG_VALID)
  		BufTableDelete(&oldTag, oldHash);
  
  	/*
  	 * Done with mapping lock.
  	 */
  	LWLockRelease(oldPartitionLock);
  
  	/*
  	 * Insert the buffer at the head of the list of free buffers.
  	 */
  	StrategyFreeBuffer(buf);
  }
  
  /*
   * MarkBufferDirty
   *
   *		Marks buffer contents as dirty (actual write happens later).
   *
   * Buffer must be pinned and exclusive-locked.  (If caller does not hold
   * exclusive lock, then somebody could be in process of writing the buffer,
   * leading to risk of bad data written to disk.)
   */
  void
  MarkBufferDirty(Buffer buffer)
  {
  	BufferDesc *bufHdr;
  	uint32		buf_state;
  	uint32		old_buf_state;
  
  	if (!BufferIsValid(buffer))
  		elog(ERROR, "bad buffer ID: %d", buffer);
  
  	if (BufferIsLocal(buffer))
  	{
  		MarkLocalBufferDirty(buffer);
  		return;
  	}
  
  	bufHdr = GetBufferDescriptor(buffer - 1);
  
  	Assert(BufferIsPinned(buffer));
  	Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
  								LW_EXCLUSIVE));
  
  	old_buf_state = pg_atomic_read_u32(&bufHdr->state);
  	for (;;)
  	{
  		if (old_buf_state & BM_LOCKED)
  			old_buf_state = WaitBufHdrUnlocked(bufHdr);
  
  		buf_state = old_buf_state;
  
  		Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
  		buf_state |= BM_DIRTY | BM_JUST_DIRTIED;
  
  		if (pg_atomic_compare_exchange_u32(&bufHdr->state, &old_buf_state,
  										   buf_state))
  			break;
  	}
  
  	/*
  	 * If the buffer was not dirty already, do vacuum accounting.
  	 */
  	if (!(old_buf_state & BM_DIRTY))
  	{
  		VacuumPageDirty++;
  		pgBufferUsage.shared_blks_dirtied++;
  		if (VacuumCostActive)
  			VacuumCostBalance += VacuumCostPageDirty;
  	}
  }
  
  /*
   * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
   *
   * Formerly, this saved one cycle of acquiring/releasing the BufMgrLock
   * compared to calling the two routines separately.  Now it's mainly just
   * a convenience function.  However, if the passed buffer is valid and
   * already contains the desired block, we just return it as-is; and that
   * does save considerable work compared to a full release and reacquire.
   *
   * Note: it is OK to pass buffer == InvalidBuffer, indicating that no old
   * buffer actually needs to be released.  This case is the same as ReadBuffer,
   * but can save some tests in the caller.
   */
  Buffer
  ReleaseAndReadBuffer(Buffer buffer,
  					 Relation relation,
  					 BlockNumber blockNum)
  {
  	ForkNumber	forkNum = MAIN_FORKNUM;
  	BufferDesc *bufHdr;
  
  	if (BufferIsValid(buffer))
  	{
  		Assert(BufferIsPinned(buffer));
  		if (BufferIsLocal(buffer))
  		{
  			bufHdr = GetLocalBufferDescriptor(-buffer - 1);
  			if (bufHdr->tag.blockNum == blockNum &&
  				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
  				bufHdr->tag.forkNum == forkNum)
  				return buffer;
  			ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
  			LocalRefCount[-buffer - 1]--;
  		}
  		else
  		{
  			bufHdr = GetBufferDescriptor(buffer - 1);
  			/* we have pin, so it's ok to examine tag without spinlock */
  			if (bufHdr->tag.blockNum == blockNum &&
  				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
  				bufHdr->tag.forkNum == forkNum)
  				return buffer;
  			UnpinBuffer(bufHdr, true);
  		}
  	}
  
  	return ReadBuffer(relation, blockNum);
  }
  
  /*
   * PinBuffer -- make buffer unavailable for replacement.
   *
   * For the default access strategy, the buffer's usage_count is incremented
   * when we first pin it; for other strategies we just make sure the usage_count
   * isn't zero.  (The idea of the latter is that we don't want synchronized
   * heap scans to inflate the count, but we need it to not be zero to discourage
   * other backends from stealing buffers from our ring.  As long as we cycle
   * through the ring faster than the global clock-sweep cycles, buffers in
   * our ring won't be chosen as victims for replacement by other backends.)
   *
   * This should be applied only to shared buffers, never local ones.
   *
   * Since buffers are pinned/unpinned very frequently, pin buffers without
   * taking the buffer header lock; instead update the state variable in loop of
   * CAS operations. Hopefully it's just a single CAS.
   *
   * Note that ResourceOwnerEnlargeBuffers must have been done already.
   *
   * Returns true if buffer is BM_VALID, else false.  This provision allows
   * some callers to avoid an extra spinlock cycle.
   */
  static bool
  PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy)
  {
  	Buffer		b = BufferDescriptorGetBuffer(buf);
  	bool		result;
  	PrivateRefCountEntry *ref;
  
  	ref = GetPrivateRefCountEntry(b, true);
  
  	if (ref == NULL)
  	{
  		uint32		buf_state;
  		uint32		old_buf_state;
  
  		ReservePrivateRefCountEntry();
  		ref = NewPrivateRefCountEntry(b);
  
  		old_buf_state = pg_atomic_read_u32(&buf->state);
  		for (;;)
  		{
  			if (old_buf_state & BM_LOCKED)
  				old_buf_state = WaitBufHdrUnlocked(buf);
  
  			buf_state = old_buf_state;
  
  			/* increase refcount */
  			buf_state += BUF_REFCOUNT_ONE;
  
  			if (strategy == NULL)
  			{
  				/* Default case: increase usagecount unless already max. */
  				if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT)
  					buf_state += BUF_USAGECOUNT_ONE;
  			}
  			else
  			{
  				/*
  				 * Ring buffers shouldn't evict others from pool.  Thus we
  				 * don't make usagecount more than 1.
  				 */
  				if (BUF_STATE_GET_USAGECOUNT(buf_state) == 0)
  					buf_state += BUF_USAGECOUNT_ONE;
  			}
  
  			if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
  											   buf_state))
  			{
  				result = (buf_state & BM_VALID) != 0;
  				break;
  			}
  		}
  	}
  	else
  	{
  		/* If we previously pinned the buffer, it must surely be valid */
  		result = true;
  	}
  
  	ref->refcount++;
  	Assert(ref->refcount > 0);
  	ResourceOwnerRememberBuffer(CurrentResourceOwner, b);
  	return result;
  }
  
  /*
   * PinBuffer_Locked -- as above, but caller already locked the buffer header.
   * The spinlock is released before return.
   *
   * As this function is called with the spinlock held, the caller has to
   * previously call ReservePrivateRefCountEntry().
   *
   * Currently, no callers of this function want to modify the buffer's
   * usage_count at all, so there's no need for a strategy parameter.
   * Also we don't bother with a BM_VALID test (the caller could check that for
   * itself).
   *
   * Also all callers only ever use this function when it's known that the
   * buffer can't have a preexisting pin by this backend. That allows us to skip
   * searching the private refcount array & hash, which is a boon, because the
   * spinlock is still held.
   *
   * Note: use of this routine is frequently mandatory, not just an optimization
   * to save a spin lock/unlock cycle, because we need to pin a buffer before
   * its state can change under us.
   */
  static void
  PinBuffer_Locked(BufferDesc *buf)
  {
  	Buffer		b;
  	PrivateRefCountEntry *ref;
  	uint32		buf_state;
  
  	/*
  	 * As explained, We don't expect any preexisting pins. That allows us to
  	 * manipulate the PrivateRefCount after releasing the spinlock
  	 */
  	Assert(GetPrivateRefCountEntry(BufferDescriptorGetBuffer(buf), false) == NULL);
  
  	/*
  	 * Since we hold the buffer spinlock, we can update the buffer state and
  	 * release the lock in one operation.
  	 */
  	buf_state = pg_atomic_read_u32(&buf->state);
  	Assert(buf_state & BM_LOCKED);
  	buf_state += BUF_REFCOUNT_ONE;
  	UnlockBufHdr(buf, buf_state);
  
  	b = BufferDescriptorGetBuffer(buf);
  
  	ref = NewPrivateRefCountEntry(b);
  	ref->refcount++;
  
  	ResourceOwnerRememberBuffer(CurrentResourceOwner, b);
  }
  
  /*
   * UnpinBuffer -- make buffer available for replacement.
   *
   * This should be applied only to shared buffers, never local ones.
   *
   * Most but not all callers want CurrentResourceOwner to be adjusted.
   * Those that don't should pass fixOwner = false.
   */
  static void
  UnpinBuffer(BufferDesc *buf, bool fixOwner)
  {
  	PrivateRefCountEntry *ref;
  	Buffer		b = BufferDescriptorGetBuffer(buf);
  
  	/* not moving as we're likely deleting it soon anyway */
  	ref = GetPrivateRefCountEntry(b, false);
  	Assert(ref != NULL);
  
  	if (fixOwner)
  		ResourceOwnerForgetBuffer(CurrentResourceOwner, b);
  
  	Assert(ref->refcount > 0);
  	ref->refcount--;
  	if (ref->refcount == 0)
  	{
  		uint32		buf_state;
  		uint32		old_buf_state;
  
  		/* I'd better not still hold any locks on the buffer */
  		Assert(!LWLockHeldByMe(BufferDescriptorGetContentLock(buf)));
  		Assert(!LWLockHeldByMe(BufferDescriptorGetIOLock(buf)));
  
  		/*
  		 * Decrement the shared reference count.
  		 *
  		 * Since buffer spinlock holder can update status using just write,
  		 * it's not safe to use atomic decrement here; thus use a CAS loop.
  		 */
  		old_buf_state = pg_atomic_read_u32(&buf->state);
  		for (;;)
  		{
  			if (old_buf_state & BM_LOCKED)
  				old_buf_state = WaitBufHdrUnlocked(buf);
  
  			buf_state = old_buf_state;
  
  			buf_state -= BUF_REFCOUNT_ONE;
  
  			if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
  											   buf_state))
  				break;
  		}
  
  		/* Support LockBufferForCleanup() */
  		if (buf_state & BM_PIN_COUNT_WAITER)
  		{
  			/*
  			 * Acquire the buffer header lock, re-check that there's a waiter.
  			 * Another backend could have unpinned this buffer, and already
  			 * woken up the waiter.  There's no danger of the buffer being
  			 * replaced after we unpinned it above, as it's pinned by the
  			 * waiter.
  			 */
  			buf_state = LockBufHdr(buf);
  
  			if ((buf_state & BM_PIN_COUNT_WAITER) &&
  				BUF_STATE_GET_REFCOUNT(buf_state) == 1)
  			{
  				/* we just released the last pin other than the waiter's */
  				int			wait_backend_pid = buf->wait_backend_pid;
  
  				buf_state &= ~BM_PIN_COUNT_WAITER;
  				UnlockBufHdr(buf, buf_state);
  				ProcSendSignal(wait_backend_pid);
  			}
  			else
  				UnlockBufHdr(buf, buf_state);
  		}
  		ForgetPrivateRefCountEntry(ref);
  	}
  }
  
  /*
   * BufferSync -- Write out all dirty buffers in the pool.
   *
   * This is called at checkpoint time to write out all dirty shared buffers.
   * The checkpoint request flags should be passed in.  If CHECKPOINT_IMMEDIATE
   * is set, we disable delays between writes; if CHECKPOINT_IS_SHUTDOWN,
   * CHECKPOINT_END_OF_RECOVERY or CHECKPOINT_FLUSH_ALL is set, we write even
   * unlogged buffers, which are otherwise skipped.  The remaining flags
   * currently have no effect here.
   */
  static void
  BufferSync(int flags)
  {
  	uint32		buf_state;
  	int			buf_id;
  	int			num_to_scan;
  	int			num_spaces;
  	int			num_processed;
  	int			num_written;
  	CkptTsStatus *per_ts_stat = NULL;
  	Oid			last_tsid;
  	binaryheap *ts_heap;
  	int			i;
  	int			mask = BM_DIRTY;
  	WritebackContext wb_context;
  
  	/* Make sure we can handle the pin inside SyncOneBuffer */
  	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
  
  	/*
  	 * Unless this is a shutdown checkpoint or we have been explicitly told,
  	 * we write only permanent, dirty buffers.  But at shutdown or end of
  	 * recovery, we write all dirty buffers.
  	 */
  	if (!((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
  					CHECKPOINT_FLUSH_ALL))))
  		mask |= BM_PERMANENT;
  
  	/*
  	 * Loop over all buffers, and mark the ones that need to be written with
  	 * BM_CHECKPOINT_NEEDED.  Count them as we go (num_to_scan), so that we
  	 * can estimate how much work needs to be done.
  	 *
  	 * This allows us to write only those pages that were dirty when the
  	 * checkpoint began, and not those that get dirtied while it proceeds.
  	 * Whenever a page with BM_CHECKPOINT_NEEDED is written out, either by us
  	 * later in this function, or by normal backends or the bgwriter cleaning
  	 * scan, the flag is cleared.  Any buffer dirtied after this point won't
  	 * have the flag set.
  	 *
  	 * Note that if we fail to write some buffer, we may leave buffers with
  	 * BM_CHECKPOINT_NEEDED still set.  This is OK since any such buffer would
  	 * certainly need to be written for the next checkpoint attempt, too.
  	 */
  	num_to_scan = 0;
  	for (buf_id = 0; buf_id < NBuffers; buf_id++)
  	{
  		BufferDesc *bufHdr = GetBufferDescriptor(buf_id);
  
  		/*
  		 * Header spinlock is enough to examine BM_DIRTY, see comment in
  		 * SyncOneBuffer.
  		 */
  		buf_state = LockBufHdr(bufHdr);
  
  		if ((buf_state & mask) == mask)
  		{
  			CkptSortItem *item;
  
  			buf_state |= BM_CHECKPOINT_NEEDED;
  
  			item = &CkptBufferIds[num_to_scan++];
  			item->buf_id = buf_id;
  			item->tsId = bufHdr->tag.rnode.spcNode;
  			item->relNode = bufHdr->tag.rnode.relNode;
  			item->forkNum = bufHdr->tag.forkNum;
  			item->blockNum = bufHdr->tag.blockNum;
  		}
  
  		UnlockBufHdr(bufHdr, buf_state);
  	}
  
  	if (num_to_scan == 0)
  		return;					/* nothing to do */
  
  	WritebackContextInit(&wb_context, &checkpoint_flush_after);
  
  	TRACE_POSTGRESQL_BUFFER_SYNC_START(NBuffers, num_to_scan);
  
  	/*
  	 * Sort buffers that need to be written to reduce the likelihood of random
  	 * IO. The sorting is also important for the implementation of balancing
  	 * writes between tablespaces. Without balancing writes we'd potentially
  	 * end up writing to the tablespaces one-by-one; possibly overloading the
  	 * underlying system.
  	 */
  	qsort(CkptBufferIds, num_to_scan, sizeof(CkptSortItem),
  		  ckpt_buforder_comparator);
  
  	num_spaces = 0;
  
  	/*
  	 * Allocate progress status for each tablespace with buffers that need to
  	 * be flushed. This requires the to-be-flushed array to be sorted.
  	 */
  	last_tsid = InvalidOid;
  	for (i = 0; i < num_to_scan; i++)
  	{
  		CkptTsStatus *s;
  		Oid			cur_tsid;
  
  		cur_tsid = CkptBufferIds[i].tsId;
  
  		/*
  		 * Grow array of per-tablespace status structs, every time a new
  		 * tablespace is found.
  		 */
  		if (last_tsid == InvalidOid || last_tsid != cur_tsid)
  		{
  			Size		sz;
  
  			num_spaces++;
  
  			/*
  			 * Not worth adding grow-by-power-of-2 logic here - even with a
  			 * few hundred tablespaces this should be fine.
  			 */
  			sz = sizeof(CkptTsStatus) * num_spaces;
  
  			if (per_ts_stat == NULL)
  				per_ts_stat = (CkptTsStatus *) palloc(sz);
  			else
  				per_ts_stat = (CkptTsStatus *) repalloc(per_ts_stat, sz);
  
  			s = &per_ts_stat[num_spaces - 1];
  			memset(s, 0, sizeof(*s));
  			s->tsId = cur_tsid;
  
  			/*
  			 * The first buffer in this tablespace. As CkptBufferIds is sorted
  			 * by tablespace all (s->num_to_scan) buffers in this tablespace
  			 * will follow afterwards.
  			 */
  			s->index = i;
  
  			/*
  			 * progress_slice will be determined once we know how many buffers
  			 * are in each tablespace, i.e. after this loop.
  			 */
  
  			last_tsid = cur_tsid;
  		}
  		else
  		{
  			s = &per_ts_stat[num_spaces - 1];
  		}
  
  		s->num_to_scan++;
  	}
  
  	Assert(num_spaces > 0);
  
  	/*
  	 * Build a min-heap over the write-progress in the individual tablespaces,
  	 * and compute how large a portion of the total progress a single
  	 * processed buffer is.
  	 */
  	ts_heap = binaryheap_allocate(num_spaces,
  								  ts_ckpt_progress_comparator,
  								  NULL);
  
  	for (i = 0; i < num_spaces; i++)
  	{
  		CkptTsStatus *ts_stat = &per_ts_stat[i];
  
  		ts_stat->progress_slice = (float8) num_to_scan / ts_stat->num_to_scan;
  
  		binaryheap_add_unordered(ts_heap, PointerGetDatum(ts_stat));
  	}
  
  	binaryheap_build(ts_heap);
  
  	/*
  	 * Iterate through to-be-checkpointed buffers and write the ones (still)
  	 * marked with BM_CHECKPOINT_NEEDED. The writes are balanced between
  	 * tablespaces; otherwise the sorting would lead to only one tablespace
  	 * receiving writes at a time, making inefficient use of the hardware.
  	 */
  	num_processed = 0;
  	num_written = 0;
  	while (!binaryheap_empty(ts_heap))
  	{
  		BufferDesc *bufHdr = NULL;
  		CkptTsStatus *ts_stat = (CkptTsStatus *)
  		DatumGetPointer(binaryheap_first(ts_heap));
  
  		buf_id = CkptBufferIds[ts_stat->index].buf_id;
  		Assert(buf_id != -1);
  
  		bufHdr = GetBufferDescriptor(buf_id);
  
  		num_processed++;
  
  		/*
  		 * We don't need to acquire the lock here, because we're only looking
  		 * at a single bit. It's possible that someone else writes the buffer
  		 * and clears the flag right after we check, but that doesn't matter
  		 * since SyncOneBuffer will then do nothing.  However, there is a
  		 * further race condition: it's conceivable that between the time we
  		 * examine the bit here and the time SyncOneBuffer acquires the lock,
  		 * someone else not only wrote the buffer but replaced it with another
  		 * page and dirtied it.  In that improbable case, SyncOneBuffer will
  		 * write the buffer though we didn't need to.  It doesn't seem worth
  		 * guarding against this, though.
  		 */
  		if (pg_atomic_read_u32(&bufHdr->state) & BM_CHECKPOINT_NEEDED)
  		{
  			if (SyncOneBuffer(buf_id, false, &wb_context) & BUF_WRITTEN)
  			{
  				TRACE_POSTGRESQL_BUFFER_SYNC_WRITTEN(buf_id);
  				BgWriterStats.m_buf_written_checkpoints++;
  				num_written++;
  			}
  		}
  
  		/*
  		 * Measure progress independent of actually having to flush the buffer
  		 * - otherwise writing become unbalanced.
  		 */
  		ts_stat->progress += ts_stat->progress_slice;
  		ts_stat->num_scanned++;
  		ts_stat->index++;
  
  		/* Have all the buffers from the tablespace been processed? */
  		if (ts_stat->num_scanned == ts_stat->num_to_scan)
  		{
  			binaryheap_remove_first(ts_heap);
  		}
  		else
  		{
  			/* update heap with the new progress */
  			binaryheap_replace_first(ts_heap, PointerGetDatum(ts_stat));
  		}
  
  		/*
  		 * Sleep to throttle our I/O rate.
  		 */
  		CheckpointWriteDelay(flags, (double) num_processed / num_to_scan);
  	}
  
  	/* issue all pending flushes */
  	IssuePendingWritebacks(&wb_context);
  
  	pfree(per_ts_stat);
  	per_ts_stat = NULL;
  	binaryheap_free(ts_heap);
  
  	/*
  	 * Update checkpoint statistics. As noted above, this doesn't include
  	 * buffers written by other backends or bgwriter scan.
  	 */
  	CheckpointStats.ckpt_bufs_written += num_written;
  
  	TRACE_POSTGRESQL_BUFFER_SYNC_DONE(NBuffers, num_written, num_to_scan);
  }
  
  /*
   * BgBufferSync -- Write out some dirty buffers in the pool.
   *
   * This is called periodically by the background writer process.
   *
   * Returns true if it's appropriate for the bgwriter process to go into
   * low-power hibernation mode.  (This happens if the strategy clock sweep
   * has been "lapped" and no buffer allocations have occurred recently,
   * or if the bgwriter has been effectively disabled by setting
   * bgwriter_lru_maxpages to 0.)
   */
  bool
  BgBufferSync(WritebackContext *wb_context)
  {
  	/* info obtained from freelist.c */
  	int			strategy_buf_id;
  	uint32		strategy_passes;
  	uint32		recent_alloc;
  
  	/*
  	 * Information saved between calls so we can determine the strategy
  	 * point's advance rate and avoid scanning already-cleaned buffers.
  	 */
  	static bool saved_info_valid = false;
  	static int	prev_strategy_buf_id;
  	static uint32 prev_strategy_passes;
  	static int	next_to_clean;
  	static uint32 next_passes;
  
  	/* Moving averages of allocation rate and clean-buffer density */
  	static float smoothed_alloc = 0;
  	static float smoothed_density = 10.0;
  
  	/* Potentially these could be tunables, but for now, not */
  	float		smoothing_samples = 16;
  	float		scan_whole_pool_milliseconds = 120000.0;
  
  	/* Used to compute how far we scan ahead */
  	long		strategy_delta;
  	int			bufs_to_lap;
  	int			bufs_ahead;
  	float		scans_per_alloc;
  	int			reusable_buffers_est;
  	int			upcoming_alloc_est;
  	int			min_scan_buffers;
  
  	/* Variables for the scanning loop proper */
  	int			num_to_scan;
  	int			num_written;
  	int			reusable_buffers;
  
  	/* Variables for final smoothed_density update */
  	long		new_strategy_delta;
  	uint32		new_recent_alloc;
  
  	/*
  	 * Find out where the freelist clock sweep currently is, and how many
  	 * buffer allocations have happened since our last call.
  	 */
  	strategy_buf_id = StrategySyncStart(&strategy_passes, &recent_alloc);
  
  	/* Report buffer alloc counts to pgstat */
  	BgWriterStats.m_buf_alloc += recent_alloc;
  
  	/*
  	 * If we're not running the LRU scan, just stop after doing the stats
  	 * stuff.  We mark the saved state invalid so that we can recover sanely
  	 * if LRU scan is turned back on later.
  	 */
  	if (bgwriter_lru_maxpages <= 0)
  	{
  		saved_info_valid = false;
  		return true;
  	}
  
  	/*
  	 * Compute strategy_delta = how many buffers have been scanned by the
  	 * clock sweep since last time.  If first time through, assume none. Then
  	 * see if we are still ahead of the clock sweep, and if so, how many
  	 * buffers we could scan before we'd catch up with it and "lap" it. Note:
  	 * weird-looking coding of xxx_passes comparisons are to avoid bogus
  	 * behavior when the passes counts wrap around.
  	 */
  	if (saved_info_valid)
  	{
  		int32		passes_delta = strategy_passes - prev_strategy_passes;
  
  		strategy_delta = strategy_buf_id - prev_strategy_buf_id;
  		strategy_delta += (long) passes_delta * NBuffers;
  
  		Assert(strategy_delta >= 0);
  
  		if ((int32) (next_passes - strategy_passes) > 0)
  		{
  			/* we're one pass ahead of the strategy point */
  			bufs_to_lap = strategy_buf_id - next_to_clean;
  #ifdef BGW_DEBUG
  			elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
  				 next_passes, next_to_clean,
  				 strategy_passes, strategy_buf_id,
  				 strategy_delta, bufs_to_lap);
  #endif
  		}
  		else if (next_passes == strategy_passes &&
  				 next_to_clean >= strategy_buf_id)
  		{
  			/* on same pass, but ahead or at least not behind */
  			bufs_to_lap = NBuffers - (next_to_clean - strategy_buf_id);
  #ifdef BGW_DEBUG
  			elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
  				 next_passes, next_to_clean,
  				 strategy_passes, strategy_buf_id,
  				 strategy_delta, bufs_to_lap);
  #endif
  		}
  		else
  		{
  			/*
  			 * We're behind, so skip forward to the strategy point and start
  			 * cleaning from there.
  			 */
  #ifdef BGW_DEBUG
  			elog(DEBUG2, "bgwriter behind: bgw %u-%u strategy %u-%u delta=%ld",
  				 next_passes, next_to_clean,
  				 strategy_passes, strategy_buf_id,
  				 strategy_delta);
  #endif
  			next_to_clean = strategy_buf_id;
  			next_passes = strategy_passes;
  			bufs_to_lap = NBuffers;
  		}
  	}
  	else
  	{
  		/*
  		 * Initializing at startup or after LRU scanning had been off. Always
  		 * start at the strategy point.
  		 */
  #ifdef BGW_DEBUG
  		elog(DEBUG2, "bgwriter initializing: strategy %u-%u",
  			 strategy_passes, strategy_buf_id);
  #endif
  		strategy_delta = 0;
  		next_to_clean = strategy_buf_id;
  		next_passes = strategy_passes;
  		bufs_to_lap = NBuffers;
  	}
  
  	/* Update saved info for next time */
  	prev_strategy_buf_id = strategy_buf_id;
  	prev_strategy_passes = strategy_passes;
  	saved_info_valid = true;
  
  	/*
  	 * Compute how many buffers had to be scanned for each new allocation, ie,
  	 * 1/density of reusable buffers, and track a moving average of that.
  	 *
  	 * If the strategy point didn't move, we don't update the density estimate
  	 */
  	if (strategy_delta > 0 && recent_alloc > 0)
  	{
  		scans_per_alloc = (float) strategy_delta / (float) recent_alloc;
  		smoothed_density += (scans_per_alloc - smoothed_density) /
  			smoothing_samples;
  	}
  
  	/*
  	 * Estimate how many reusable buffers there are between the current
  	 * strategy point and where we've scanned ahead to, based on the smoothed
  	 * density estimate.
  	 */
  	bufs_ahead = NBuffers - bufs_to_lap;
  	reusable_buffers_est = (float) bufs_ahead / smoothed_density;
  
  	/*
  	 * Track a moving average of recent buffer allocations.  Here, rather than
  	 * a true average we want a fast-attack, slow-decline behavior: we
  	 * immediately follow any increase.
  	 */
  	if (smoothed_alloc <= (float) recent_alloc)
  		smoothed_alloc = recent_alloc;
  	else
  		smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
  			smoothing_samples;
  
  	/* Scale the estimate by a GUC to allow more aggressive tuning. */
  	upcoming_alloc_est = (int) (smoothed_alloc * bgwriter_lru_multiplier);
  
  	/*
  	 * If recent_alloc remains at zero for many cycles, smoothed_alloc will
  	 * eventually underflow to zero, and the underflows produce annoying
  	 * kernel warnings on some platforms.  Once upcoming_alloc_est has gone to
  	 * zero, there's no point in tracking smaller and smaller values of
  	 * smoothed_alloc, so just reset it to exactly zero to avoid this
  	 * syndrome.  It will pop back up as soon as recent_alloc increases.
  	 */
  	if (upcoming_alloc_est == 0)
  		smoothed_alloc = 0;
  
  	/*
  	 * Even in cases where there's been little or no buffer allocation
  	 * activity, we want to make a small amount of progress through the buffer
  	 * cache so that as many reusable buffers as possible are clean after an
  	 * idle period.
  	 *
  	 * (scan_whole_pool_milliseconds / BgWriterDelay) computes how many times
  	 * the BGW will be called during the scan_whole_pool time; slice the
  	 * buffer pool into that many sections.
  	 */
  	min_scan_buffers = (int) (NBuffers / (scan_whole_pool_milliseconds / BgWriterDelay));
  
  	if (upcoming_alloc_est < (min_scan_buffers + reusable_buffers_est))
  	{
  #ifdef BGW_DEBUG
  		elog(DEBUG2, "bgwriter: alloc_est=%d too small, using min=%d + reusable_est=%d",
  			 upcoming_alloc_est, min_scan_buffers, reusable_buffers_est);
  #endif
  		upcoming_alloc_est = min_scan_buffers + reusable_buffers_est;
  	}
  
  	/*
  	 * Now write out dirty reusable buffers, working forward from the
  	 * next_to_clean point, until we have lapped the strategy scan, or cleaned
  	 * enough buffers to match our estimate of the next cycle's allocation
  	 * requirements, or hit the bgwriter_lru_maxpages limit.
  	 */
  
  	/* Make sure we can handle the pin inside SyncOneBuffer */
  	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
  
  	num_to_scan = bufs_to_lap;
  	num_written = 0;
  	reusable_buffers = reusable_buffers_est;
  
  	/* Execute the LRU scan */
  	while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
  	{
  		int			sync_state = SyncOneBuffer(next_to_clean, true,
  											   wb_context);
  
  		if (++next_to_clean >= NBuffers)
  		{
  			next_to_clean = 0;
  			next_passes++;
  		}
  		num_to_scan--;
  
  		if (sync_state & BUF_WRITTEN)
  		{
  			reusable_buffers++;
  			if (++num_written >= bgwriter_lru_maxpages)
  			{
  				BgWriterStats.m_maxwritten_clean++;
  				break;
  			}
  		}
  		else if (sync_state & BUF_REUSABLE)
  			reusable_buffers++;
  	}
  
  	BgWriterStats.m_buf_written_clean += num_written;
  
  #ifdef BGW_DEBUG
  	elog(DEBUG1, "bgwriter: recent_alloc=%u smoothed=%.2f delta=%ld ahead=%d density=%.2f reusable_est=%d upcoming_est=%d scanned=%d wrote=%d reusable=%d",
  		 recent_alloc, smoothed_alloc, strategy_delta, bufs_ahead,
  		 smoothed_density, reusable_buffers_est, upcoming_alloc_est,
  		 bufs_to_lap - num_to_scan,
  		 num_written,
  		 reusable_buffers - reusable_buffers_est);
  #endif
  
  	/*
  	 * Consider the above scan as being like a new allocation scan.
  	 * Characterize its density and update the smoothed one based on it. This
  	 * effectively halves the moving average period in cases where both the
  	 * strategy and the background writer are doing some useful scanning,
  	 * which is helpful because a long memory isn't as desirable on the
  	 * density estimates.
  	 */
  	new_strategy_delta = bufs_to_lap - num_to_scan;
  	new_recent_alloc = reusable_buffers - reusable_buffers_est;
  	if (new_strategy_delta > 0 && new_recent_alloc > 0)
  	{
  		scans_per_alloc = (float) new_strategy_delta / (float) new_recent_alloc;
  		smoothed_density += (scans_per_alloc - smoothed_density) /
  			smoothing_samples;
  
  #ifdef BGW_DEBUG
  		elog(DEBUG2, "bgwriter: cleaner density alloc=%u scan=%ld density=%.2f new smoothed=%.2f",
  			 new_recent_alloc, new_strategy_delta,
  			 scans_per_alloc, smoothed_density);
  #endif
  	}
  
  	/* Return true if OK to hibernate */
  	return (bufs_to_lap == 0 && recent_alloc == 0);
  }
  
  /*
   * SyncOneBuffer -- process a single buffer during syncing.
   *
   * If skip_recently_used is true, we don't write currently-pinned buffers, nor
   * buffers marked recently used, as these are not replacement candidates.
   *
   * Returns a bitmask containing the following flag bits:
   *	BUF_WRITTEN: we wrote the buffer.
   *	BUF_REUSABLE: buffer is available for replacement, ie, it has
   *		pin count 0 and usage count 0.
   *
   * (BUF_WRITTEN could be set in error if FlushBuffers finds the buffer clean
   * after locking it, but we don't care all that much.)
   *
   * Note: caller must have done ResourceOwnerEnlargeBuffers.
   */
  static int
  SyncOneBuffer(int buf_id, bool skip_recently_used, WritebackContext *wb_context)
  {
  	BufferDesc *bufHdr = GetBufferDescriptor(buf_id);
  	int			result = 0;
  	uint32		buf_state;
  	BufferTag	tag;
  
  	ReservePrivateRefCountEntry();
  
  	/*
  	 * Check whether buffer needs writing.
  	 *
  	 * We can make this check without taking the buffer content lock so long
  	 * as we mark pages dirty in access methods *before* logging changes with
  	 * XLogInsert(): if someone marks the buffer dirty just after our check we
  	 * don't worry because our checkpoint.redo points before log record for
  	 * upcoming changes and so we are not required to write such dirty buffer.
  	 */
  	buf_state = LockBufHdr(bufHdr);
  
  	if (BUF_STATE_GET_REFCOUNT(buf_state) == 0 &&
  		BUF_STATE_GET_USAGECOUNT(buf_state) == 0)
  	{
  		result |= BUF_REUSABLE;
  	}
  	else if (skip_recently_used)
  	{
  		/* Caller told us not to write recently-used buffers */
  		UnlockBufHdr(bufHdr, buf_state);
  		return result;
  	}
  
  	if (!(buf_state & BM_VALID) || !(buf_state & BM_DIRTY))
  	{
  		/* It's clean, so nothing to do */
  		UnlockBufHdr(bufHdr, buf_state);
  		return result;
  	}
  
  	/*
  	 * Pin it, share-lock it, write it.  (FlushBuffer will do nothing if the
  	 * buffer is clean by the time we've locked it.)
  	 */
  	PinBuffer_Locked(bufHdr);
  	LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
  
  	FlushBuffer(bufHdr, NULL);
  
  	LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
  
  	tag = bufHdr->tag;
  
  	UnpinBuffer(bufHdr, true);
  
  	ScheduleBufferTagForWriteback(wb_context, &tag);
  
  	return result | BUF_WRITTEN;
  }
  
  /*
   *		AtEOXact_Buffers - clean up at end of transaction.
   *
   *		As of PostgreSQL 8.0, buffer pins should get released by the
   *		ResourceOwner mechanism.  This routine is just a debugging
   *		cross-check that no pins remain.
   */
  void
  AtEOXact_Buffers(bool isCommit)
  {
  	CheckForBufferLeaks();
  
  	AtEOXact_LocalBuffers(isCommit);
  
  	Assert(PrivateRefCountOverflowed == 0);
  }
  
  /*
   * Initialize access to shared buffer pool
   *
   * This is called during backend startup (whether standalone or under the
   * postmaster).  It sets up for this backend's access to the already-existing
   * buffer pool.
   *
   * NB: this is called before InitProcess(), so we do not have a PGPROC and
   * cannot do LWLockAcquire; hence we can't actually access stuff in
   * shared memory yet.  We are only initializing local data here.
   * (See also InitBufferPoolBackend)
   */
  void
  InitBufferPoolAccess(void)
  {
  	HASHCTL		hash_ctl;
  
  	memset(&PrivateRefCountArray, 0, sizeof(PrivateRefCountArray));
  
  	MemSet(&hash_ctl, 0, sizeof(hash_ctl));
  	hash_ctl.keysize = sizeof(int32);
  	hash_ctl.entrysize = sizeof(PrivateRefCountEntry);
  
  	PrivateRefCountHash = hash_create("PrivateRefCount", 100, &hash_ctl,
  									  HASH_ELEM | HASH_BLOBS);
  }
  
  /*
   * InitBufferPoolBackend --- second-stage initialization of a new backend
   *
   * This is called after we have acquired a PGPROC and so can safely get
   * LWLocks.  We don't currently need to do anything at this stage ...
   * except register a shmem-exit callback.  AtProcExit_Buffers needs LWLock
   * access, and thereby has to be called at the corresponding phase of
   * backend shutdown.
   */
  void
  InitBufferPoolBackend(void)
  {
  	on_shmem_exit(AtProcExit_Buffers, 0);
  }
  
  /*
   * During backend exit, ensure that we released all shared-buffer locks and
   * assert that we have no remaining pins.
   */
  static void
  AtProcExit_Buffers(int code, Datum arg)
  {
  	AbortBufferIO();
  	UnlockBuffers();
  
  	CheckForBufferLeaks();
  
  	/* localbuf.c needs a chance too */
  	AtProcExit_LocalBuffers();
  }
  
  /*
   *		CheckForBufferLeaks - ensure this backend holds no buffer pins
   *
   *		As of PostgreSQL 8.0, buffer pins should get released by the
   *		ResourceOwner mechanism.  This routine is just a debugging
   *		cross-check that no pins remain.
   */
  static void
  CheckForBufferLeaks(void)
  {
  #ifdef USE_ASSERT_CHECKING
  	int			RefCountErrors = 0;
  	PrivateRefCountEntry *res;
  	int			i;
  
  	/* check the array */
  	for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
  	{
  		res = &PrivateRefCountArray[i];
  
  		if (res->buffer != InvalidBuffer)
  		{
  			PrintBufferLeakWarning(res->buffer);
  			RefCountErrors++;
  		}
  	}
  
  	/* if necessary search the hash */
  	if (PrivateRefCountOverflowed)
  	{
  		HASH_SEQ_STATUS hstat;
  
  		hash_seq_init(&hstat, PrivateRefCountHash);
  		while ((res = (PrivateRefCountEntry *) hash_seq_search(&hstat)) != NULL)
  		{
  			PrintBufferLeakWarning(res->buffer);
  			RefCountErrors++;
  		}
  
  	}
  
  	Assert(RefCountErrors == 0);
  #endif
  }
  
  /*
   * Helper routine to issue warnings when a buffer is unexpectedly pinned
   */
  void
  PrintBufferLeakWarning(Buffer buffer)
  {
  	BufferDesc *buf;
  	int32		loccount;
  	char	   *path;
  	BackendId	backend;
  	uint32		buf_state;
  
  	Assert(BufferIsValid(buffer));
  	if (BufferIsLocal(buffer))
  	{
  		buf = GetLocalBufferDescriptor(-buffer - 1);
  		loccount = LocalRefCount[-buffer - 1];
  		backend = MyBackendId;
  	}
  	else
  	{
  		buf = GetBufferDescriptor(buffer - 1);
  		loccount = GetPrivateRefCount(buffer);
  		backend = InvalidBackendId;
  	}
  
  	/* theoretically we should lock the bufhdr here */
  	path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
  	buf_state = pg_atomic_read_u32(&buf->state);
  	elog(WARNING,
  		 "buffer refcount leak: [%03d] "
  		 "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
  		 buffer, path,
  		 buf->tag.blockNum, buf_state & BUF_FLAG_MASK,
  		 BUF_STATE_GET_REFCOUNT(buf_state), loccount);
  	pfree(path);
  }
  
  /*
   * CheckPointBuffers
   *
   * Flush all dirty blocks in buffer pool to disk at checkpoint time.
   *
   * Note: temporary relations do not participate in checkpoints, so they don't
   * need to be flushed.
   */
  void
  CheckPointBuffers(int flags)
  {
  	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_START(flags);
  	CheckpointStats.ckpt_write_t = GetCurrentTimestamp();
  	BufferSync(flags);
  	CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
  	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_SYNC_START();
  	smgrsync();
  	CheckpointStats.ckpt_sync_end_t = GetCurrentTimestamp();
  	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_DONE();
  }
  
  
  /*
   * Do whatever is needed to prepare for commit at the bufmgr and smgr levels
   */
  void
  BufmgrCommit(void)
  {
  	/* Nothing to do in bufmgr anymore... */
  }
  
  /*
   * BufferGetBlockNumber
   *		Returns the block number associated with a buffer.
   *
   * Note:
   *		Assumes that the buffer is valid and pinned, else the
   *		value may be obsolete immediately...
   */
  BlockNumber
  BufferGetBlockNumber(Buffer buffer)
  {
  	BufferDesc *bufHdr;
  
  	Assert(BufferIsPinned(buffer));
  
  	if (BufferIsLocal(buffer))
  		bufHdr = GetLocalBufferDescriptor(-buffer - 1);
  	else
  		bufHdr = GetBufferDescriptor(buffer - 1);
  
  	/* pinned, so OK to read tag without spinlock */
  	return bufHdr->tag.blockNum;
  }
  
  /*
   * BufferGetTag
   *		Returns the relfilenode, fork number and block number associated with
   *		a buffer.
   */
  void
  BufferGetTag(Buffer buffer, RelFileNode *rnode, ForkNumber *forknum,
  			 BlockNumber *blknum)
  {
  	BufferDesc *bufHdr;
  
  	/* Do the same checks as BufferGetBlockNumber. */
  	Assert(BufferIsPinned(buffer));
  
  	if (BufferIsLocal(buffer))
  		bufHdr = GetLocalBufferDescriptor(-buffer - 1);
  	else
  		bufHdr = GetBufferDescriptor(buffer - 1);
  
  	/* pinned, so OK to read tag without spinlock */
  	*rnode = bufHdr->tag.rnode;
  	*forknum = bufHdr->tag.forkNum;
  	*blknum = bufHdr->tag.blockNum;
  }
  
  /*
   * FlushBuffer
   *		Physically write out a shared buffer.
   *
   * NOTE: this actually just passes the buffer contents to the kernel; the
   * real write to disk won't happen until the kernel feels like it.  This
   * is okay from our point of view since we can redo the changes from WAL.
   * However, we will need to force the changes to disk via fsync before
   * we can checkpoint WAL.
   *
   * The caller must hold a pin on the buffer and have share-locked the
   * buffer contents.  (Note: a share-lock does not prevent updates of
   * hint bits in the buffer, so the page could change while the write
   * is in progress, but we assume that that will not invalidate the data
   * written.)
   *
   * If the caller has an smgr reference for the buffer's relation, pass it
   * as the second parameter.  If not, pass NULL.
   */
  static void
  FlushBuffer(BufferDesc *buf, SMgrRelation reln)
  {
  	XLogRecPtr	recptr;
  	ErrorContextCallback errcallback;
  	instr_time	io_start,
  				io_time;
  	Block		bufBlock;
  	char	   *bufToWrite;
  	uint32		buf_state;
  
  	/*
  	 * Acquire the buffer's io_in_progress lock.  If StartBufferIO returns
  	 * false, then someone else flushed the buffer before we could, so we need
  	 * not do anything.
  	 */
  	if (!StartBufferIO(buf, false))
  		return;
  
  	/* Setup error traceback support for ereport() */
  	errcallback.callback = shared_buffer_write_error_callback;
  	errcallback.arg = (void *) buf;
  	errcallback.previous = error_context_stack;
  	error_context_stack = &errcallback;
  
  	/* Find smgr relation for buffer */
  	if (reln == NULL)
  		reln = smgropen(buf->tag.rnode, InvalidBackendId);
  
  	TRACE_POSTGRESQL_BUFFER_FLUSH_START(buf->tag.forkNum,
  										buf->tag.blockNum,
  										reln->smgr_rnode.node.spcNode,
  										reln->smgr_rnode.node.dbNode,
  										reln->smgr_rnode.node.relNode);
  
  	buf_state = LockBufHdr(buf);
  
  	/*
  	 * Run PageGetLSN while holding header lock, since we don't have the
  	 * buffer locked exclusively in all cases.
  	 */
  	recptr = BufferGetLSN(buf);
  
  	/* To check if block content changes while flushing. - vadim 01/17/97 */
  	buf_state &= ~BM_JUST_DIRTIED;
  	UnlockBufHdr(buf, buf_state);
  
  	/*
  	 * Force XLOG flush up to buffer's LSN.  This implements the basic WAL
  	 * rule that log updates must hit disk before any of the data-file changes
  	 * they describe do.
  	 *
  	 * However, this rule does not apply to unlogged relations, which will be
  	 * lost after a crash anyway.  Most unlogged relation pages do not bear
  	 * LSNs since we never emit WAL records for them, and therefore flushing
  	 * up through the buffer LSN would be useless, but harmless.  However,
  	 * GiST indexes use LSNs internally to track page-splits, and therefore
  	 * unlogged GiST pages bear "fake" LSNs generated by
  	 * GetFakeLSNForUnloggedRel.  It is unlikely but possible that the fake
  	 * LSN counter could advance past the WAL insertion point; and if it did
  	 * happen, attempting to flush WAL through that location would fail, with
  	 * disastrous system-wide consequences.  To make sure that can't happen,
  	 * skip the flush if the buffer isn't permanent.
  	 */
  	if (buf_state & BM_PERMANENT)
  		XLogFlush(recptr);
  
  	/*
  	 * Now it's safe to write buffer to disk. Note that no one else should
  	 * have been able to write it while we were busy with log flushing because
  	 * we have the io_in_progress lock.
  	 */
  	bufBlock = BufHdrGetBlock(buf);
  
  	/*
  	 * Update page checksum if desired.  Since we have only shared lock on the
  	 * buffer, other processes might be updating hint bits in it, so we must
  	 * copy the page to private storage if we do checksumming.
  	 */
  	bufToWrite = PageSetChecksumCopy((Page) bufBlock, buf->tag.blockNum);
  
  	if (track_io_timing)
  		INSTR_TIME_SET_CURRENT(io_start);
  
  	/*
  	 * bufToWrite is either the shared buffer or a copy, as appropriate.
  	 */
  	smgrwrite(reln,
  			  buf->tag.forkNum,
  			  buf->tag.blockNum,
  			  bufToWrite,
  			  false);
  
  	if (track_io_timing)
  	{
  		INSTR_TIME_SET_CURRENT(io_time);
  		INSTR_TIME_SUBTRACT(io_time, io_start);
  		pgstat_count_buffer_write_time(INSTR_TIME_GET_MICROSEC(io_time));
  		INSTR_TIME_ADD(pgBufferUsage.blk_write_time, io_time);
  	}
  
  	pgBufferUsage.shared_blks_written++;
  
  	/*
  	 * Mark the buffer as clean (unless BM_JUST_DIRTIED has become set) and
  	 * end the io_in_progress state.
  	 */
  	TerminateBufferIO(buf, true, 0);
  
  	TRACE_POSTGRESQL_BUFFER_FLUSH_DONE(buf->tag.forkNum,
  									   buf->tag.blockNum,
  									   reln->smgr_rnode.node.spcNode,
  									   reln->smgr_rnode.node.dbNode,
  									   reln->smgr_rnode.node.relNode);
  
  	/* Pop the error context stack */
  	error_context_stack = errcallback.previous;
  }
  
  /*
   * RelationGetNumberOfBlocksInFork
   *		Determines the current number of pages in the specified relation fork.
   */
  BlockNumber
  RelationGetNumberOfBlocksInFork(Relation relation, ForkNumber forkNum)
  {
  	if (IsYBRelation(relation))
  	{
  		return 0;
  	}
  
  	/* Open it at the smgr level if not already done */
  	RelationOpenSmgr(relation);
  
  	return smgrnblocks(relation->rd_smgr, forkNum);
  }
  
  /*
   * BufferIsPermanent
   *		Determines whether a buffer will potentially still be around after
   *		a crash.  Caller must hold a buffer pin.
   */
  bool
  BufferIsPermanent(Buffer buffer)
  {
  	BufferDesc *bufHdr;
  
  	/* Local buffers are used only for temp relations. */
  	if (BufferIsLocal(buffer))
  		return false;
  
  	/* Make sure we've got a real buffer, and that we hold a pin on it. */
  	Assert(BufferIsValid(buffer));
  	Assert(BufferIsPinned(buffer));
  
  	/*
  	 * BM_PERMANENT can't be changed while we hold a pin on the buffer, so we
  	 * need not bother with the buffer header spinlock.  Even if someone else
  	 * changes the buffer header state while we're doing this, the state is
  	 * changed atomically, so we'll read the old value or the new value, but
  	 * not random garbage.
  	 */
  	bufHdr = GetBufferDescriptor(buffer - 1);
  	return (pg_atomic_read_u32(&bufHdr->state) & BM_PERMANENT) != 0;
  }
  
  /*
   * BufferGetLSNAtomic
   *		Retrieves the LSN of the buffer atomically using a buffer header lock.
   *		This is necessary for some callers who may not have an exclusive lock
   *		on the buffer.
   */
  XLogRecPtr
  BufferGetLSNAtomic(Buffer buffer)
  {
  	BufferDesc *bufHdr = GetBufferDescriptor(buffer - 1);
  	char	   *page = BufferGetPage(buffer);
  	XLogRecPtr	lsn;
  	uint32		buf_state;
  
  	/*
  	 * If we don't need locking for correctness, fastpath out.
  	 */
  	if (!XLogHintBitIsNeeded() || BufferIsLocal(buffer))
  		return PageGetLSN(page);
  
  	/* Make sure we've got a real buffer, and that we hold a pin on it. */
  	Assert(BufferIsValid(buffer));
  	Assert(BufferIsPinned(buffer));
  
  	buf_state = LockBufHdr(bufHdr);
  	lsn = PageGetLSN(page);
  	UnlockBufHdr(bufHdr, buf_state);
  
  	return lsn;
  }
  
  /* ---------------------------------------------------------------------
   *		DropRelFileNodeBuffers
   *
   *		This function removes from the buffer pool all the pages of the
   *		specified relation fork that have block numbers >= firstDelBlock.
   *		(In particular, with firstDelBlock = 0, all pages are removed.)
   *		Dirty pages are simply dropped, without bothering to write them
   *		out first.  Therefore, this is NOT rollback-able, and so should be
   *		used only with extreme caution!
   *
   *		Currently, this is called only from smgr.c when the underlying file
   *		is about to be deleted or truncated (firstDelBlock is needed for
   *		the truncation case).  The data in the affected pages would therefore
   *		be deleted momentarily anyway, and there is no point in writing it.
   *		It is the responsibility of higher-level code to ensure that the
   *		deletion or truncation does not lose any data that could be needed
   *		later.  It is also the responsibility of higher-level code to ensure
   *		that no other process could be trying to load more pages of the
   *		relation into buffers.
   *
   *		XXX currently it sequentially searches the buffer pool, should be
   *		changed to more clever ways of searching.  However, this routine
   *		is used only in code paths that aren't very performance-critical,
   *		and we shouldn't slow down the hot paths to make it faster ...
   * --------------------------------------------------------------------
   */
  void
  DropRelFileNodeBuffers(RelFileNodeBackend rnode, ForkNumber forkNum,
  					   BlockNumber firstDelBlock)
  {
  	int			i;
  
  	/* If it's a local relation, it's localbuf.c's problem. */
  	if (RelFileNodeBackendIsTemp(rnode))
  	{
  		if (rnode.backend == MyBackendId)
  			DropRelFileNodeLocalBuffers(rnode.node, forkNum, firstDelBlock);
  		return;
  	}
  
  	for (i = 0; i < NBuffers; i++)
  	{
  		BufferDesc *bufHdr = GetBufferDescriptor(i);
  		uint32		buf_state;
  
  		/*
  		 * We can make this a tad faster by prechecking the buffer tag before
  		 * we attempt to lock the buffer; this saves a lot of lock
  		 * acquisitions in typical cases.  It should be safe because the
  		 * caller must have AccessExclusiveLock on the relation, or some other
  		 * reason to be certain that no one is loading new pages of the rel
  		 * into the buffer pool.  (Otherwise we might well miss such pages
  		 * entirely.)  Therefore, while the tag might be changing while we
  		 * look at it, it can't be changing *to* a value we care about, only
  		 * *away* from such a value.  So false negatives are impossible, and
  		 * false positives are safe because we'll recheck after getting the
  		 * buffer lock.
  		 *
  		 * We could check forkNum and blockNum as well as the rnode, but the
  		 * incremental win from doing so seems small.
  		 */
  		if (!RelFileNodeEquals(bufHdr->tag.rnode, rnode.node))
  			continue;
  
  		buf_state = LockBufHdr(bufHdr);
  		if (RelFileNodeEquals(bufHdr->tag.rnode, rnode.node) &&
  			bufHdr->tag.forkNum == forkNum &&
  			bufHdr->tag.blockNum >= firstDelBlock)
  			InvalidateBuffer(bufHdr);	/* releases spinlock */
  		else
  			UnlockBufHdr(bufHdr, buf_state);
  	}
  }
  
  /* ---------------------------------------------------------------------
   *		DropRelFileNodesAllBuffers
   *
   *		This function removes from the buffer pool all the pages of all
   *		forks of the specified relations.  It's equivalent to calling
   *		DropRelFileNodeBuffers once per fork per relation with
   *		firstDelBlock = 0.
   * --------------------------------------------------------------------
   */
  void
  DropRelFileNodesAllBuffers(RelFileNodeBackend *rnodes, int nnodes)
  {
  	int			i,
  				n = 0;
  	RelFileNode *nodes;
  	bool		use_bsearch;
  
  	if (nnodes == 0)
  		return;
  
  	nodes = palloc(sizeof(RelFileNode) * nnodes);	/* non-local relations */
  
  	/* If it's a local relation, it's localbuf.c's problem. */
  	for (i = 0; i < nnodes; i++)
  	{
  		if (RelFileNodeBackendIsTemp(rnodes[i]))
  		{
  			if (rnodes[i].backend == MyBackendId)
  				DropRelFileNodeAllLocalBuffers(rnodes[i].node);
  		}
  		else
  			nodes[n++] = rnodes[i].node;
  	}
  
  	/*
  	 * If there are no non-local relations, then we're done. Release the
  	 * memory and return.
  	 */
  	if (n == 0)
  	{
  		pfree(nodes);
  		return;
  	}
  
  	/*
  	 * For low number of relations to drop just use a simple walk through, to
  	 * save the bsearch overhead. The threshold to use is rather a guess than
  	 * an exactly determined value, as it depends on many factors (CPU and RAM
  	 * speeds, amount of shared buffers etc.).
  	 */
  	use_bsearch = n > DROP_RELS_BSEARCH_THRESHOLD;
  
  	/* sort the list of rnodes if necessary */
  	if (use_bsearch)
  		pg_qsort(nodes, n, sizeof(RelFileNode), rnode_comparator);
  
  	for (i = 0; i < NBuffers; i++)
  	{
  		RelFileNode *rnode = NULL;
  		BufferDesc *bufHdr = GetBufferDescriptor(i);
  		uint32		buf_state;
  
  		/*
  		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
  		 * and saves some cycles.
  		 */
  
  		if (!use_bsearch)
  		{
  			int			j;
  
  			for (j = 0; j < n; j++)
  			{
  				if (RelFileNodeEquals(bufHdr->tag.rnode, nodes[j]))
  				{
  					rnode = &nodes[j];
  					break;
  				}
  			}
  		}
  		else
  		{
  			rnode = bsearch((const void *) &(bufHdr->tag.rnode),
  							nodes, n, sizeof(RelFileNode),
  							rnode_comparator);
  		}
  
  		/* buffer doesn't belong to any of the given relfilenodes; skip it */
  		if (rnode == NULL)
  			continue;
  
  		buf_state = LockBufHdr(bufHdr);
  		if (RelFileNodeEquals(bufHdr->tag.rnode, (*rnode)))
  			InvalidateBuffer(bufHdr);	/* releases spinlock */
  		else
  			UnlockBufHdr(bufHdr, buf_state);
  	}
  
  	pfree(nodes);
  }
  
  /* ---------------------------------------------------------------------
   *		DropDatabaseBuffers
   *
   *		This function removes all the buffers in the buffer cache for a
   *		particular database.  Dirty pages are simply dropped, without
   *		bothering to write them out first.  This is used when we destroy a
   *		database, to avoid trying to flush data to disk when the directory
   *		tree no longer exists.  Implementation is pretty similar to
   *		DropRelFileNodeBuffers() which is for destroying just one relation.
   * --------------------------------------------------------------------
   */
  void
  DropDatabaseBuffers(Oid dbid)
  {
  	int			i;
  
  	/*
  	 * We needn't consider local buffers, since by assumption the target
  	 * database isn't our own.
  	 */
  
  	for (i = 0; i < NBuffers; i++)
  	{
  		BufferDesc *bufHdr = GetBufferDescriptor(i);
  		uint32		buf_state;
  
  		/*
  		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
  		 * and saves some cycles.
  		 */
  		if (bufHdr->tag.rnode.dbNode != dbid)
  			continue;
  
  		buf_state = LockBufHdr(bufHdr);
  		if (bufHdr->tag.rnode.dbNode == dbid)
  			InvalidateBuffer(bufHdr);	/* releases spinlock */
  		else
  			UnlockBufHdr(bufHdr, buf_state);
  	}
  }
  
  /* -----------------------------------------------------------------
   *		PrintBufferDescs
   *
   *		this function prints all the buffer descriptors, for debugging
   *		use only.
   * -----------------------------------------------------------------
   */
  #ifdef NOT_USED
  void
  PrintBufferDescs(void)
  {
  	int			i;
  
  	for (i = 0; i < NBuffers; ++i)
  	{
  		BufferDesc *buf = GetBufferDescriptor(i);
  		Buffer		b = BufferDescriptorGetBuffer(buf);
  
  		/* theoretically we should lock the bufhdr here */
  		elog(LOG,
  			 "[%02d] (freeNext=%d, rel=%s, "
  			 "blockNum=%u, flags=0x%x, refcount=%u %d)",
  			 i, buf->freeNext,
  			 relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum),
  			 buf->tag.blockNum, buf->flags,
  			 buf->refcount, GetPrivateRefCount(b));
  	}
  }
  #endif
  
  #ifdef NOT_USED
  void
  PrintPinnedBufs(void)
  {
  	int			i;
  
  	for (i = 0; i < NBuffers; ++i)
  	{
  		BufferDesc *buf = GetBufferDescriptor(i);
  		Buffer		b = BufferDescriptorGetBuffer(buf);
  
  		if (GetPrivateRefCount(b) > 0)
  		{
  			/* theoretically we should lock the bufhdr here */
  			elog(LOG,
  				 "[%02d] (freeNext=%d, rel=%s, "
  				 "blockNum=%u, flags=0x%x, refcount=%u %d)",
  				 i, buf->freeNext,
  				 relpathperm(buf->tag.rnode, buf->tag.forkNum),
  				 buf->tag.blockNum, buf->flags,
  				 buf->refcount, GetPrivateRefCount(b));
  		}
  	}
  }
  #endif
  
  /* ---------------------------------------------------------------------
   *		FlushRelationBuffers
   *
   *		This function writes all dirty pages of a relation out to disk
   *		(or more accurately, out to kernel disk buffers), ensuring that the
   *		kernel has an up-to-date view of the relation.
   *
   *		Generally, the caller should be holding AccessExclusiveLock on the
   *		target relation to ensure that no other backend is busy dirtying
   *		more blocks of the relation; the effects can't be expected to last
   *		after the lock is released.
   *
   *		XXX currently it sequentially searches the buffer pool, should be
   *		changed to more clever ways of searching.  This routine is not
   *		used in any performance-critical code paths, so it's not worth
   *		adding additional overhead to normal paths to make it go faster;
   *		but see also DropRelFileNodeBuffers.
   * --------------------------------------------------------------------
   */
  void
  FlushRelationBuffers(Relation rel)
  {
  	int			i;
  	BufferDesc *bufHdr;
  
  	/* Open rel at the smgr level if not already done */
  	RelationOpenSmgr(rel);
  
  	if (RelationUsesLocalBuffers(rel))
  	{
  		for (i = 0; i < NLocBuffer; i++)
  		{
  			uint32		buf_state;
  
  			bufHdr = GetLocalBufferDescriptor(i);
  			if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
  				((buf_state = pg_atomic_read_u32(&bufHdr->state)) &
  				 (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
  			{
  				ErrorContextCallback errcallback;
  				Page		localpage;
  
  				localpage = (char *) LocalBufHdrGetBlock(bufHdr);
  
  				/* Setup error traceback support for ereport() */
  				errcallback.callback = local_buffer_write_error_callback;
  				errcallback.arg = (void *) bufHdr;
  				errcallback.previous = error_context_stack;
  				error_context_stack = &errcallback;
  
  				PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
  
  				smgrwrite(rel->rd_smgr,
  						  bufHdr->tag.forkNum,
  						  bufHdr->tag.blockNum,
  						  localpage,
  						  false);
  
  				buf_state &= ~(BM_DIRTY | BM_JUST_DIRTIED);
  				pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
  
  				/* Pop the error context stack */
  				error_context_stack = errcallback.previous;
  			}
  		}
  
  		return;
  	}
  
  	/* Make sure we can handle the pin inside the loop */
  	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
  
  	for (i = 0; i < NBuffers; i++)
  	{
  		uint32		buf_state;
  
  		bufHdr = GetBufferDescriptor(i);
  
  		/*
  		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
  		 * and saves some cycles.
  		 */
  		if (!RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node))
  			continue;
  
  		ReservePrivateRefCountEntry();
  
  		buf_state = LockBufHdr(bufHdr);
  		if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
  			(buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
  		{
  			PinBuffer_Locked(bufHdr);
  			LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
  			FlushBuffer(bufHdr, rel->rd_smgr);
  			LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
  			UnpinBuffer(bufHdr, true);
  		}
  		else
  			UnlockBufHdr(bufHdr, buf_state);
  	}
  }
  
  /* ---------------------------------------------------------------------
   *		FlushDatabaseBuffers
   *
   *		This function writes all dirty pages of a database out to disk
   *		(or more accurately, out to kernel disk buffers), ensuring that the
   *		kernel has an up-to-date view of the database.
   *
   *		Generally, the caller should be holding an appropriate lock to ensure
   *		no other backend is active in the target database; otherwise more
   *		pages could get dirtied.
   *
   *		Note we don't worry about flushing any pages of temporary relations.
   *		It's assumed these wouldn't be interesting.
   * --------------------------------------------------------------------
   */
  void
  FlushDatabaseBuffers(Oid dbid)
  {
  	int			i;
  	BufferDesc *bufHdr;
  
  	/* Make sure we can handle the pin inside the loop */
  	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
  
  	for (i = 0; i < NBuffers; i++)
  	{
  		uint32		buf_state;
  
  		bufHdr = GetBufferDescriptor(i);
  
  		/*
  		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
  		 * and saves some cycles.
  		 */
  		if (bufHdr->tag.rnode.dbNode != dbid)
  			continue;
  
  		ReservePrivateRefCountEntry();
  
  		buf_state = LockBufHdr(bufHdr);
  		if (bufHdr->tag.rnode.dbNode == dbid &&
  			(buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
  		{
  			PinBuffer_Locked(bufHdr);
  			LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
  			FlushBuffer(bufHdr, NULL);
  			LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
  			UnpinBuffer(bufHdr, true);
  		}
  		else
  			UnlockBufHdr(bufHdr, buf_state);
  	}
  }
  
  /*
   * Flush a previously, shared or exclusively, locked and pinned buffer to the
   * OS.
   */
  void
  FlushOneBuffer(Buffer buffer)
  {
  	BufferDesc *bufHdr;
  
  	/* currently not needed, but no fundamental reason not to support */
  	Assert(!BufferIsLocal(buffer));
  
  	Assert(BufferIsPinned(buffer));
  
  	bufHdr = GetBufferDescriptor(buffer - 1);
  
  	Assert(LWLockHeldByMe(BufferDescriptorGetContentLock(bufHdr)));
  
  	FlushBuffer(bufHdr, NULL);
  }
  
  /*
   * ReleaseBuffer -- release the pin on a buffer
   */
  void
  ReleaseBuffer(Buffer buffer)
  {
  	if (!BufferIsValid(buffer))
  		elog(ERROR, "bad buffer ID: %d", buffer);
  
  	if (BufferIsLocal(buffer))
  	{
  		ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
  
  		Assert(LocalRefCount[-buffer - 1] > 0);
  		LocalRefCount[-buffer - 1]--;
  		return;
  	}
  
  	UnpinBuffer(GetBufferDescriptor(buffer - 1), true);
  }
  
  /*
   * UnlockReleaseBuffer -- release the content lock and pin on a buffer
   *
   * This is just a shorthand for a common combination.
   */
  void
  UnlockReleaseBuffer(Buffer buffer)
  {
  	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
  	ReleaseBuffer(buffer);
  }
  
  /*
   * IncrBufferRefCount
   *		Increment the pin count on a buffer that we have *already* pinned
   *		at least once.
   *
   *		This function cannot be used on a buffer we do not have pinned,
   *		because it doesn't change the shared buffer state.
   */
  void
  IncrBufferRefCount(Buffer buffer)
  {
  	Assert(BufferIsPinned(buffer));
  	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
  	if (BufferIsLocal(buffer))
  		LocalRefCount[-buffer - 1]++;
  	else
  	{
  		PrivateRefCountEntry *ref;
  
  		ref = GetPrivateRefCountEntry(buffer, true);
  		Assert(ref != NULL);
  		ref->refcount++;
  	}
  	ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
  }
  
  /*
   * MarkBufferDirtyHint
   *
   *	Mark a buffer dirty for non-critical changes.
   *
   * This is essentially the same as MarkBufferDirty, except:
   *
   * 1. The caller does not write WAL; so if checksums are enabled, we may need
   *	  to write an XLOG_FPI WAL record to protect against torn pages.
   * 2. The caller might have only share-lock instead of exclusive-lock on the
   *	  buffer's content lock.
   * 3. This function does not guarantee that the buffer is always marked dirty
   *	  (due to a race condition), so it cannot be used for important changes.
   */
  void
  MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
  {
  	BufferDesc *bufHdr;
  	Page		page = BufferGetPage(buffer);
  
  	if (!BufferIsValid(buffer))
  		elog(ERROR, "bad buffer ID: %d", buffer);
  
  	if (BufferIsLocal(buffer))
  	{
  		MarkLocalBufferDirty(buffer);
  		return;
  	}
  
  	bufHdr = GetBufferDescriptor(buffer - 1);
  
  	Assert(GetPrivateRefCount(buffer) > 0);
  	/* here, either share or exclusive lock is OK */
  	Assert(LWLockHeldByMe(BufferDescriptorGetContentLock(bufHdr)));
  
  	/*
  	 * This routine might get called many times on the same page, if we are
  	 * making the first scan after commit of an xact that added/deleted many
  	 * tuples. So, be as quick as we can if the buffer is already dirty.  We
  	 * do this by not acquiring spinlock if it looks like the status bits are
  	 * already set.  Since we make this test unlocked, there's a chance we
  	 * might fail to notice that the flags have just been cleared, and failed
  	 * to reset them, due to memory-ordering issues.  But since this function
  	 * is only intended to be used in cases where failing to write out the
  	 * data would be harmless anyway, it doesn't really matter.
  	 */
  	if ((pg_atomic_read_u32(&bufHdr->state) & (BM_DIRTY | BM_JUST_DIRTIED)) !=
  		(BM_DIRTY | BM_JUST_DIRTIED))
  	{
  		XLogRecPtr	lsn = InvalidXLogRecPtr;
  		bool		dirtied = false;
  		bool		delayChkpt = false;
  		uint32		buf_state;
  
  		/*
  		 * If we need to protect hint bit updates from torn writes, WAL-log a
  		 * full page image of the page. This full page image is only necessary
  		 * if the hint bit update is the first change to the page since the
  		 * last checkpoint.
  		 *
  		 * We don't check full_page_writes here because that logic is included
  		 * when we call XLogInsert() since the value changes dynamically.
  		 */
  		if (XLogHintBitIsNeeded() &&
  			(pg_atomic_read_u32(&bufHdr->state) & BM_PERMANENT))
  		{
  			/*
  			 * If we're in recovery we cannot dirty a page because of a hint.
  			 * We can set the hint, just not dirty the page as a result so the
  			 * hint is lost when we evict the page or shutdown.
  			 *
  			 * See src/backend/storage/page/README for longer discussion.
  			 */
  			if (RecoveryInProgress())
  				return;
  
  			/*
  			 * If the block is already dirty because we either made a change
  			 * or set a hint already, then we don't need to write a full page
  			 * image.  Note that aggressive cleaning of blocks dirtied by hint
  			 * bit setting would increase the call rate. Bulk setting of hint
  			 * bits would reduce the call rate...
  			 *
  			 * We must issue the WAL record before we mark the buffer dirty.
  			 * Otherwise we might write the page before we write the WAL. That
  			 * causes a race condition, since a checkpoint might occur between
  			 * writing the WAL record and marking the buffer dirty. We solve
  			 * that with a kluge, but one that is already in use during
  			 * transaction commit to prevent race conditions. Basically, we
  			 * simply prevent the checkpoint WAL record from being written
  			 * until we have marked the buffer dirty. We don't start the
  			 * checkpoint flush until we have marked dirty, so our checkpoint
  			 * must flush the change to disk successfully or the checkpoint
  			 * never gets written, so crash recovery will fix.
  			 *
  			 * It's possible we may enter here without an xid, so it is
  			 * essential that CreateCheckpoint waits for virtual transactions
  			 * rather than full transactionids.
  			 */
  			MyPgXact->delayChkpt = delayChkpt = true;
  			lsn = XLogSaveBufferForHint(buffer, buffer_std);
  		}
  
  		buf_state = LockBufHdr(bufHdr);
  
  		Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
  
  		if (!(buf_state & BM_DIRTY))
  		{
  			dirtied = true;		/* Means "will be dirtied by this action" */
  
  			/*
  			 * Set the page LSN if we wrote a backup block. We aren't supposed
  			 * to set this when only holding a share lock but as long as we
  			 * serialise it somehow we're OK. We choose to set LSN while
  			 * holding the buffer header lock, which causes any reader of an
  			 * LSN who holds only a share lock to also obtain a buffer header
  			 * lock before using PageGetLSN(), which is enforced in
  			 * BufferGetLSNAtomic().
  			 *
  			 * If checksums are enabled, you might think we should reset the
  			 * checksum here. That will happen when the page is written
  			 * sometime later in this checkpoint cycle.
  			 */
  			if (!XLogRecPtrIsInvalid(lsn))
  				PageSetLSN(page, lsn);
  		}
  
  		buf_state |= BM_DIRTY | BM_JUST_DIRTIED;
  		UnlockBufHdr(bufHdr, buf_state);
  
  		if (delayChkpt)
  			MyPgXact->delayChkpt = false;
  
  		if (dirtied)
  		{
  			VacuumPageDirty++;
  			pgBufferUsage.shared_blks_dirtied++;
  			if (VacuumCostActive)
  				VacuumCostBalance += VacuumCostPageDirty;
  		}
  	}
  }
  
  /*
   * Release buffer content locks for shared buffers.
   *
   * Used to clean up after errors.
   *
   * Currently, we can expect that lwlock.c's LWLockReleaseAll() took care
   * of releasing buffer content locks per se; the only thing we need to deal
   * with here is clearing any PIN_COUNT request that was in progress.
   */
  void
  UnlockBuffers(void)
  {
  	BufferDesc *buf = PinCountWaitBuf;
  
  	if (buf)
  	{
  		uint32		buf_state;
  
  		buf_state = LockBufHdr(buf);
  
  		/*
  		 * Don't complain if flag bit not set; it could have been reset but we
  		 * got a cancel/die interrupt before getting the signal.
  		 */
  		if ((buf_state & BM_PIN_COUNT_WAITER) != 0 &&
  			buf->wait_backend_pid == MyProcPid)
  			buf_state &= ~BM_PIN_COUNT_WAITER;
  
  		UnlockBufHdr(buf, buf_state);
  
  		PinCountWaitBuf = NULL;
  	}
  }
  
  /*
   * Acquire or release the content_lock for the buffer.
   */
  void
  LockBuffer(Buffer buffer, int mode)
  {
  	BufferDesc *buf;
  
  	Assert(BufferIsValid(buffer));
  	if (BufferIsLocal(buffer))
  		return;					/* local buffers need no lock */
  
  	buf = GetBufferDescriptor(buffer - 1);
  
  	if (mode == BUFFER_LOCK_UNLOCK)
  		LWLockRelease(BufferDescriptorGetContentLock(buf));
  	else if (mode == BUFFER_LOCK_SHARE)
  		LWLockAcquire(BufferDescriptorGetContentLock(buf), LW_SHARED);
  	else if (mode == BUFFER_LOCK_EXCLUSIVE)
  		LWLockAcquire(BufferDescriptorGetContentLock(buf), LW_EXCLUSIVE);
  	else
  		elog(ERROR, "unrecognized buffer lock mode: %d", mode);
  }
  
  /*
   * Acquire the content_lock for the buffer, but only if we don't have to wait.
   *
   * This assumes the caller wants BUFFER_LOCK_EXCLUSIVE mode.
   */
  bool
  ConditionalLockBuffer(Buffer buffer)
  {
  	BufferDesc *buf;
  
  	Assert(BufferIsValid(buffer));
  	if (BufferIsLocal(buffer))
  		return true;			/* act as though we got it */
  
  	buf = GetBufferDescriptor(buffer - 1);
  
  	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
  									LW_EXCLUSIVE);
  }
  
  /*
   * LockBufferForCleanup - lock a buffer in preparation for deleting items
   *
   * Items may be deleted from a disk page only when the caller (a) holds an
   * exclusive lock on the buffer and (b) has observed that no other backend
   * holds a pin on the buffer.  If there is a pin, then the other backend
   * might have a pointer into the buffer (for example, a heapscan reference
   * to an item --- see README for more details).  It's OK if a pin is added
   * after the cleanup starts, however; the newly-arrived backend will be
   * unable to look at the page until we release the exclusive lock.
   *
   * To implement this protocol, a would-be deleter must pin the buffer and
   * then call LockBufferForCleanup().  LockBufferForCleanup() is similar to
   * LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE), except that it loops until
   * it has successfully observed pin count = 1.
   */
  void
  LockBufferForCleanup(Buffer buffer)
  {
  	BufferDesc *bufHdr;
  
  	Assert(BufferIsValid(buffer));
  	Assert(PinCountWaitBuf == NULL);
  
  	if (BufferIsLocal(buffer))
  	{
  		/* There should be exactly one pin */
  		if (LocalRefCount[-buffer - 1] != 1)
  			elog(ERROR, "incorrect local pin count: %d",
  				 LocalRefCount[-buffer - 1]);
  		/* Nobody else to wait for */
  		return;
  	}
  
  	/* There should be exactly one local pin */
  	if (GetPrivateRefCount(buffer) != 1)
  		elog(ERROR, "incorrect local pin count: %d",
  			 GetPrivateRefCount(buffer));
  
  	bufHdr = GetBufferDescriptor(buffer - 1);
  
  	for (;;)
  	{
  		uint32		buf_state;
  
  		/* Try to acquire lock */
  		LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
  		buf_state = LockBufHdr(bufHdr);
  
  		Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
  		if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
  		{
  			/* Successfully acquired exclusive lock with pincount 1 */
  			UnlockBufHdr(bufHdr, buf_state);
  			return;
  		}
  		/* Failed, so mark myself as waiting for pincount 1 */
  		if (buf_state & BM_PIN_COUNT_WAITER)
  		{
  			UnlockBufHdr(bufHdr, buf_state);
  			LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
  			elog(ERROR, "multiple backends attempting to wait for pincount 1");
  		}
  		bufHdr->wait_backend_pid = MyProcPid;
  		PinCountWaitBuf = bufHdr;
  		buf_state |= BM_PIN_COUNT_WAITER;
  		UnlockBufHdr(bufHdr, buf_state);
  		LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
  
  		/* Wait to be signaled by UnpinBuffer() */
  		if (InHotStandby)
  		{
  			/* Publish the bufid that Startup process waits on */
  			SetStartupBufferPinWaitBufId(buffer - 1);
  			/* Set alarm and then wait to be signaled by UnpinBuffer() */
  			ResolveRecoveryConflictWithBufferPin();
  			/* Reset the published bufid */
  			SetStartupBufferPinWaitBufId(-1);
  		}
  		else
  			ProcWaitForSignal(PG_WAIT_BUFFER_PIN);
  
  		/*
  		 * Remove flag marking us as waiter. Normally this will not be set
  		 * anymore, but ProcWaitForSignal() can return for other signals as
  		 * well.  We take care to only reset the flag if we're the waiter, as
  		 * theoretically another backend could have started waiting. That's
  		 * impossible with the current usages due to table level locking, but
  		 * better be safe.
  		 */
  		buf_state = LockBufHdr(bufHdr);
  		if ((buf_state & BM_PIN_COUNT_WAITER) != 0 &&
  			bufHdr->wait_backend_pid == MyProcPid)
  			buf_state &= ~BM_PIN_COUNT_WAITER;
  		UnlockBufHdr(bufHdr, buf_state);
  
  		PinCountWaitBuf = NULL;
  		/* Loop back and try again */
  	}
  }
  
  /*
   * Check called from RecoveryConflictInterrupt handler when Startup
   * process requests cancellation of all pin holders that are blocking it.
   */
  bool
  HoldingBufferPinThatDelaysRecovery(void)
  {
  	int			bufid = GetStartupBufferPinWaitBufId();
  
  	/*
  	 * If we get woken slowly then it's possible that the Startup process was
  	 * already woken by other backends before we got here. Also possible that
  	 * we get here by multiple interrupts or interrupts at inappropriate
  	 * times, so make sure we do nothing if the bufid is not set.
  	 */
  	if (bufid < 0)
  		return false;
  
  	if (GetPrivateRefCount(bufid + 1) > 0)
  		return true;
  
  	return false;
  }
  
  /*
   * ConditionalLockBufferForCleanup - as above, but don't wait to get the lock
   *
   * We won't loop, but just check once to see if the pin count is OK.  If
   * not, return false with no lock held.
   */
  bool
  ConditionalLockBufferForCleanup(Buffer buffer)
  {
  	BufferDesc *bufHdr;
  	uint32		buf_state,
  				refcount;
  
  	Assert(BufferIsValid(buffer));
  
  	if (BufferIsLocal(buffer))
  	{
  		refcount = LocalRefCount[-buffer - 1];
  		/* There should be exactly one pin */
  		Assert(refcount > 0);
  		if (refcount != 1)
  			return false;
  		/* Nobody else to wait for */
  		return true;
  	}
  
  	/* There should be exactly one local pin */
  	refcount = GetPrivateRefCount(buffer);
  	Assert(refcount);
  	if (refcount != 1)
  		return false;
  
  	/* Try to acquire lock */
  	if (!ConditionalLockBuffer(buffer))
  		return false;
  
  	bufHdr = GetBufferDescriptor(buffer - 1);
  	buf_state = LockBufHdr(bufHdr);
  	refcount = BUF_STATE_GET_REFCOUNT(buf_state);
  
  	Assert(refcount > 0);
  	if (refcount == 1)
  	{
  		/* Successfully acquired exclusive lock with pincount 1 */
  		UnlockBufHdr(bufHdr, buf_state);
  		return true;
  	}
  
  	/* Failed, so release the lock */
  	UnlockBufHdr(bufHdr, buf_state);
  	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
  	return false;
  }
  
  /*
   * IsBufferCleanupOK - as above, but we already have the lock
   *
   * Check whether it's OK to perform cleanup on a buffer we've already
   * locked.  If we observe that the pin count is 1, our exclusive lock
   * happens to be a cleanup lock, and we can proceed with anything that
   * would have been allowable had we sought a cleanup lock originally.
   */
  bool
  IsBufferCleanupOK(Buffer buffer)
  {
  	BufferDesc *bufHdr;
  	uint32		buf_state;
  
  	Assert(BufferIsValid(buffer));
  
  	if (BufferIsLocal(buffer))
  	{
  		/* There should be exactly one pin */
  		if (LocalRefCount[-buffer - 1] != 1)
  			return false;
  		/* Nobody else to wait for */
  		return true;
  	}
  
  	/* There should be exactly one local pin */
  	if (GetPrivateRefCount(buffer) != 1)
  		return false;
  
  	bufHdr = GetBufferDescriptor(buffer - 1);
  
  	/* caller must hold exclusive lock on buffer */
  	Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
  								LW_EXCLUSIVE));
  
  	buf_state = LockBufHdr(bufHdr);
  
  	Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
  	if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
  	{
  		/* pincount is OK. */
  		UnlockBufHdr(bufHdr, buf_state);
  		return true;
  	}
  
  	UnlockBufHdr(bufHdr, buf_state);
  	return false;
  }
  
  
  /*
   *	Functions for buffer I/O handling
   *
   *	Note: We assume that nested buffer I/O never occurs.
   *	i.e at most one io_in_progress lock is held per proc.
   *
   *	Also note that these are used only for shared buffers, not local ones.
   */
  
  /*
   * WaitIO -- Block until the IO_IN_PROGRESS flag on 'buf' is cleared.
   */
  static void
  WaitIO(BufferDesc *buf)
  {
  	/*
  	 * Changed to wait until there's no IO - Inoue 01/13/2000
  	 *
  	 * Note this is *necessary* because an error abort in the process doing
  	 * I/O could release the io_in_progress_lock prematurely. See
  	 * AbortBufferIO.
  	 */
  	for (;;)
  	{
  		uint32		buf_state;
  
  		/*
  		 * It may not be necessary to acquire the spinlock to check the flag
  		 * here, but since this test is essential for correctness, we'd better
  		 * play it safe.
  		 */
  		buf_state = LockBufHdr(buf);
  		UnlockBufHdr(buf, buf_state);
  
  		if (!(buf_state & BM_IO_IN_PROGRESS))
  			break;
  		LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_SHARED);
  		LWLockRelease(BufferDescriptorGetIOLock(buf));
  	}
  }
  
  /*
   * StartBufferIO: begin I/O on this buffer
   *	(Assumptions)
   *	My process is executing no IO
   *	The buffer is Pinned
   *
   * In some scenarios there are race conditions in which multiple backends
   * could attempt the same I/O operation concurrently.  If someone else
   * has already started I/O on this buffer then we will block on the
   * io_in_progress lock until he's done.
   *
   * Input operations are only attempted on buffers that are not BM_VALID,
   * and output operations only on buffers that are BM_VALID and BM_DIRTY,
   * so we can always tell if the work is already done.
   *
   * Returns true if we successfully marked the buffer as I/O busy,
   * false if someone else already did the work.
   */
  static bool
  StartBufferIO(BufferDesc *buf, bool forInput)
  {
  	uint32		buf_state;
  
  	Assert(!InProgressBuf);
  
  	for (;;)
  	{
  		/*
  		 * Grab the io_in_progress lock so that other processes can wait for
  		 * me to finish the I/O.
  		 */
  		LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_EXCLUSIVE);
  
  		buf_state = LockBufHdr(buf);
  
  		if (!(buf_state & BM_IO_IN_PROGRESS))
  			break;
  
  		/*
  		 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
  		 * lock isn't held is if the process doing the I/O is recovering from
  		 * an error (see AbortBufferIO).  If that's the case, we must wait for
  		 * him to get unwedged.
  		 */
  		UnlockBufHdr(buf, buf_state);
  		LWLockRelease(BufferDescriptorGetIOLock(buf));
  		WaitIO(buf);
  	}
  
  	/* Once we get here, there is definitely no I/O active on this buffer */
  
  	if (forInput ? (buf_state & BM_VALID) : !(buf_state & BM_DIRTY))
  	{
  		/* someone else already did the I/O */
  		UnlockBufHdr(buf, buf_state);
  		LWLockRelease(BufferDescriptorGetIOLock(buf));
  		return false;
  	}
  
  	buf_state |= BM_IO_IN_PROGRESS;
  	UnlockBufHdr(buf, buf_state);
  
  	InProgressBuf = buf;
  	IsForInput = forInput;
  
  	return true;
  }
  
  /*
   * TerminateBufferIO: release a buffer we were doing I/O on
   *	(Assumptions)
   *	My process is executing IO for the buffer
   *	BM_IO_IN_PROGRESS bit is set for the buffer
   *	We hold the buffer's io_in_progress lock
   *	The buffer is Pinned
   *
   * If clear_dirty is true and BM_JUST_DIRTIED is not set, we clear the
   * buffer's BM_DIRTY flag.  This is appropriate when terminating a
   * successful write.  The check on BM_JUST_DIRTIED is necessary to avoid
   * marking the buffer clean if it was re-dirtied while we were writing.
   *
   * set_flag_bits gets ORed into the buffer's flags.  It must include
   * BM_IO_ERROR in a failure case.  For successful completion it could
   * be 0, or BM_VALID if we just finished reading in the page.
   */
  static void
  TerminateBufferIO(BufferDesc *buf, bool clear_dirty, uint32 set_flag_bits)
  {
  	uint32		buf_state;
  
  	Assert(buf == InProgressBuf);
  
  	buf_state = LockBufHdr(buf);
  
  	Assert(buf_state & BM_IO_IN_PROGRESS);
  
  	buf_state &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
  	if (clear_dirty && !(buf_state & BM_JUST_DIRTIED))
  		buf_state &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
  
  	buf_state |= set_flag_bits;
  	UnlockBufHdr(buf, buf_state);
  
  	InProgressBuf = NULL;
  
  	LWLockRelease(BufferDescriptorGetIOLock(buf));
  }
  
  /*
   * AbortBufferIO: Clean up any active buffer I/O after an error.
   *
   *	All LWLocks we might have held have been released,
   *	but we haven't yet released buffer pins, so the buffer is still pinned.
   *
   *	If I/O was in progress, we always set BM_IO_ERROR, even though it's
   *	possible the error condition wasn't related to the I/O.
   */
  void
  AbortBufferIO(void)
  {
  	BufferDesc *buf = InProgressBuf;
  
  	if (buf)
  	{
  		uint32		buf_state;
  
  		/*
  		 * Since LWLockReleaseAll has already been called, we're not holding
  		 * the buffer's io_in_progress_lock. We have to re-acquire it so that
  		 * we can use TerminateBufferIO. Anyone who's executing WaitIO on the
  		 * buffer will be in a busy spin until we succeed in doing this.
  		 */
  		LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_EXCLUSIVE);
  
  		buf_state = LockBufHdr(buf);
  		Assert(buf_state & BM_IO_IN_PROGRESS);
  		if (IsForInput)
  		{
  			Assert(!(buf_state & BM_DIRTY));
  
  			/* We'd better not think buffer is valid yet */
  			Assert(!(buf_state & BM_VALID));
  			UnlockBufHdr(buf, buf_state);
  		}
  		else
  		{
  			Assert(buf_state & BM_DIRTY);
  			UnlockBufHdr(buf, buf_state);
  			/* Issue notice if this is not the first failure... */
  			if (buf_state & BM_IO_ERROR)
  			{
  				/* Buffer is pinned, so we can read tag without spinlock */
  				char	   *path;
  
  				path = relpathperm(buf->tag.rnode, buf->tag.forkNum);
  				ereport(WARNING,
  						(errcode(ERRCODE_IO_ERROR),
  						 errmsg("could not write block %u of %s",
  								buf->tag.blockNum, path),
  						 errdetail("Multiple failures --- write error might be permanent.")));
  				pfree(path);
  			}
  		}
  		TerminateBufferIO(buf, false, BM_IO_ERROR);
  	}
  }
  
  /*
   * Error context callback for errors occurring during shared buffer writes.
   */
  static void
  shared_buffer_write_error_callback(void *arg)
  {
  	BufferDesc *bufHdr = (BufferDesc *) arg;
  
  	/* Buffer is pinned, so we can read the tag without locking the spinlock */
  	if (bufHdr != NULL)
  	{
  		char	   *path = relpathperm(bufHdr->tag.rnode, bufHdr->tag.forkNum);
  
  		errcontext("writing block %u of relation %s",
  				   bufHdr->tag.blockNum, path);
  		pfree(path);
  	}
  }
  
  /*
   * Error context callback for errors occurring during local buffer writes.
   */
  static void
  local_buffer_write_error_callback(void *arg)
  {
  	BufferDesc *bufHdr = (BufferDesc *) arg;
  
  	if (bufHdr != NULL)
  	{
  		char	   *path = relpathbackend(bufHdr->tag.rnode, MyBackendId,
  										  bufHdr->tag.forkNum);
  
  		errcontext("writing block %u of relation %s",
  				   bufHdr->tag.blockNum, path);
  		pfree(path);
  	}
  }
  
  /*
   * RelFileNode qsort/bsearch comparator; see RelFileNodeEquals.
   */
  static int
  rnode_comparator(const void *p1, const void *p2)
  {
  	RelFileNode n1 = *(const RelFileNode *) p1;
  	RelFileNode n2 = *(const RelFileNode *) p2;
  
  	if (n1.relNode < n2.relNode)
  		return -1;
  	else if (n1.relNode > n2.relNode)
  		return 1;
  
  	if (n1.dbNode < n2.dbNode)
  		return -1;
  	else if (n1.dbNode > n2.dbNode)
  		return 1;
  
  	if (n1.spcNode < n2.spcNode)
  		return -1;
  	else if (n1.spcNode > n2.spcNode)
  		return 1;
  	else
  		return 0;
  }
  
  /*
   * Lock buffer header - set BM_LOCKED in buffer state.
   */
  uint32
  LockBufHdr(BufferDesc *desc)
  {
  	SpinDelayStatus delayStatus;
  	uint32		old_buf_state;
  
  	init_local_spin_delay(&delayStatus);
  
  	while (true)
  	{
  		/* set BM_LOCKED flag */
  		old_buf_state = pg_atomic_fetch_or_u32(&desc->state, BM_LOCKED);
  		/* if it wasn't set before we're OK */
  		if (!(old_buf_state & BM_LOCKED))
  			break;
  		perform_spin_delay(&delayStatus);
  	}
  	finish_spin_delay(&delayStatus);
  	return old_buf_state | BM_LOCKED;
  }
  
  /*
   * Wait until the BM_LOCKED flag isn't set anymore and return the buffer's
   * state at that point.
   *
   * Obviously the buffer could be locked by the time the value is returned, so
   * this is primarily useful in CAS style loops.
   */
  static uint32
  WaitBufHdrUnlocked(BufferDesc *buf)
  {
  	SpinDelayStatus delayStatus;
  	uint32		buf_state;
  
  	init_local_spin_delay(&delayStatus);
  
  	buf_state = pg_atomic_read_u32(&buf->state);
  
  	while (buf_state & BM_LOCKED)
  	{
  		perform_spin_delay(&delayStatus);
  		buf_state = pg_atomic_read_u32(&buf->state);
  	}
  
  	finish_spin_delay(&delayStatus);
  
  	return buf_state;
  }
  
  /*
   * BufferTag comparator.
   */
  static int
  buffertag_comparator(const void *a, const void *b)
  {
  	const BufferTag *ba = (const BufferTag *) a;
  	const BufferTag *bb = (const BufferTag *) b;
  	int			ret;
  
  	ret = rnode_comparator(&ba->rnode, &bb->rnode);
  
  	if (ret != 0)
  		return ret;
  
  	if (ba->forkNum < bb->forkNum)
  		return -1;
  	if (ba->forkNum > bb->forkNum)
  		return 1;
  
  	if (ba->blockNum < bb->blockNum)
  		return -1;
  	if (ba->blockNum > bb->blockNum)
  		return 1;
  
  	return 0;
  }
  
  /*
   * Comparator determining the writeout order in a checkpoint.
   *
   * It is important that tablespaces are compared first, the logic balancing
   * writes between tablespaces relies on it.
   */
  static int
  ckpt_buforder_comparator(const void *pa, const void *pb)
  {
  	const CkptSortItem *a = (const CkptSortItem *) pa;
  	const CkptSortItem *b = (const CkptSortItem *) pb;
  
  	/* compare tablespace */
  	if (a->tsId < b->tsId)
  		return -1;
  	else if (a->tsId > b->tsId)
  		return 1;
  	/* compare relation */
  	if (a->relNode < b->relNode)
  		return -1;
  	else if (a->relNode > b->relNode)
  		return 1;
  	/* compare fork */
  	else if (a->forkNum < b->forkNum)
  		return -1;
  	else if (a->forkNum > b->forkNum)
  		return 1;
  	/* compare block number */
  	else if (a->blockNum < b->blockNum)
  		return -1;
  	else if (a->blockNum > b->blockNum)
  		return 1;
  	/* equal page IDs are unlikely, but not impossible */
  	return 0;
  }
  
  /*
   * Comparator for a Min-Heap over the per-tablespace checkpoint completion
   * progress.
   */
  static int
  ts_ckpt_progress_comparator(Datum a, Datum b, void *arg)
  {
  	CkptTsStatus *sa = (CkptTsStatus *) a;
  	CkptTsStatus *sb = (CkptTsStatus *) b;
  
  	/* we want a min-heap, so return 1 for the a < b */
  	if (sa->progress < sb->progress)
  		return 1;
  	else if (sa->progress == sb->progress)
  		return 0;
  	else
  		return -1;
  }
  
  /*
   * Initialize a writeback context, discarding potential previous state.
   *
   * *max_pending is a pointer instead of an immediate value, so the coalesce
   * limits can easily changed by the GUC mechanism, and so calling code does
   * not have to check the current configuration. A value is 0 means that no
   * writeback control will be performed.
   */
  void
  WritebackContextInit(WritebackContext *context, int *max_pending)
  {
  	Assert(*max_pending <= WRITEBACK_MAX_PENDING_FLUSHES);
  
  	context->max_pending = max_pending;
  	context->nr_pending = 0;
  }
  
  /*
   * Add buffer to list of pending writeback requests.
   */
  void
  ScheduleBufferTagForWriteback(WritebackContext *context, BufferTag *tag)
  {
  	PendingWriteback *pending;
  
  	/*
  	 * Add buffer to the pending writeback array, unless writeback control is
  	 * disabled.
  	 */
  	if (*context->max_pending > 0)
  	{
  		Assert(*context->max_pending <= WRITEBACK_MAX_PENDING_FLUSHES);
  
  		pending = &context->pending_writebacks[context->nr_pending++];
  
  		pending->tag = *tag;
  	}
  
  	/*
  	 * Perform pending flushes if the writeback limit is exceeded. This
  	 * includes the case where previously an item has been added, but control
  	 * is now disabled.
  	 */
  	if (context->nr_pending >= *context->max_pending)
  		IssuePendingWritebacks(context);
  }
  
  /*
   * Issue all pending writeback requests, previously scheduled with
   * ScheduleBufferTagForWriteback, to the OS.
   *
   * Because this is only used to improve the OSs IO scheduling we try to never
   * error out - it's just a hint.
   */
  void
  IssuePendingWritebacks(WritebackContext *context)
  {
  	int			i;
  
  	if (context->nr_pending == 0)
  		return;
  
  	/*
  	 * Executing the writes in-order can make them a lot faster, and allows to
  	 * merge writeback requests to consecutive blocks into larger writebacks.
  	 */
  	qsort(&context->pending_writebacks, context->nr_pending,
  		  sizeof(PendingWriteback), buffertag_comparator);
  
  	/*
  	 * Coalesce neighbouring writes, but nothing else. For that we iterate
  	 * through the, now sorted, array of pending flushes, and look forward to
  	 * find all neighbouring (or identical) writes.
  	 */
  	for (i = 0; i < context->nr_pending; i++)
  	{
  		PendingWriteback *cur;
  		PendingWriteback *next;
  		SMgrRelation reln;
  		int			ahead;
  		BufferTag	tag;
  		Size		nblocks = 1;
  
  		cur = &context->pending_writebacks[i];
  		tag = cur->tag;
  
  		/*
  		 * Peek ahead, into following writeback requests, to see if they can
  		 * be combined with the current one.
  		 */
  		for (ahead = 0; i + ahead + 1 < context->nr_pending; ahead++)
  		{
  			next = &context->pending_writebacks[i + ahead + 1];
  
  			/* different file, stop */
  			if (!RelFileNodeEquals(cur->tag.rnode, next->tag.rnode) ||
  				cur->tag.forkNum != next->tag.forkNum)
  				break;
  
  			/* ok, block queued twice, skip */
  			if (cur->tag.blockNum == next->tag.blockNum)
  				continue;
  
  			/* only merge consecutive writes */
  			if (cur->tag.blockNum + 1 != next->tag.blockNum)
  				break;
  
  			nblocks++;
  			cur = next;
  		}
  
  		i += ahead;
  
  		/* and finally tell the kernel to write the data to storage */
  		reln = smgropen(tag.rnode, InvalidBackendId);
  		smgrwriteback(reln, tag.forkNum, tag.blockNum, nblocks);
  	}
  
  	context->nr_pending = 0;
  }
  
  
  /*
   * Implement slower/larger portions of TestForOldSnapshot
   *
   * Smaller/faster portions are put inline, but the entire set of logic is too
   * big for that.
   */
  void
  TestForOldSnapshot_impl(Snapshot snapshot, Relation relation)
  {
  	if (RelationAllowsEarlyPruning(relation)
  		&& (snapshot)->whenTaken < GetOldSnapshotThresholdTimestamp())
  		ereport(ERROR,
  				(errcode(ERRCODE_SNAPSHOT_TOO_OLD),
  				 errmsg("snapshot too old")));
  }
  