text: /*-------------------------------------------------------------------------
   *
   * xlogutils.c
   *
   * PostgreSQL transaction log manager utility routines
   *
   * This file contains support routines that are used by XLOG replay functions.
   * None of this code is used during normal system operation.
   *
   *
   * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   * src/backend/access/transam/xlogutils.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include <unistd.h>
  
  #include "access/xlog.h"
  #include "access/xlog_internal.h"
  #include "access/xlogutils.h"
  #include "catalog/catalog.h"
  #include "miscadmin.h"
  #include "storage/smgr.h"
  #include "utils/guc.h"
  #include "utils/hsearch.h"
  #include "utils/rel.h"
  
  
  /*
   * During XLOG replay, we may see XLOG records for incremental updates of
   * pages that no longer exist, because their relation was later dropped or
   * truncated.  (Note: this is only possible when full_page_writes = OFF,
   * since when it's ON, the first reference we see to a page should always
   * be a full-page rewrite not an incremental update.)  Rather than simply
   * ignoring such records, we make a note of the referenced page, and then
   * complain if we don't actually see a drop or truncate covering the page
   * later in replay.
   */
  typedef struct xl_invalid_page_key
  {
  	RelFileNode node;			/* the relation */
  	ForkNumber	forkno;			/* the fork number */
  	BlockNumber blkno;			/* the page */
  } xl_invalid_page_key;
  
  typedef struct xl_invalid_page
  {
  	xl_invalid_page_key key;	/* hash key ... must be first */
  	bool		present;		/* page existed but contained zeroes */
  } xl_invalid_page;
  
  static HTAB *invalid_page_tab = NULL;
  
  
  /* Report a reference to an invalid page */
  static void
  report_invalid_page(int elevel, RelFileNode node, ForkNumber forkno,
  					BlockNumber blkno, bool present)
  {
  	char	   *path = relpathperm(node, forkno);
  
  	if (present)
  		elog(elevel, "page %u of relation %s is uninitialized",
  			 blkno, path);
  	else
  		elog(elevel, "page %u of relation %s does not exist",
  			 blkno, path);
  	pfree(path);
  }
  
  /* Log a reference to an invalid page */
  static void
  log_invalid_page(RelFileNode node, ForkNumber forkno, BlockNumber blkno,
  				 bool present)
  {
  	xl_invalid_page_key key;
  	xl_invalid_page *hentry;
  	bool		found;
  
  	/*
  	 * Once recovery has reached a consistent state, the invalid-page table
  	 * should be empty and remain so. If a reference to an invalid page is
  	 * found after consistency is reached, PANIC immediately. This might seem
  	 * aggressive, but it's better than letting the invalid reference linger
  	 * in the hash table until the end of recovery and PANIC there, which
  	 * might come only much later if this is a standby server.
  	 */
  	if (reachedConsistency)
  	{
  		report_invalid_page(WARNING, node, forkno, blkno, present);
  		elog(PANIC, "WAL contains references to invalid pages");
  	}
  
  	/*
  	 * Log references to invalid pages at DEBUG1 level.  This allows some
  	 * tracing of the cause (note the elog context mechanism will tell us
  	 * something about the XLOG record that generated the reference).
  	 */
  	if (log_min_messages <= DEBUG1 || client_min_messages <= DEBUG1)
  		report_invalid_page(DEBUG1, node, forkno, blkno, present);
  
  	if (invalid_page_tab == NULL)
  	{
  		/* create hash table when first needed */
  		HASHCTL		ctl;
  
  		memset(&ctl, 0, sizeof(ctl));
  		ctl.keysize = sizeof(xl_invalid_page_key);
  		ctl.entrysize = sizeof(xl_invalid_page);
  
  		invalid_page_tab = hash_create("XLOG invalid-page table",
  									   100,
  									   &ctl,
  									   HASH_ELEM | HASH_BLOBS);
  	}
  
  	/* we currently assume xl_invalid_page_key contains no padding */
  	key.node = node;
  	key.forkno = forkno;
  	key.blkno = blkno;
  	hentry = (xl_invalid_page *)
  		hash_search(invalid_page_tab, (void *) &key, HASH_ENTER, &found);
  
  	if (!found)
  	{
  		/* hash_search already filled in the key */
  		hentry->present = present;
  	}
  	else
  	{
  		/* repeat reference ... leave "present" as it was */
  	}
  }
  
  /* Forget any invalid pages >= minblkno, because they've been dropped */
  static void
  forget_invalid_pages(RelFileNode node, ForkNumber forkno, BlockNumber minblkno)
  {
  	HASH_SEQ_STATUS status;
  	xl_invalid_page *hentry;
  
  	if (invalid_page_tab == NULL)
  		return;					/* nothing to do */
  
  	hash_seq_init(&status, invalid_page_tab);
  
  	while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
  	{
  		if (RelFileNodeEquals(hentry->key.node, node) &&
  			hentry->key.forkno == forkno &&
  			hentry->key.blkno >= minblkno)
  		{
  			if (log_min_messages <= DEBUG2 || client_min_messages <= DEBUG2)
  			{
  				char	   *path = relpathperm(hentry->key.node, forkno);
  
  				elog(DEBUG2, "page %u of relation %s has been dropped",
  					 hentry->key.blkno, path);
  				pfree(path);
  			}
  
  			if (hash_search(invalid_page_tab,
  							(void *) &hentry->key,
  							HASH_REMOVE, NULL) == NULL)
  				elog(ERROR, "hash table corrupted");
  		}
  	}
  }
  
  /* Forget any invalid pages in a whole database */
  static void
  forget_invalid_pages_db(Oid dbid)
  {
  	HASH_SEQ_STATUS status;
  	xl_invalid_page *hentry;
  
  	if (invalid_page_tab == NULL)
  		return;					/* nothing to do */
  
  	hash_seq_init(&status, invalid_page_tab);
  
  	while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
  	{
  		if (hentry->key.node.dbNode == dbid)
  		{
  			if (log_min_messages <= DEBUG2 || client_min_messages <= DEBUG2)
  			{
  				char	   *path = relpathperm(hentry->key.node, hentry->key.forkno);
  
  				elog(DEBUG2, "page %u of relation %s has been dropped",
  					 hentry->key.blkno, path);
  				pfree(path);
  			}
  
  			if (hash_search(invalid_page_tab,
  							(void *) &hentry->key,
  							HASH_REMOVE, NULL) == NULL)
  				elog(ERROR, "hash table corrupted");
  		}
  	}
  }
  
  /* Are there any unresolved references to invalid pages? */
  bool
  XLogHaveInvalidPages(void)
  {
  	if (invalid_page_tab != NULL &&
  		hash_get_num_entries(invalid_page_tab) > 0)
  		return true;
  	return false;
  }
  
  /* Complain about any remaining invalid-page entries */
  void
  XLogCheckInvalidPages(void)
  {
  	HASH_SEQ_STATUS status;
  	xl_invalid_page *hentry;
  	bool		foundone = false;
  
  	if (invalid_page_tab == NULL)
  		return;					/* nothing to do */
  
  	hash_seq_init(&status, invalid_page_tab);
  
  	/*
  	 * Our strategy is to emit WARNING messages for all remaining entries and
  	 * only PANIC after we've dumped all the available info.
  	 */
  	while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
  	{
  		report_invalid_page(WARNING, hentry->key.node, hentry->key.forkno,
  							hentry->key.blkno, hentry->present);
  		foundone = true;
  	}
  
  	if (foundone)
  		elog(PANIC, "WAL contains references to invalid pages");
  
  	hash_destroy(invalid_page_tab);
  	invalid_page_tab = NULL;
  }
  
  
  /*
   * XLogReadBufferForRedo
   *		Read a page during XLOG replay
   *
   * Reads a block referenced by a WAL record into shared buffer cache, and
   * determines what needs to be done to redo the changes to it.  If the WAL
   * record includes a full-page image of the page, it is restored.
   *
   * 'lsn' is the LSN of the record being replayed.  It is compared with the
   * page's LSN to determine if the record has already been replayed.
   * 'block_id' is the ID number the block was registered with, when the WAL
   * record was created.
   *
   * Returns one of the following:
   *
   *	BLK_NEEDS_REDO	- changes from the WAL record need to be applied
   *	BLK_DONE		- block doesn't need replaying
   *	BLK_RESTORED	- block was restored from a full-page image included in
   *					  the record
   *	BLK_NOTFOUND	- block was not found (because it was truncated away by
   *					  an operation later in the WAL stream)
   *
   * On return, the buffer is locked in exclusive-mode, and returned in *buf.
   * Note that the buffer is locked and returned even if it doesn't need
   * replaying.  (Getting the buffer lock is not really necessary during
   * single-process crash recovery, but some subroutines such as MarkBufferDirty
   * will complain if we don't have the lock.  In hot standby mode it's
   * definitely necessary.)
   *
   * Note: when a backup block is available in XLOG, we restore it
   * unconditionally, even if the page in the database appears newer.  This is
   * to protect ourselves against database pages that were partially or
   * incorrectly written during a crash.  We assume that the XLOG data must be
   * good because it has passed a CRC check, while the database page might not
   * be.  This will force us to replay all subsequent modifications of the page
   * that appear in XLOG, rather than possibly ignoring them as already
   * applied, but that's not a huge drawback.
   */
  XLogRedoAction
  XLogReadBufferForRedo(XLogReaderState *record, uint8 block_id,
  					  Buffer *buf)
  {
  	return XLogReadBufferForRedoExtended(record, block_id, RBM_NORMAL,
  										 false, buf);
  }
  
  /*
   * Pin and lock a buffer referenced by a WAL record, for the purpose of
   * re-initializing it.
   */
  Buffer
  XLogInitBufferForRedo(XLogReaderState *record, uint8 block_id)
  {
  	Buffer		buf;
  
  	XLogReadBufferForRedoExtended(record, block_id, RBM_ZERO_AND_LOCK, false,
  								  &buf);
  	return buf;
  }
  
  /*
   * XLogReadBufferForRedoExtended
   *		Like XLogReadBufferForRedo, but with extra options.
   *
   * In RBM_ZERO_* modes, if the page doesn't exist, the relation is extended
   * with all-zeroes pages up to the referenced block number.  In
   * RBM_ZERO_AND_LOCK and RBM_ZERO_AND_CLEANUP_LOCK modes, the return value
   * is always BLK_NEEDS_REDO.
   *
   * (The RBM_ZERO_AND_CLEANUP_LOCK mode is redundant with the get_cleanup_lock
   * parameter. Do not use an inconsistent combination!)
   *
   * If 'get_cleanup_lock' is true, a "cleanup lock" is acquired on the buffer
   * using LockBufferForCleanup(), instead of a regular exclusive lock.
   */
  XLogRedoAction
  XLogReadBufferForRedoExtended(XLogReaderState *record,
  							  uint8 block_id,
  							  ReadBufferMode mode, bool get_cleanup_lock,
  							  Buffer *buf)
  {
  	XLogRecPtr	lsn = record->EndRecPtr;
  	RelFileNode rnode;
  	ForkNumber	forknum;
  	BlockNumber blkno;
  	Page		page;
  	bool		zeromode;
  	bool		willinit;
  
  	if (!XLogRecGetBlockTag(record, block_id, &rnode, &forknum, &blkno))
  	{
  		/* Caller specified a bogus block_id */
  		elog(PANIC, "failed to locate backup block with ID %d", block_id);
  	}
  
  	/*
  	 * Make sure that if the block is marked with WILL_INIT, the caller is
  	 * going to initialize it. And vice versa.
  	 */
  	zeromode = (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK);
  	willinit = (record->blocks[block_id].flags & BKPBLOCK_WILL_INIT) != 0;
  	if (willinit && !zeromode)
  		elog(PANIC, "block with WILL_INIT flag in WAL record must be zeroed by redo routine");
  	if (!willinit && zeromode)
  		elog(PANIC, "block to be initialized in redo routine must be marked with WILL_INIT flag in the WAL record");
  
  	/* If it's a full-page image, restore it. */
  	if (XLogRecHasBlockImage(record, block_id))
  	{
  		*buf = XLogReadBufferExtended(rnode, forknum, blkno,
  		   get_cleanup_lock ? RBM_ZERO_AND_CLEANUP_LOCK : RBM_ZERO_AND_LOCK);
  		page = BufferGetPage(*buf);
  		if (!RestoreBlockImage(record, block_id, page))
  			elog(ERROR, "failed to restore block image");
  
  		/*
  		 * The page may be uninitialized. If so, we can't set the LSN because
  		 * that would corrupt the page.
  		 */
  		if (!PageIsNew(page))
  		{
  			PageSetLSN(page, lsn);
  		}
  
  		MarkBufferDirty(*buf);
  
  		/*
  		 * At the end of crash recovery the init forks of unlogged relations
  		 * are copied, without going through shared buffers. So we need to
  		 * force the on-disk state of init forks to always be in sync with the
  		 * state in shared buffers.
  		 */
  		if (forknum == INIT_FORKNUM)
  			FlushOneBuffer(*buf);
  
  		return BLK_RESTORED;
  	}
  	else
  	{
  		*buf = XLogReadBufferExtended(rnode, forknum, blkno, mode);
  		if (BufferIsValid(*buf))
  		{
  			if (mode != RBM_ZERO_AND_LOCK && mode != RBM_ZERO_AND_CLEANUP_LOCK)
  			{
  				if (get_cleanup_lock)
  					LockBufferForCleanup(*buf);
  				else
  					LockBuffer(*buf, BUFFER_LOCK_EXCLUSIVE);
  			}
  			if (lsn <= PageGetLSN(BufferGetPage(*buf)))
  				return BLK_DONE;
  			else
  				return BLK_NEEDS_REDO;
  		}
  		else
  			return BLK_NOTFOUND;
  	}
  }
  
  /*
   * XLogReadBufferExtended
   *		Read a page during XLOG replay
   *
   * This is functionally comparable to ReadBufferExtended. There's some
   * differences in the behavior wrt. the "mode" argument:
   *
   * In RBM_NORMAL mode, if the page doesn't exist, or contains all-zeroes, we
   * return InvalidBuffer. In this case the caller should silently skip the
   * update on this page. (In this situation, we expect that the page was later
   * dropped or truncated. If we don't see evidence of that later in the WAL
   * sequence, we'll complain at the end of WAL replay.)
   *
   * In RBM_ZERO_* modes, if the page doesn't exist, the relation is extended
   * with all-zeroes pages up to the given block number.
   *
   * In RBM_NORMAL_NO_LOG mode, we return InvalidBuffer if the page doesn't
   * exist, and we don't check for all-zeroes.  Thus, no log entry is made
   * to imply that the page should be dropped or truncated later.
   *
   * NB: A redo function should normally not call this directly. To get a page
   * to modify, use XLogReadBufferForRedoExtended instead. It is important that
   * all pages modified by a WAL record are registered in the WAL records, or
   * they will be invisible to tools that that need to know which pages are
   * modified.
   */
  Buffer
  XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,
  					   BlockNumber blkno, ReadBufferMode mode)
  {
  	BlockNumber lastblock;
  	Buffer		buffer;
  	SMgrRelation smgr;
  
  	Assert(blkno != P_NEW);
  
  	/* Open the relation at smgr level */
  	smgr = smgropen(rnode, InvalidBackendId);
  
  	/*
  	 * Create the target file if it doesn't already exist.  This lets us cope
  	 * if the replay sequence contains writes to a relation that is later
  	 * deleted.  (The original coding of this routine would instead suppress
  	 * the writes, but that seems like it risks losing valuable data if the
  	 * filesystem loses an inode during a crash.  Better to write the data
  	 * until we are actually told to delete the file.)
  	 */
  	smgrcreate(smgr, forknum, true);
  
  	lastblock = smgrnblocks(smgr, forknum);
  
  	if (blkno < lastblock)
  	{
  		/* page exists in file */
  		buffer = ReadBufferWithoutRelcache(rnode, forknum, blkno,
  										   mode, NULL);
  	}
  	else
  	{
  		/* hm, page doesn't exist in file */
  		if (mode == RBM_NORMAL)
  		{
  			log_invalid_page(rnode, forknum, blkno, false);
  			return InvalidBuffer;
  		}
  		if (mode == RBM_NORMAL_NO_LOG)
  			return InvalidBuffer;
  		/* OK to extend the file */
  		/* we do this in recovery only - no rel-extension lock needed */
  		Assert(InRecovery);
  		buffer = InvalidBuffer;
  		do
  		{
  			if (buffer != InvalidBuffer)
  			{
  				if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
  					LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
  				ReleaseBuffer(buffer);
  			}
  			buffer = ReadBufferWithoutRelcache(rnode, forknum,
  											   P_NEW, mode, NULL);
  		}
  		while (BufferGetBlockNumber(buffer) < blkno);
  		/* Handle the corner case that P_NEW returns non-consecutive pages */
  		if (BufferGetBlockNumber(buffer) != blkno)
  		{
  			if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
  				LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
  			ReleaseBuffer(buffer);
  			buffer = ReadBufferWithoutRelcache(rnode, forknum, blkno,
  											   mode, NULL);
  		}
  	}
  
  	if (mode == RBM_NORMAL)
  	{
  		/* check that page has been initialized */
  		Page		page = (Page) BufferGetPage(buffer);
  
  		/*
  		 * We assume that PageIsNew is safe without a lock. During recovery,
  		 * there should be no other backends that could modify the buffer at
  		 * the same time.
  		 */
  		if (PageIsNew(page))
  		{
  			ReleaseBuffer(buffer);
  			log_invalid_page(rnode, forknum, blkno, true);
  			return InvalidBuffer;
  		}
  	}
  
  	return buffer;
  }
  
  /*
   * Struct actually returned by XLogFakeRelcacheEntry, though the declared
   * return type is Relation.
   */
  typedef struct
  {
  	RelationData reldata;		/* Note: this must be first */
  	FormData_pg_class pgc;
  } FakeRelCacheEntryData;
  
  typedef FakeRelCacheEntryData *FakeRelCacheEntry;
  
  /*
   * Create a fake relation cache entry for a physical relation
   *
   * It's often convenient to use the same functions in XLOG replay as in the
   * main codepath, but those functions typically work with a relcache entry.
   * We don't have a working relation cache during XLOG replay, but this
   * function can be used to create a fake relcache entry instead. Only the
   * fields related to physical storage, like rd_rel, are initialized, so the
   * fake entry is only usable in low-level operations like ReadBuffer().
   *
   * Caller must free the returned entry with FreeFakeRelcacheEntry().
   */
  Relation
  CreateFakeRelcacheEntry(RelFileNode rnode)
  {
  	FakeRelCacheEntry fakeentry;
  	Relation	rel;
  
  	Assert(InRecovery);
  
  	/* Allocate the Relation struct and all related space in one block. */
  	fakeentry = palloc0(sizeof(FakeRelCacheEntryData));
  	rel = (Relation) fakeentry;
  
  	rel->rd_rel = &fakeentry->pgc;
  	rel->rd_node = rnode;
  	/* We will never be working with temp rels during recovery */
  	rel->rd_backend = InvalidBackendId;
  
  	/* It must be a permanent table if we're in recovery. */
  	rel->rd_rel->relpersistence = RELPERSISTENCE_PERMANENT;
  
  	/* We don't know the name of the relation; use relfilenode instead */
  	sprintf(RelationGetRelationName(rel), "%u", rnode.relNode);
  
  	/*
  	 * We set up the lockRelId in case anything tries to lock the dummy
  	 * relation.  Note that this is fairly bogus since relNode may be
  	 * different from the relation's OID.  It shouldn't really matter though,
  	 * since we are presumably running by ourselves and can't have any lock
  	 * conflicts ...
  	 */
  	rel->rd_lockInfo.lockRelId.dbId = rnode.dbNode;
  	rel->rd_lockInfo.lockRelId.relId = rnode.relNode;
  
  	rel->rd_smgr = NULL;
  
  	return rel;
  }
  
  /*
   * Free a fake relation cache entry.
   */
  void
  FreeFakeRelcacheEntry(Relation fakerel)
  {
  	/* make sure the fakerel is not referenced by the SmgrRelation anymore */
  	if (fakerel->rd_smgr != NULL)
  		smgrclearowner(&fakerel->rd_smgr, fakerel->rd_smgr);
  	pfree(fakerel);
  }
  
  /*
   * Drop a relation during XLOG replay
   *
   * This is called when the relation is about to be deleted; we need to remove
   * any open "invalid-page" records for the relation.
   */
  void
  XLogDropRelation(RelFileNode rnode, ForkNumber forknum)
  {
  	forget_invalid_pages(rnode, forknum, 0);
  }
  
  /*
   * Drop a whole database during XLOG replay
   *
   * As above, but for DROP DATABASE instead of dropping a single rel
   */
  void
  XLogDropDatabase(Oid dbid)
  {
  	/*
  	 * This is unnecessarily heavy-handed, as it will close SMgrRelation
  	 * objects for other databases as well. DROP DATABASE occurs seldom enough
  	 * that it's not worth introducing a variant of smgrclose for just this
  	 * purpose. XXX: Or should we rather leave the smgr entries dangling?
  	 */
  	smgrcloseall();
  
  	forget_invalid_pages_db(dbid);
  }
  
  /*
   * Truncate a relation during XLOG replay
   *
   * We need to clean up any open "invalid-page" records for the dropped pages.
   */
  void
  XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
  					 BlockNumber nblocks)
  {
  	forget_invalid_pages(rnode, forkNum, nblocks);
  }
  
  /*
   * Read 'count' bytes from WAL into 'buf', starting at location 'startptr'
   * in timeline 'tli'.
   *
   * Will open, and keep open, one WAL segment stored in the static file
   * descriptor 'sendFile'. This means if XLogRead is used once, there will
   * always be one descriptor left open until the process ends, but never
   * more than one.
   *
   * XXX This is very similar to pg_xlogdump's XLogDumpXLogRead and to XLogRead
   * in walsender.c but for small differences (such as lack of elog() in
   * frontend).  Probably these should be merged at some point.
   */
  static void
  XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
  {
  	char	   *p;
  	XLogRecPtr	recptr;
  	Size		nbytes;
  
  	/* state maintained across calls */
  	static int	sendFile = -1;
  	static XLogSegNo sendSegNo = 0;
  	static uint32 sendOff = 0;
  
  	p = buf;
  	recptr = startptr;
  	nbytes = count;
  
  	while (nbytes > 0)
  	{
  		uint32		startoff;
  		int			segbytes;
  		int			readbytes;
  
  		startoff = recptr % XLogSegSize;
  
  		/* Do we need to switch to a different xlog segment? */
  		if (sendFile < 0 || !XLByteInSeg(recptr, sendSegNo))
  		{
  			char		path[MAXPGPATH];
  
  			if (sendFile >= 0)
  				close(sendFile);
  
  			XLByteToSeg(recptr, sendSegNo);
  
  			XLogFilePath(path, tli, sendSegNo);
  
  			sendFile = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
  
  			if (sendFile < 0)
  			{
  				if (errno == ENOENT)
  					ereport(ERROR,
  							(errcode_for_file_access(),
  							 errmsg("requested WAL segment %s has already been removed",
  									path)));
  				else
  					ereport(ERROR,
  							(errcode_for_file_access(),
  							 errmsg("could not open file \"%s\": %m",
  									path)));
  			}
  			sendOff = 0;
  		}
  
  		/* Need to seek in the file? */
  		if (sendOff != startoff)
  		{
  			if (lseek(sendFile, (off_t) startoff, SEEK_SET) < 0)
  			{
  				char		path[MAXPGPATH];
  				int			save_errno = errno;
  
  				XLogFilePath(path, tli, sendSegNo);
  
  				errno = save_errno;
  				ereport(ERROR,
  						(errcode_for_file_access(),
  				  errmsg("could not seek in log segment %s to offset %u: %m",
  						 path, startoff)));
  			}
  			sendOff = startoff;
  		}
  
  		/* How many bytes are within this segment? */
  		if (nbytes > (XLogSegSize - startoff))
  			segbytes = XLogSegSize - startoff;
  		else
  			segbytes = nbytes;
  
  		readbytes = read(sendFile, p, segbytes);
  		if (readbytes <= 0)
  		{
  			char		path[MAXPGPATH];
  			int			save_errno = errno;
  
  			XLogFilePath(path, tli, sendSegNo);
  
  			errno = save_errno;
  			ereport(ERROR,
  					(errcode_for_file_access(),
  					 errmsg("could not read from log segment %s, offset %u, length %lu: %m",
  							path, sendOff, (unsigned long) segbytes)));
  		}
  
  		/* Update state for read */
  		recptr += readbytes;
  
  		sendOff += readbytes;
  		nbytes -= readbytes;
  		p += readbytes;
  	}
  }
  
  /*
   * read_page callback for reading local xlog files
   *
   * Public because it would likely be very helpful for someone writing another
   * output method outside walsender, e.g. in a bgworker.
   *
   * TODO: The walsender has its own version of this, but it relies on the
   * walsender's latch being set whenever WAL is flushed. No such infrastructure
   * exists for normal backends, so we have to do a check/sleep/repeat style of
   * loop for now.
   */
  int
  read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
  					 int reqLen, XLogRecPtr targetRecPtr, char *cur_page,
  					 TimeLineID *pageTLI)
  {
  	XLogRecPtr	read_upto,
  				loc;
  	int			count;
  
  	loc = targetPagePtr + reqLen;
  	while (1)
  	{
  		/*
  		 * TODO: we're going to have to do something more intelligent about
  		 * timelines on standbys. Use readTimeLineHistory() and
  		 * tliOfPointInHistory() to get the proper LSN? For now we'll catch
  		 * that case earlier, but the code and TODO is left in here for when
  		 * that changes.
  		 */
  		if (!RecoveryInProgress())
  		{
  			*pageTLI = ThisTimeLineID;
  			read_upto = GetFlushRecPtr();
  		}
  		else
  			read_upto = GetXLogReplayRecPtr(pageTLI);
  
  		if (loc <= read_upto)
  			break;
  
  		CHECK_FOR_INTERRUPTS();
  		pg_usleep(1000L);
  	}
  
  	if (targetPagePtr + XLOG_BLCKSZ <= read_upto)
  	{
  		/*
  		 * more than one block available; read only that block, have caller
  		 * come back if they need more.
  		 */
  		count = XLOG_BLCKSZ;
  	}
  	else if (targetPagePtr + reqLen > read_upto)
  	{
  		/* not enough data there */
  		return -1;
  	}
  	else
  	{
  		/* enough bytes available to satisfy the request */
  		count = read_upto - targetPagePtr;
  	}
  
  	/*
  	 * Even though we just determined how much of the page can be validly read
  	 * as 'count', read the whole page anyway. It's guaranteed to be
  	 * zero-padded up to the page boundary if it's incomplete.
  	 */
  	XLogRead(cur_page, *pageTLI, targetPagePtr, XLOG_BLCKSZ);
  
  	/* number of valid bytes in the buffer */
  	return count;
  }
  