text: /*-------------------------------------------------------------------------
   *
   * xlogutils.c
   *
   * PostgreSQL write-ahead 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) 1996-2021, 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/timeline.h"
  #include "access/xlog.h"
  #include "access/xlog_internal.h"
  #include "access/xlogutils.h"
  #include "miscadmin.h"
  #include "pgstat.h"
  #include "storage/smgr.h"
  #include "utils/guc.h"
  #include "utils/hsearch.h"
  #include "utils/rel.h"
  
  
  /* GUC variable */
  bool		ignore_invalid_pages = false;
  
  /*
   * 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(ignore_invalid_pages ? WARNING : 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 (message_level_is_interesting(DEBUG1))
  		report_invalid_page(DEBUG1, node, forkno, blkno, present);
  
  	if (invalid_page_tab == NULL)
  	{
  		/* create hash table when first needed */
  		HASHCTL		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 (message_level_is_interesting(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 (message_level_is_interesting(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(ignore_invalid_pages ? WARNING : 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.
   *
   * 'record.EndRecPtr' is compared to 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 with the BKPIMAGE_APPLY flag
   * set, we restore it, 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 has a full-page image and it should be restored, do it. */
  	if (XLogRecBlockImageApply(record, block_id))
  	{
  		Assert(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 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 CreateFakeRelcacheEntry, 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().
   *
   * This is also used for syncing WAL-skipped files.
   *
   * Caller must free the returned entry with FreeFakeRelcacheEntry().
   */
  Relation
  CreateFakeRelcacheEntry(RelFileNode rnode)
  {
  	FakeRelCacheEntry fakeentry;
  	Relation	rel;
  
  	/* 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 or while
  	 * syncing WAL-skipped files.
  	 */
  	rel->rd_backend = InvalidBackendId;
  
  	/* It must be a permanent table here */
  	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.
  	 * In recovery, we are running by ourselves and can't have any lock
  	 * conflicts.  While syncing, we already hold AccessExclusiveLock.
  	 */
  	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);
  }
  
  /*
   * Determine which timeline to read an xlog page from and set the
   * XLogReaderState's currTLI to that timeline ID.
   *
   * We care about timelines in xlogreader when we might be reading xlog
   * generated prior to a promotion, either if we're currently a standby in
   * recovery or if we're a promoted primary reading xlogs generated by the old
   * primary before our promotion.
   *
   * wantPage must be set to the start address of the page to read and
   * wantLength to the amount of the page that will be read, up to
   * XLOG_BLCKSZ. If the amount to be read isn't known, pass XLOG_BLCKSZ.
   *
   * We switch to an xlog segment from the new timeline eagerly when on a
   * historical timeline, as soon as we reach the start of the xlog segment
   * containing the timeline switch.  The server copied the segment to the new
   * timeline so all the data up to the switch point is the same, but there's no
   * guarantee the old segment will still exist. It may have been deleted or
   * renamed with a .partial suffix so we can't necessarily keep reading from
   * the old TLI even though tliSwitchPoint says it's OK.
   *
   * We can't just check the timeline when we read a page on a different segment
   * to the last page. We could've received a timeline switch from a cascading
   * upstream, so the current segment ends abruptly (possibly getting renamed to
   * .partial) and we have to switch to a new one.  Even in the middle of reading
   * a page we could have to dump the cached page and switch to a new TLI.
   *
   * Because of this, callers MAY NOT assume that currTLI is the timeline that
   * will be in a page's xlp_tli; the page may begin on an older timeline or we
   * might be reading from historical timeline data on a segment that's been
   * copied to a new timeline.
   *
   * The caller must also make sure it doesn't read past the current replay
   * position (using GetXLogReplayRecPtr) if executing in recovery, so it
   * doesn't fail to notice that the current timeline became historical. The
   * caller must also update ThisTimeLineID with the result of
   * GetXLogReplayRecPtr and must check RecoveryInProgress().
   */
  void
  XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wantLength)
  {
  	const XLogRecPtr lastReadPage = (state->seg.ws_segno *
  									 state->segcxt.ws_segsize + state->segoff);
  
  	Assert(wantPage != InvalidXLogRecPtr && wantPage % XLOG_BLCKSZ == 0);
  	Assert(wantLength <= XLOG_BLCKSZ);
  	Assert(state->readLen == 0 || state->readLen <= XLOG_BLCKSZ);
  
  	/*
  	 * If the desired page is currently read in and valid, we have nothing to
  	 * do.
  	 *
  	 * The caller should've ensured that it didn't previously advance readOff
  	 * past the valid limit of this timeline, so it doesn't matter if the
  	 * current TLI has since become historical.
  	 */
  	if (lastReadPage == wantPage &&
  		state->readLen != 0 &&
  		lastReadPage + state->readLen >= wantPage + Min(wantLength, XLOG_BLCKSZ - 1))
  		return;
  
  	/*
  	 * If we're reading from the current timeline, it hasn't become historical
  	 * and the page we're reading is after the last page read, we can again
  	 * just carry on. (Seeking backwards requires a check to make sure the
  	 * older page isn't on a prior timeline).
  	 *
  	 * ThisTimeLineID might've become historical since we last looked, but the
  	 * caller is required not to read past the flush limit it saw at the time
  	 * it looked up the timeline. There's nothing we can do about it if
  	 * StartupXLOG() renames it to .partial concurrently.
  	 */
  	if (state->currTLI == ThisTimeLineID && wantPage >= lastReadPage)
  	{
  		Assert(state->currTLIValidUntil == InvalidXLogRecPtr);
  		return;
  	}
  
  	/*
  	 * If we're just reading pages from a previously validated historical
  	 * timeline and the timeline we're reading from is valid until the end of
  	 * the current segment we can just keep reading.
  	 */
  	if (state->currTLIValidUntil != InvalidXLogRecPtr &&
  		state->currTLI != ThisTimeLineID &&
  		state->currTLI != 0 &&
  		((wantPage + wantLength) / state->segcxt.ws_segsize) <
  		(state->currTLIValidUntil / state->segcxt.ws_segsize))
  		return;
  
  	/*
  	 * If we reach this point we're either looking up a page for random
  	 * access, the current timeline just became historical, or we're reading
  	 * from a new segment containing a timeline switch. In all cases we need
  	 * to determine the newest timeline on the segment.
  	 *
  	 * If it's the current timeline we can just keep reading from here unless
  	 * we detect a timeline switch that makes the current timeline historical.
  	 * If it's a historical timeline we can read all the segment on the newest
  	 * timeline because it contains all the old timelines' data too. So only
  	 * one switch check is required.
  	 */
  	{
  		/*
  		 * We need to re-read the timeline history in case it's been changed
  		 * by a promotion or replay from a cascaded replica.
  		 */
  		List	   *timelineHistory = readTimeLineHistory(ThisTimeLineID);
  		XLogRecPtr	endOfSegment;
  
  		endOfSegment = ((wantPage / state->segcxt.ws_segsize) + 1) *
  			state->segcxt.ws_segsize - 1;
  		Assert(wantPage / state->segcxt.ws_segsize ==
  			   endOfSegment / state->segcxt.ws_segsize);
  
  		/*
  		 * Find the timeline of the last LSN on the segment containing
  		 * wantPage.
  		 */
  		state->currTLI = tliOfPointInHistory(endOfSegment, timelineHistory);
  		state->currTLIValidUntil = tliSwitchPoint(state->currTLI, timelineHistory,
  												  &state->nextTLI);
  
  		Assert(state->currTLIValidUntil == InvalidXLogRecPtr ||
  			   wantPage + wantLength < state->currTLIValidUntil);
  
  		list_free_deep(timelineHistory);
  
  		elog(DEBUG3, "switched to timeline %u valid until %X/%X",
  			 state->currTLI,
  			 LSN_FORMAT_ARGS(state->currTLIValidUntil));
  	}
  }
  
  /* XLogReaderRoutine->segment_open callback for local pg_wal files */
  void
  wal_segment_open(XLogReaderState *state, XLogSegNo nextSegNo,
  				 TimeLineID *tli_p)
  {
  	TimeLineID	tli = *tli_p;
  	char		path[MAXPGPATH];
  
  	XLogFilePath(path, tli, nextSegNo, state->segcxt.ws_segsize);
  	state->seg.ws_file = BasicOpenFile(path, O_RDONLY | PG_BINARY);
  	if (state->seg.ws_file >= 0)
  		return;
  
  	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)));
  }
  
  /* stock XLogReaderRoutine->segment_close callback */
  void
  wal_segment_close(XLogReaderState *state)
  {
  	close(state->seg.ws_file);
  	/* need to check errno? */
  	state->seg.ws_file = -1;
  }
  
  /*
   * XLogReaderRoutine->page_read 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)
  {
  	XLogRecPtr	read_upto,
  				loc;
  	TimeLineID	tli;
  	int			count;
  	WALReadError errinfo;
  
  	loc = targetPagePtr + reqLen;
  
  	/* Loop waiting for xlog to be available if necessary */
  	while (1)
  	{
  		/*
  		 * Determine the limit of xlog we can currently read to, and what the
  		 * most recent timeline is.
  		 *
  		 * RecoveryInProgress() will update ThisTimeLineID when it first
  		 * notices recovery finishes, so we only have to maintain it for the
  		 * local process until recovery ends.
  		 */
  		if (!RecoveryInProgress())
  			read_upto = GetFlushRecPtr();
  		else
  			read_upto = GetXLogReplayRecPtr(&ThisTimeLineID);
  		tli = ThisTimeLineID;
  
  		/*
  		 * Check which timeline to get the record from.
  		 *
  		 * We have to do it each time through the loop because if we're in
  		 * recovery as a cascading standby, the current timeline might've
  		 * become historical. We can't rely on RecoveryInProgress() because in
  		 * a standby configuration like
  		 *
  		 * A => B => C
  		 *
  		 * if we're a logical decoding session on C, and B gets promoted, our
  		 * timeline will change while we remain in recovery.
  		 *
  		 * We can't just keep reading from the old timeline as the last WAL
  		 * archive in the timeline will get renamed to .partial by
  		 * StartupXLOG().
  		 *
  		 * If that happens after our caller updated ThisTimeLineID but before
  		 * we actually read the xlog page, we might still try to read from the
  		 * old (now renamed) segment and fail. There's not much we can do
  		 * about this, but it can only happen when we're a leaf of a cascading
  		 * standby whose primary gets promoted while we're decoding, so a
  		 * one-off ERROR isn't too bad.
  		 */
  		XLogReadDetermineTimeline(state, targetPagePtr, reqLen);
  
  		if (state->currTLI == ThisTimeLineID)
  		{
  
  			if (loc <= read_upto)
  				break;
  
  			CHECK_FOR_INTERRUPTS();
  			pg_usleep(1000L);
  		}
  		else
  		{
  			/*
  			 * We're on a historical timeline, so limit reading to the switch
  			 * point where we moved to the next timeline.
  			 *
  			 * We don't need to GetFlushRecPtr or GetXLogReplayRecPtr. We know
  			 * about the new timeline, so we must've received past the end of
  			 * it.
  			 */
  			read_upto = state->currTLIValidUntil;
  
  			/*
  			 * Setting tli to our wanted record's TLI is slightly wrong; the
  			 * page might begin on an older timeline if it contains a timeline
  			 * switch, since its xlog segment will have been copied from the
  			 * prior timeline. This is pretty harmless though, as nothing
  			 * cares so long as the timeline doesn't go backwards.  We should
  			 * read the page header instead; FIXME someday.
  			 */
  			tli = state->currTLI;
  
  			/* No need to wait on a historical timeline */
  			break;
  		}
  	}
  
  	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.
  	 */
  	if (!WALRead(state, cur_page, targetPagePtr, XLOG_BLCKSZ, tli,
  				 &errinfo))
  		WALReadRaiseError(&errinfo);
  
  	/* number of valid bytes in the buffer */
  	return count;
  }
  
  /*
   * Backend-specific convenience code to handle read errors encountered by
   * WALRead().
   */
  void
  WALReadRaiseError(WALReadError *errinfo)
  {
  	WALOpenSegment *seg = &errinfo->wre_seg;
  	char		fname[MAXFNAMELEN];
  
  	XLogFileName(fname, seg->ws_tli, seg->ws_segno, wal_segment_size);
  
  	if (errinfo->wre_read < 0)
  	{
  		errno = errinfo->wre_errno;
  		ereport(ERROR,
  				(errcode_for_file_access(),
  				 errmsg("could not read from log segment %s, offset %u: %m",
  						fname, errinfo->wre_off)));
  	}
  	else if (errinfo->wre_read == 0)
  	{
  		ereport(ERROR,
  				(errcode(ERRCODE_DATA_CORRUPTED),
  				 errmsg("could not read from log segment %s, offset %u: read %d of %zu",
  						fname, errinfo->wre_off, errinfo->wre_read,
  						(Size) errinfo->wre_req)));
  	}
  }
  