text: /*-------------------------------------------------------------------------
   *
   * md.c
   *	  This code manages relations that reside on magnetic disk.
   *
   * Or at least, that was what the Berkeley folk had in mind when they named
   * this file.  In reality, what this code provides is an interface from
   * the smgr API to Unix-like filesystem APIs, so it will work with any type
   * of device for which the operating system provides filesystem support.
   * It doesn't matter whether the bits are on spinning rust or some other
   * storage technology.
   *
   * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   *
   * IDENTIFICATION
   *	  src/backend/storage/smgr/md.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include <unistd.h>
  #include <fcntl.h>
  #include <sys/file.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  
  #include "access/aomd.h"
  #include "access/htup_details.h"
  #include "catalog/catalog.h"
  #include "miscadmin.h"
  #include "access/xlogutils.h"
  #include "access/xlog.h"
  #include "pgstat.h"
  #include "postmaster/bgwriter.h"
  #include "storage/fd.h"
  #include "storage/bufmgr.h"
  #include "storage/md.h"
  #include "storage/relfilenode.h"
  #include "storage/smgr.h"
  #include "storage/sync.h"
  #include "utils/hsearch.h"
  #include "utils/memutils.h"
  #include "pg_trace.h"
  
  #include "catalog/pg_tablespace.h"
  #include "utils/faultinjector.h"
  
  /*
   *	The magnetic disk storage manager keeps track of open file
   *	descriptors in its own descriptor pool.  This is done to make it
   *	easier to support relations that are larger than the operating
   *	system's file size limit (often 2GBytes).  In order to do that,
   *	we break relations up into "segment" files that are each shorter than
   *	the OS file size limit.  The segment size is set by the RELSEG_SIZE
   *	configuration constant in pg_config.h.
   *
   *	On disk, a relation must consist of consecutively numbered segment
   *	files in the pattern
   *		-- Zero or more full segments of exactly RELSEG_SIZE blocks each
   *		-- Exactly one partial segment of size 0 <= size < RELSEG_SIZE blocks
   *		-- Optionally, any number of inactive segments of size 0 blocks.
   *	The full and partial segments are collectively the "active" segments.
   *	Inactive segments are those that once contained data but are currently
   *	not needed because of an mdtruncate() operation.  The reason for leaving
   *	them present at size zero, rather than unlinking them, is that other
   *	backends and/or the checkpointer might be holding open file references to
   *	such segments.  If the relation expands again after mdtruncate(), such
   *	that a deactivated segment becomes active again, it is important that
   *	such file references still be valid --- else data might get written
   *	out to an unlinked old copy of a segment file that will eventually
   *	disappear.
   *
   *	File descriptors are stored in the per-fork md_seg_fds arrays inside
   *	SMgrRelation. The length of these arrays is stored in md_num_open_segs.
   *	Note that a fork's md_num_open_segs having a specific value does not
   *	necessarily mean the relation doesn't have additional segments; we may
   *	just not have opened the next segment yet.  (We could not have "all
   *	segments are in the array" as an invariant anyway, since another backend
   *	could extend the relation while we aren't looking.)  We do not have
   *	entries for inactive segments, however; as soon as we find a partial
   *	segment, we assume that any subsequent segments are inactive.
   *
   *	The entire MdfdVec array is palloc'd in the MdCxt memory context.
   */
  
  typedef struct _MdfdVec
  {
  	File		mdfd_vfd;		/* fd number in fd.c's pool */
  	BlockNumber mdfd_segno;		/* segment number, from 0 */
  } MdfdVec;
  
  static MemoryContext MdCxt;		/* context for all MdfdVec objects */
  
  
  /* Populate a file tag describing an md.c segment file. */
  #define INIT_MD_FILETAG(a,xx_rnode,xx_forknum,xx_segno) \
  ( \
  	memset(&(a), 0, sizeof(FileTag)), \
  	(a).handler = SYNC_HANDLER_MD, \
  	(a).rnode = (xx_rnode), \
  	(a).forknum = (xx_forknum), \
  	(a).segno = (xx_segno) \
  )
  
  
  /*** behavior for mdopen & _mdfd_getseg ***/
  /* ereport if segment not present */
  #define EXTENSION_FAIL				(1 << 0)
  /* return NULL if segment not present */
  #define EXTENSION_RETURN_NULL		(1 << 1)
  /* create new segments as needed */
  #define EXTENSION_CREATE			(1 << 2)
  /* create new segments if needed during recovery */
  #define EXTENSION_CREATE_RECOVERY	(1 << 3)
  /*
   * Allow opening segments which are preceded by segments smaller than
   * RELSEG_SIZE, e.g. inactive segments (see above). Note that this breaks
   * mdnblocks() and related functionality henceforth - which currently is ok,
   * because this is only required in the checkpointer which never uses
   * mdnblocks().
   */
  #define EXTENSION_DONT_CHECK_SIZE	(1 << 4)
  
  
  /* local routines */
  static void mdunlinkfork(RelFileNodeBackend rnode, ForkNumber forkNum,
  						 bool isRedo);
  static MdfdVec *mdopen(SMgrRelation reln, ForkNumber forknum, int behavior);
  static void register_dirty_segment(SMgrRelation reln, ForkNumber forknum,
  								   MdfdVec *seg);
  static void register_unlink_segment(RelFileNodeBackend rnode, ForkNumber forknum,
  									BlockNumber segno);
  static void register_forget_request(RelFileNodeBackend rnode, ForkNumber forknum,
  									BlockNumber segno);
  static void _fdvec_resize(SMgrRelation reln,
  						  ForkNumber forknum,
  						  int nseg);
  static char *_mdfd_segpath(SMgrRelation reln, ForkNumber forknum,
  						   BlockNumber segno);
  static MdfdVec *_mdfd_openseg(SMgrRelation reln, ForkNumber forkno,
  							  BlockNumber segno, int oflags);
  static MdfdVec *_mdfd_getseg(SMgrRelation reln, ForkNumber forkno,
  							 BlockNumber blkno, bool skipFsync, int behavior);
  static BlockNumber _mdnblocks(SMgrRelation reln, ForkNumber forknum,
  							  MdfdVec *seg);
  
  
  /*
   *	mdinit() -- Initialize private state for magnetic disk storage manager.
   */
  void
  mdinit(void)
  {
  	MdCxt = AllocSetContextCreate(TopMemoryContext,
  								  "MdSmgr",
  								  ALLOCSET_DEFAULT_SIZES);
  }
  
  /*
   *	mdexists() -- Does the physical file exist?
   *
   * Note: this will return true for lingering files, with pending deletions
   */
  bool
  mdexists(SMgrRelation reln, ForkNumber forkNum)
  {
  	/*
  	 * Close it first, to ensure that we notice if the fork has been unlinked
  	 * since we opened it.
  	 */
  	mdclose(reln, forkNum);
  
  	return (mdopen(reln, forkNum, EXTENSION_RETURN_NULL) != NULL);
  }
  
  /*
   *	mdcreate() -- Create a new relation on magnetic disk.
   *
   * If isRedo is true, it's okay for the relation to exist already.
   */
  void
  mdcreate(SMgrRelation reln, ForkNumber forkNum, bool isRedo)
  {
  	MdfdVec    *mdfd;
  	char	   *path;
  	File		fd;
  
  	if (isRedo && reln->md_num_open_segs[forkNum] > 0)
  		return;					/* created and opened already... */
  
  	Assert(reln->md_num_open_segs[forkNum] == 0);
  
  	path = relpath(reln->smgr_rnode, forkNum);
  
  	fd = PathNameOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
  
  	if (fd < 0)
  	{
  		int			save_errno = errno;
  
  		if (isRedo)
  			fd = PathNameOpenFile(path, O_RDWR | PG_BINARY);
  		if (fd < 0)
  		{
  			/* be sure to report the error reported by create, not open */
  			errno = save_errno;
  			ereport(ERROR,
  					(errcode_for_file_access(),
  					 errmsg("could not create file \"%s\": %m", path)));
  		}
  	}
  
  	pfree(path);
  
  	_fdvec_resize(reln, forkNum, 1);
  	mdfd = &reln->md_seg_fds[forkNum][0];
  	mdfd->mdfd_vfd = fd;
  	mdfd->mdfd_segno = 0;
  }
  
  /*
   *	mdcreate_ao() -- Create a AO segfile
   *
   * If isRedo is true, it's okay for the file to exist already.
   */
  void
  mdcreate_ao(RelFileNodeBackend rnode, int32 segmentFileNum, bool isRedo)
  {
  	char	   *path;
  	char		buf[MAXPGPATH];
  	File		fd;
  
  	path = aorelpath(rnode, segmentFileNum);
  
  	fd = PathNameOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
  
  	if (fd < 0)
  	{
  		int			save_errno = errno;
  
  		/*
  		 * During bootstrap, there are cases where a system relation will be
  		 * accessed (by internal backend processes) before the bootstrap
  		 * script nominally creates it.  Therefore, allow the file to exist
  		 * already, even if isRedo is not set.	(See also mdopen)
  		 */
  		if (isRedo || IsBootstrapProcessingMode())
  			fd = PathNameOpenFile(path, O_RDWR | PG_BINARY);
  		if (fd < 0)
  		{
  			/* be sure to report the error reported by create, not open */
  			errno = save_errno;
  			ereport(ERROR,
  					(errcode_for_file_access(),
  					 errmsg("could not create relation %s: %m", path)));
  		}
  	}
  
  	if (path != buf)
  		pfree(path);
  }
  
  /*
   *	mdunlink() -- Unlink a relation.
   *
   * Note that we're passed a RelFileNodeBackend --- by the time this is called,
   * there won't be an SMgrRelation hashtable entry anymore.
   *
   * forkNum can be a fork number to delete a specific fork, or InvalidForkNumber
   * to delete all forks.
   *
   * For regular relations, we don't unlink the first segment file of the rel,
   * but just truncate it to zero length, and record a request to unlink it after
   * the next checkpoint.  Additional segments can be unlinked immediately,
   * however.  Leaving the empty file in place prevents that relfilenode
   * number from being reused.  The scenario this protects us from is:
   * 1. We delete a relation (and commit, and actually remove its file).
   * 2. We create a new relation, which by chance gets the same relfilenode as
   *	  the just-deleted one (OIDs must've wrapped around for that to happen).
   * 3. We crash before another checkpoint occurs.
   * During replay, we would delete the file and then recreate it, which is fine
   * if the contents of the file were repopulated by subsequent WAL entries.
   * But if we didn't WAL-log insertions, but instead relied on fsyncing the
   * file after populating it (as for instance CLUSTER and CREATE INDEX do),
   * the contents of the file would be lost forever.  By leaving the empty file
   * until after the next checkpoint, we prevent reassignment of the relfilenode
   * number until it's safe, because relfilenode assignment skips over any
   * existing file.
   *
   * We do not need to go through this dance for temp relations, though, because
   * we never make WAL entries for temp rels, and so a temp rel poses no threat
   * to the health of a regular rel that has taken over its relfilenode number.
   * The fact that temp rels and regular rels have different file naming
   * patterns provides additional safety.
   *
   * All the above applies only to the relation's main fork; other forks can
   * just be removed immediately, since they are not needed to prevent the
   * relfilenode number from being recycled.  Also, we do not carefully
   * track whether other forks have been created or not, but just attempt to
   * unlink them unconditionally; so we should never complain about ENOENT.
   *
   * If isRedo is true, it's unsurprising for the relation to be already gone.
   * Also, we should remove the file immediately instead of queuing a request
   * for later, since during redo there's no possibility of creating a
   * conflicting relation.
   *
   * Note: any failure should be reported as WARNING not ERROR, because
   * we are usually not in a transaction anymore when this is called.
   */
  void
  mdunlink(RelFileNodeBackend rnode, ForkNumber forkNum, bool isRedo)
  {
  	/* Now do the per-fork work */
  	if (forkNum == InvalidForkNumber)
  	{
  		for (forkNum = 0; forkNum <= MAX_FORKNUM; forkNum++)
  			mdunlinkfork(rnode, forkNum, isRedo);
  	}
  	else
  		mdunlinkfork(rnode, forkNum, isRedo);
  }
  
  static void
  mdunlinkfork(RelFileNodeBackend rnode, ForkNumber forkNum, bool isRedo)
  {
  	char	   *path;
  	int			ret;
  
  	path = relpath(rnode, forkNum);
  
  	/*
  	 * Delete or truncate the first segment.
  	 */
  	if (isRedo || forkNum != MAIN_FORKNUM || RelFileNodeBackendIsTemp(rnode))
  	{
  		/* First, forget any pending sync requests for the first segment */
  		if (!RelFileNodeBackendIsTemp(rnode))
  			register_forget_request(rnode, forkNum, 0 /* first seg */ );
  
  		/* Next unlink the file */
  		ret = unlink(path);
  		if (ret < 0 && errno != ENOENT)
  			ereport(WARNING,
  					(errcode_for_file_access(),
  					 errmsg("could not remove file \"%s\": %m", path)));
  	}
  	else
  	{
  		/* truncate(2) would be easier here, but Windows hasn't got it */
  		int			fd;
  
  		fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
  		if (fd >= 0)
  		{
  			int			save_errno;
  
  			ret = ftruncate(fd, 0);
  			save_errno = errno;
  			CloseTransientFile(fd);
  			errno = save_errno;
  		}
  		else
  			ret = -1;
  		if (ret < 0 && errno != ENOENT)
  			ereport(WARNING,
  					(errcode_for_file_access(),
  					 errmsg("could not truncate file \"%s\": %m", path)));
  
  		/* Register request to unlink first segment later */
  		register_unlink_segment(rnode, forkNum, 0 /* first seg */ );
  	}
  
  	/*
  	 * Delete any additional segments.
  	 */
  	if (ret >= 0)
  	{
  		char	   *segpath = (char *) palloc(strlen(path) + 12);
  		BlockNumber segno;
  
  		/*
  		 * Note that because we loop until getting ENOENT, we will correctly
  		 * remove all inactive segments as well as active ones.
  		 */
  		for (segno = 1;; segno++)
  		{
  			/*
  			 * Forget any pending sync requests for this segment before we try
  			 * to unlink.
  			 */
  			if (!RelFileNodeBackendIsTemp(rnode))
  				register_forget_request(rnode, forkNum, segno);
  
  			sprintf(segpath, "%s.%u", path, segno);
  			if (unlink(segpath) < 0)
  			{
  				/* ENOENT is expected after the last segment... */
  				if (errno != ENOENT)
  					ereport(WARNING,
  							(errcode_for_file_access(),
  							 errmsg("could not remove file \"%s\": %m", segpath)));
  				break;
  			}
  		}
  		pfree(segpath);
  	}
  
  	pfree(path);
  }
  
  /*
   *	mdextend() -- Add a block to the specified relation.
   *
   *		The semantics are nearly the same as mdwrite(): write at the
   *		specified position.  However, this is to be used for the case of
   *		extending a relation (i.e., blocknum is at or beyond the current
   *		EOF).  Note that we assume writing a block beyond current EOF
   *		causes intervening file space to become filled with zeroes.
   */
  void
  mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
  		 char *buffer, bool skipFsync)
  {
  	off_t		seekpos;
  	int			nbytes;
  	MdfdVec    *v;
  
  	/* This assert is too expensive to have on normally ... */
  #ifdef CHECK_WRITE_VS_EXTEND
  	Assert(blocknum >= mdnblocks(reln, forknum));
  #endif
  
  	/*
  	 * If a relation manages to grow to 2^32-1 blocks, refuse to extend it any
  	 * more --- we mustn't create a block whose number actually is
  	 * InvalidBlockNumber.
  	 */
  	if (blocknum == InvalidBlockNumber)
  		ereport(ERROR,
  				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
  				 errmsg("cannot extend file \"%s\" beyond %u blocks",
  						relpath(reln->smgr_rnode, forknum),
  						InvalidBlockNumber)));
  
  	v = _mdfd_getseg(reln, forknum, blocknum, skipFsync, EXTENSION_CREATE);
  
  	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
  
  	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
  
  	if ((nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ, seekpos, WAIT_EVENT_DATA_FILE_EXTEND)) != BLCKSZ)
  	{
  		if (nbytes < 0)
  			ereport(ERROR,
  					(errcode_for_file_access(),
  					 errmsg("could not extend file \"%s\": %m",
  							FilePathName(v->mdfd_vfd)),
  					 errhint("Check free disk space.")));
  		/* short write: complain appropriately */
  		ereport(ERROR,
  				(errcode(ERRCODE_DISK_FULL),
  				 errmsg("could not extend file \"%s\": wrote only %d of %d bytes at block %u",
  						FilePathName(v->mdfd_vfd),
  						nbytes, BLCKSZ, blocknum),
  				 errhint("Check free disk space.")));
  	}
  
  	if (!skipFsync && !SmgrIsTemp(reln))
  		register_dirty_segment(reln, forknum, v);
  
  	Assert(_mdnblocks(reln, forknum, v) <= ((BlockNumber) RELSEG_SIZE));
  }
  
  /*
   *	mdopen() -- Open the specified relation.
   *
   * Note we only open the first segment, when there are multiple segments.
   *
   * If first segment is not present, either ereport or return NULL according
   * to "behavior".  We treat EXTENSION_CREATE the same as EXTENSION_FAIL;
   * EXTENSION_CREATE means it's OK to extend an existing relation, not to
   * invent one out of whole cloth.
   */
  static MdfdVec *
  mdopen(SMgrRelation reln, ForkNumber forknum, int behavior)
  {
  	MdfdVec    *mdfd;
  	char	   *path;
  	File		fd;
  
  	/* No work if already open */
  	if (reln->md_num_open_segs[forknum] > 0)
  		return &reln->md_seg_fds[forknum][0];
  
  	path = relpath(reln->smgr_rnode, forknum);
  
  	fd = PathNameOpenFile(path, O_RDWR | PG_BINARY);
  
  	if (fd < 0)
  	{
  		if ((behavior & EXTENSION_RETURN_NULL) &&
  			FILE_POSSIBLY_DELETED(errno))
  		{
  			pfree(path);
  			return NULL;
  		}
  		ereport(ERROR,
  				(errcode_for_file_access(),
  				 errmsg("could not open file \"%s\": %m", path)));
  	}
  
  	pfree(path);
  
  	_fdvec_resize(reln, forknum, 1);
  	mdfd = &reln->md_seg_fds[forknum][0];
  	mdfd->mdfd_vfd = fd;
  	mdfd->mdfd_segno = 0;
  
  	Assert(_mdnblocks(reln, forknum, mdfd) <= ((BlockNumber) RELSEG_SIZE));
  
  	return mdfd;
  }
  
  /*
   *	mdclose() -- Close the specified relation, if it isn't closed already.
   */
  void
  mdclose(SMgrRelation reln, ForkNumber forknum)
  {
  	int			nopensegs = reln->md_num_open_segs[forknum];
  
  	/* No work if already closed */
  	if (nopensegs == 0)
  		return;
  
  	/* close segments starting from the end */
  	while (nopensegs > 0)
  	{
  		MdfdVec    *v = &reln->md_seg_fds[forknum][nopensegs - 1];
  
  		/* if not closed already */
  		if (v->mdfd_vfd >= 0)
  		{
  			FileClose(v->mdfd_vfd);
  			v->mdfd_vfd = -1;
  		}
  
  		nopensegs--;
  	}
  
  	/* resize just once, avoids pointless reallocations */
  	_fdvec_resize(reln, forknum, 0);
  }
  
  /*
   *	mdprefetch() -- Initiate asynchronous read of the specified block of a relation
   */
  void
  mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum)
  {
  #ifdef USE_PREFETCH
  	off_t		seekpos;
  	MdfdVec    *v;
  
  	v = _mdfd_getseg(reln, forknum, blocknum, false, EXTENSION_FAIL);
  
  	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
  
  	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
  
  	(void) FilePrefetch(v->mdfd_vfd, seekpos, BLCKSZ, WAIT_EVENT_DATA_FILE_PREFETCH);
  #endif							/* USE_PREFETCH */
  }
  
  /*
   * mdwriteback() -- Tell the kernel to write pages back to storage.
   *
   * This accepts a range of blocks because flushing several pages at once is
   * considerably more efficient than doing so individually.
   */
  void
  mdwriteback(SMgrRelation reln, ForkNumber forknum,
  			BlockNumber blocknum, BlockNumber nblocks)
  {
  	/*
  	 * Issue flush requests in as few requests as possible; have to split at
  	 * segment boundaries though, since those are actually separate files.
  	 */
  	while (nblocks > 0)
  	{
  		BlockNumber nflush = nblocks;
  		off_t		seekpos;
  		MdfdVec    *v;
  		int			segnum_start,
  					segnum_end;
  
  		v = _mdfd_getseg(reln, forknum, blocknum, true /* not used */ ,
  						 EXTENSION_RETURN_NULL);
  
  		/*
  		 * We might be flushing buffers of already removed relations, that's
  		 * ok, just ignore that case.
  		 */
  		if (!v)
  			return;
  
  		/* compute offset inside the current segment */
  		segnum_start = blocknum / RELSEG_SIZE;
  
  		/* compute number of desired writes within the current segment */
  		segnum_end = (blocknum + nblocks - 1) / RELSEG_SIZE;
  		if (segnum_start != segnum_end)
  			nflush = RELSEG_SIZE - (blocknum % ((BlockNumber) RELSEG_SIZE));
  
  		Assert(nflush >= 1);
  		Assert(nflush <= nblocks);
  
  		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
  
  		FileWriteback(v->mdfd_vfd, seekpos, (off_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
  
  		nblocks -= nflush;
  		blocknum += nflush;
  	}
  }
  
  /*
   *	mdread() -- Read the specified block from a relation.
   */
  void
  mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
  	   char *buffer)
  {
  	off_t		seekpos;
  	int			nbytes;
  	MdfdVec    *v;
  
  	TRACE_POSTGRESQL_SMGR_MD_READ_START(forknum, blocknum,
  										reln->smgr_rnode.node.spcNode,
  										reln->smgr_rnode.node.dbNode,
  										reln->smgr_rnode.node.relNode,
  										reln->smgr_rnode.backend);
  
  	v = _mdfd_getseg(reln, forknum, blocknum, false,
  					 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
  
  	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
  
  	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
  
  	nbytes = FileRead(v->mdfd_vfd, buffer, BLCKSZ, seekpos, WAIT_EVENT_DATA_FILE_READ);
  
  	TRACE_POSTGRESQL_SMGR_MD_READ_DONE(forknum, blocknum,
  									   reln->smgr_rnode.node.spcNode,
  									   reln->smgr_rnode.node.dbNode,
  									   reln->smgr_rnode.node.relNode,
  									   reln->smgr_rnode.backend,
  									   nbytes,
  									   BLCKSZ);
  
  	if (nbytes != BLCKSZ)
  	{
  		if (nbytes < 0)
  			ereport(ERROR,
  					(errcode_for_file_access(),
  					 errmsg("could not read block %u in file \"%s\": %m",
  							blocknum, FilePathName(v->mdfd_vfd))));
  
  		/*
  		 * Short read: we are at or past EOF, or we read a partial block at
  		 * EOF.  Normally this is an error; upper levels should never try to
  		 * read a nonexistent block.  However, if zero_damaged_pages is ON or
  		 * we are InRecovery, we should instead return zeroes without
  		 * complaining.  This allows, for example, the case of trying to
  		 * update a block that was later truncated away.
  		 */
  		if (zero_damaged_pages || InRecovery)
  			MemSet(buffer, 0, BLCKSZ);
  		else
  			ereport(ERROR,
  					(errcode(ERRCODE_DATA_CORRUPTED),
  					 errmsg("could not read block %u in file \"%s\": read only %d of %d bytes",
  							blocknum, FilePathName(v->mdfd_vfd),
  							nbytes, BLCKSZ)));
  	}
  }
  
  /*
   *	mdwrite() -- Write the supplied block at the appropriate location.
   *
   *		This is to be used only for updating already-existing blocks of a
   *		relation (ie, those before the current EOF).  To extend a relation,
   *		use mdextend().
   */
  void
  mdwrite(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
  		char *buffer, bool skipFsync)
  {
  	off_t		seekpos;
  	int			nbytes;
  	MdfdVec    *v;
  
  	/* This assert is too expensive to have on normally ... */
  #ifdef CHECK_WRITE_VS_EXTEND
  	Assert(blocknum < mdnblocks(reln, forknum));
  #endif
  
  	TRACE_POSTGRESQL_SMGR_MD_WRITE_START(forknum, blocknum,
  										 reln->smgr_rnode.node.spcNode,
  										 reln->smgr_rnode.node.dbNode,
  										 reln->smgr_rnode.node.relNode,
  										 reln->smgr_rnode.backend);
  
  	v = _mdfd_getseg(reln, forknum, blocknum, skipFsync,
  					 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
  
  	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
  
  	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
  
  	nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ, seekpos, WAIT_EVENT_DATA_FILE_WRITE);
  
  	TRACE_POSTGRESQL_SMGR_MD_WRITE_DONE(forknum, blocknum,
  										reln->smgr_rnode.node.spcNode,
  										reln->smgr_rnode.node.dbNode,
  										reln->smgr_rnode.node.relNode,
  										reln->smgr_rnode.backend,
  										nbytes,
  										BLCKSZ);
  
  	if (nbytes != BLCKSZ)
  	{
  		if (nbytes < 0)
  			ereport(ERROR,
  					(errcode_for_file_access(),
  					 errmsg("could not write block %u in file \"%s\": %m",
  							blocknum, FilePathName(v->mdfd_vfd))));
  		/* short write: complain appropriately */
  		ereport(ERROR,
  				(errcode(ERRCODE_DISK_FULL),
  				 errmsg("could not write block %u in file \"%s\": wrote only %d of %d bytes",
  						blocknum,
  						FilePathName(v->mdfd_vfd),
  						nbytes, BLCKSZ),
  				 errhint("Check free disk space.")));
  	}
  
  	if (!skipFsync && !SmgrIsTemp(reln))
  		register_dirty_segment(reln, forknum, v);
  }
  
  /*
   *	mdnblocks() -- Get the number of blocks stored in a relation.
   *
   *		Important side effect: all active segments of the relation are opened
   *		and added to the mdfd_seg_fds array.  If this routine has not been
   *		called, then only segments up to the last one actually touched
   *		are present in the array.
   */
  BlockNumber
  mdnblocks(SMgrRelation reln, ForkNumber forknum)
  {
  	MdfdVec    *v = mdopen(reln, forknum, EXTENSION_FAIL);
  	BlockNumber nblocks;
  	BlockNumber segno = 0;
  
  	/* mdopen has opened the first segment */
  	Assert(reln->md_num_open_segs[forknum] > 0);
  
  	/*
  	 * Start from the last open segments, to avoid redundant seeks.  We have
  	 * previously verified that these segments are exactly RELSEG_SIZE long,
  	 * and it's useless to recheck that each time.
  	 *
  	 * NOTE: this assumption could only be wrong if another backend has
  	 * truncated the relation.  We rely on higher code levels to handle that
  	 * scenario by closing and re-opening the md fd, which is handled via
  	 * relcache flush.  (Since the checkpointer doesn't participate in
  	 * relcache flush, it could have segment entries for inactive segments;
  	 * that's OK because the checkpointer never needs to compute relation
  	 * size.)
  	 */
  	segno = reln->md_num_open_segs[forknum] - 1;
  	v = &reln->md_seg_fds[forknum][segno];
  
  	for (;;)
  	{
  		nblocks = _mdnblocks(reln, forknum, v);
  		if (nblocks > ((BlockNumber) RELSEG_SIZE))
  			elog(FATAL, "segment too big");
  		if (nblocks < ((BlockNumber) RELSEG_SIZE))
  			return (segno * ((BlockNumber) RELSEG_SIZE)) + nblocks;
  
  		/*
  		 * If segment is exactly RELSEG_SIZE, advance to next one.
  		 */
  		segno++;
  
  		/*
  		 * We used to pass O_CREAT here, but that has the disadvantage that it
  		 * might create a segment which has vanished through some operating
  		 * system misadventure.  In such a case, creating the segment here
  		 * undermines _mdfd_getseg's attempts to notice and report an error
  		 * upon access to a missing segment.
  		 */
  		v = _mdfd_openseg(reln, forknum, segno, 0);
  		if (v == NULL)
  			return segno * ((BlockNumber) RELSEG_SIZE);
  	}
  }
  
  /*
   *	mdtruncate() -- Truncate relation to specified number of blocks.
   */
  void
  mdtruncate(SMgrRelation reln, ForkNumber forknum, BlockNumber nblocks)
  {
  	BlockNumber curnblk;
  	BlockNumber priorblocks;
  	int			curopensegs;
  
  	/*
  	 * NOTE: mdnblocks makes sure we have opened all active segments, so that
  	 * truncation loop will get them all!
  	 */
  	curnblk = mdnblocks(reln, forknum);
  	if (nblocks > curnblk)
  	{
  		/* Bogus request ... but no complaint if InRecovery */
  		if (InRecovery)
  			return;
  		ereport(ERROR,
  				(errmsg("could not truncate file \"%s\" to %u blocks: it's only %u blocks now",
  						relpath(reln->smgr_rnode, forknum),
  						nblocks, curnblk)));
  	}
  
  	if (nblocks == curnblk && (forknum != MAIN_FORKNUM))
  		return;					/* no work */
  
  	/*
  	 * Truncate segments, starting at the last one. Starting at the end makes
  	 * managing the memory for the fd array easier, should there be errors.
  	 */
  	curopensegs = reln->md_num_open_segs[forknum];
  	while (curopensegs > 0)
  	{
  		MdfdVec    *v;
  
  		priorblocks = (curopensegs - 1) * RELSEG_SIZE;
  
  		v = &reln->md_seg_fds[forknum][curopensegs - 1];
  
  		if (priorblocks > nblocks)
  		{
  			/*
  			 * This segment is no longer active. We truncate the file, but do
  			 * not delete it, for reasons explained in the header comments.
  			 */
  			if (FileTruncate(v->mdfd_vfd, 0, WAIT_EVENT_DATA_FILE_TRUNCATE) < 0)
  				ereport(ERROR,
  						(errcode_for_file_access(),
  						 errmsg("could not truncate file \"%s\": %m",
  								FilePathName(v->mdfd_vfd))));
  
  			if (!SmgrIsTemp(reln))
  				register_dirty_segment(reln, forknum, v);
  
  			/* we never drop the 1st segment */
  			Assert(v != &reln->md_seg_fds[forknum][0]);
  
  			FileClose(v->mdfd_vfd);
  			_fdvec_resize(reln, forknum, curopensegs - 1);
  		}
  		else if (priorblocks + ((BlockNumber) RELSEG_SIZE) > nblocks)
  		{
  			/*
  			 * This is the last segment we want to keep. Truncate the file to
  			 * the right length. NOTE: if nblocks is exactly a multiple K of
  			 * RELSEG_SIZE, we will truncate the K+1st segment to 0 length but
  			 * keep it. This adheres to the invariant given in the header
  			 * comments.
  			 */
  			BlockNumber lastsegblocks = nblocks - priorblocks;
  
  			if (FileTruncate(v->mdfd_vfd, (off_t) lastsegblocks * BLCKSZ, WAIT_EVENT_DATA_FILE_TRUNCATE) < 0)
  				ereport(ERROR,
  						(errcode_for_file_access(),
  						 errmsg("could not truncate file \"%s\" to %u blocks: %m",
  								FilePathName(v->mdfd_vfd),
  								nblocks)));
  			if (!SmgrIsTemp(reln))
  				register_dirty_segment(reln, forknum, v);
  		}
  		else
  		{
  			/*
  			 * We still need this segment, so nothing to do for this and any
  			 * earlier segment.
  			 */
  			break;
  		}
  		curopensegs--;
  	}
  }
  
  /*
   *	mdimmedsync() -- Immediately sync a relation to stable storage.
   *
   * Note that only writes already issued are synced; this routine knows
   * nothing of dirty buffers that may exist inside the buffer manager.
   */
  void
  mdimmedsync(SMgrRelation reln, ForkNumber forknum)
  {
  	int			segno;
  
  	/*
  	 * NOTE: mdnblocks makes sure we have opened all active segments, so that
  	 * fsync loop will get them all!
  	 */
  	mdnblocks(reln, forknum);
  
  	segno = reln->md_num_open_segs[forknum];
  
  	while (segno > 0)
  	{
  		MdfdVec    *v = &reln->md_seg_fds[forknum][segno - 1];
  
  		if (FileSync(v->mdfd_vfd, WAIT_EVENT_DATA_FILE_IMMEDIATE_SYNC) < 0)
  			ereport(data_sync_elevel(ERROR),
  					(errcode_for_file_access(),
  					 errmsg("could not fsync file \"%s\": %m",
  							FilePathName(v->mdfd_vfd))));
  		segno--;
  	}
  }
  
  /*
   * register_dirty_segment() -- Mark a relation segment as needing fsync
   *
   * If there is a local pending-ops table, just make an entry in it for
   * ProcessSyncRequests to process later.  Otherwise, try to pass off the
   * fsync request to the checkpointer process.  If that fails, just do the
   * fsync locally before returning (we hope this will not happen often
   * enough to be a performance problem).
   */
  static void
  register_dirty_segment(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
  {
  	FileTag		tag;
  
  	INIT_MD_FILETAG(tag, reln->smgr_rnode.node, forknum, seg->mdfd_segno);
  
  	/* Temp relations should never be fsync'd */
  	Assert(!SmgrIsTemp(reln));
  
  	if (!RegisterSyncRequest(&tag, SYNC_REQUEST, false /* retryOnError */ ))
  	{
  		ereport(DEBUG1,
  				(errmsg("could not forward fsync request because request queue is full")));
  
  		if (FileSync(seg->mdfd_vfd, WAIT_EVENT_DATA_FILE_SYNC) < 0)
  			ereport(data_sync_elevel(ERROR),
  					(errcode_for_file_access(),
  					 errmsg("could not fsync file \"%s\": %m",
  							FilePathName(seg->mdfd_vfd))));
  	}
  }
  
  /*
   * register_dirty_segment_ao()
   *
   * Similar to register_dirty_segment() but it is for append optimized tables.
   * The API definition is different because (1) relation forks are not used for
   * AO tables, it's always MAIN_FORKNUM and (2) there is no MdfdVec equivalent
   * for AO segment files.
   */
  void
  register_dirty_segment_ao(RelFileNode rnode, int segno, File vfd)
  {
  	FileTag		tag;
  
  	INIT_FILETAG(tag, rnode, MAIN_FORKNUM, segno, SYNC_HANDLER_AO);
  
  	if (!RegisterSyncRequest(&tag, SYNC_REQUEST, false /* retryOnError */ ))
  	{
  		ereport(DEBUG1,
  				(errmsg("could not forward AO fsync request because request queue is full")));
  
  		if (FileSync(vfd, WAIT_EVENT_DATA_FILE_SYNC) < 0)
  			ereport(data_sync_elevel(ERROR),
  					(errcode_for_file_access(),
  					 errmsg("could not fsync AO file \"%s\": %m",
  							FilePathName(vfd))));
  	}
  }
  
  /*
   * register_unlink_segment() -- Schedule a file to be deleted after next checkpoint
   */
  static void
  register_unlink_segment(RelFileNodeBackend rnode, ForkNumber forknum,
  						BlockNumber segno)
  {
  	FileTag		tag;
  
  	INIT_MD_FILETAG(tag, rnode.node, forknum, segno);
  
  	/* Should never be used with temp relations */
  	Assert(!RelFileNodeBackendIsTemp(rnode));
  
  	RegisterSyncRequest(&tag, SYNC_UNLINK_REQUEST, true /* retryOnError */ );
  }
  
  /*
   * register_forget_request() -- forget any fsyncs for a relation fork's segment
   */
  static void
  register_forget_request(RelFileNodeBackend rnode, ForkNumber forknum,
  						BlockNumber segno)
  {
  	FileTag		tag;
  
  	INIT_MD_FILETAG(tag, rnode.node, forknum, segno);
  
  	RegisterSyncRequest(&tag, SYNC_FORGET_REQUEST, true /* retryOnError */ );
  }
  
  /*
   * ForgetDatabaseSyncRequests -- forget any fsyncs and unlinks for a DB
   */
  void
  ForgetDatabaseSyncRequests(Oid dbid)
  {
  	FileTag		tag;
  	RelFileNode rnode;
  
  	rnode.dbNode = dbid;
  	rnode.spcNode = 0;
  	rnode.relNode = 0;
  
  	INIT_MD_FILETAG(tag, rnode, InvalidForkNumber, InvalidBlockNumber);
  
  	RegisterSyncRequest(&tag, SYNC_FILTER_REQUEST, true /* retryOnError */ );
  
  	/*
  	 * Need to register filter requests for all the handlers because handler
  	 * is part of the key that is used to determine equivalence among two
  	 * pending entries.
  	 */
  	INIT_FILETAG(tag, rnode, InvalidForkNumber, InvalidBlockNumber, SYNC_HANDLER_AO);
  
  	RegisterSyncRequest(&tag, SYNC_FILTER_REQUEST, true /* retryOnError */ );
  }
  
  /*
   * DropRelationFiles -- drop files of all given relations
   */
  void
  DropRelationFiles(RelFileNodePendingDelete *delrels, int ndelrels, bool isRedo)
  {
  	SMgrRelation *srels;
  	int			i;
  
  	srels = palloc(sizeof(SMgrRelation) * ndelrels);
  	for (i = 0; i < ndelrels; i++)
  	{
  		/* GPDB: backend can only be TempRelBackendId or InvalidBackendId for a
  		 * given relfile since we don't tie temp relations to their backends. */
  		SMgrRelation srel = smgropen(delrels[i].node,
  									 delrels[i].isTempRelation ?
  									 TempRelBackendId : InvalidBackendId,
  									 delrels[i].smgr_which);
  
  		if (isRedo)
  		{
  			ForkNumber	fork;
  
  			for (fork = 0; fork <= MAX_FORKNUM; fork++)
  				XLogDropRelation(delrels[i].node, fork);
  		}
  		srels[i] = srel;
  	}
  
  	smgrdounlinkall(srels, ndelrels, isRedo);
  
  	/*
  	 * Call smgrclose() in reverse order as when smgropen() is called.
  	 * This trick enables remove_from_unowned_list() in smgrclose()
  	 * to search the SMgrRelation from the unowned list,
  	 * with O(1) performance.
  	 */
  	for (i = ndelrels - 1; i >= 0; i--)
  		smgrclose(srels[i]);
  	pfree(srels);
  }
  
  /*
   *	_fdvec_resize() -- Resize the fork's open segments array
   */
  static void
  _fdvec_resize(SMgrRelation reln,
  			  ForkNumber forknum,
  			  int nseg)
  {
  	if (nseg == 0)
  	{
  		if (reln->md_num_open_segs[forknum] > 0)
  		{
  			pfree(reln->md_seg_fds[forknum]);
  			reln->md_seg_fds[forknum] = NULL;
  		}
  	}
  	else if (reln->md_num_open_segs[forknum] == 0)
  	{
  		reln->md_seg_fds[forknum] =
  			MemoryContextAlloc(MdCxt, sizeof(MdfdVec) * nseg);
  	}
  	else
  	{
  		/*
  		 * It doesn't seem worthwhile complicating the code by having a more
  		 * aggressive growth strategy here; the number of segments doesn't
  		 * grow that fast, and the memory context internally will sometimes
  		 * avoid doing an actual reallocation.
  		 */
  		reln->md_seg_fds[forknum] =
  			repalloc(reln->md_seg_fds[forknum],
  					 sizeof(MdfdVec) * nseg);
  	}
  
  	reln->md_num_open_segs[forknum] = nseg;
  }
  
  /*
   * Return the filename for the specified segment of the relation. The
   * returned string is palloc'd.
   */
  static char *
  _mdfd_segpath(SMgrRelation reln, ForkNumber forknum, BlockNumber segno)
  {
  	char	   *path,
  			   *fullpath;
  
  	path = relpath(reln->smgr_rnode, forknum);
  
  	if (segno > 0)
  	{
  		fullpath = psprintf("%s.%u", path, segno);
  		pfree(path);
  	}
  	else
  		fullpath = path;
  
  	return fullpath;
  }
  
  /*
   * Open the specified segment of the relation,
   * and make a MdfdVec object for it.  Returns NULL on failure.
   */
  static MdfdVec *
  _mdfd_openseg(SMgrRelation reln, ForkNumber forknum, BlockNumber segno,
  			  int oflags)
  {
  	MdfdVec    *v;
  	int			fd;
  	char	   *fullpath;
  
  	fullpath = _mdfd_segpath(reln, forknum, segno);
  
  	/* open the file */
  	fd = PathNameOpenFile(fullpath, O_RDWR | PG_BINARY | oflags);
  
  	pfree(fullpath);
  
  	if (fd < 0)
  		return NULL;
  
  	if (segno <= reln->md_num_open_segs[forknum])
  		_fdvec_resize(reln, forknum, segno + 1);
  
  	/* fill the entry */
  	v = &reln->md_seg_fds[forknum][segno];
  	v->mdfd_vfd = fd;
  	v->mdfd_segno = segno;
  
  	Assert(_mdnblocks(reln, forknum, v) <= ((BlockNumber) RELSEG_SIZE));
  
  	/* all done */
  	return v;
  }
  
  /*
   *	_mdfd_getseg() -- Find the segment of the relation holding the
   *		specified block.
   *
   * If the segment doesn't exist, we ereport, return NULL, or create the
   * segment, according to "behavior".  Note: skipFsync is only used in the
   * EXTENSION_CREATE case.
   */
  static MdfdVec *
  _mdfd_getseg(SMgrRelation reln, ForkNumber forknum, BlockNumber blkno,
  			 bool skipFsync, int behavior)
  {
  	MdfdVec    *v;
  	BlockNumber targetseg;
  	BlockNumber nextsegno;
  
  	/* some way to handle non-existent segments needs to be specified */
  	Assert(behavior &
  		   (EXTENSION_FAIL | EXTENSION_CREATE | EXTENSION_RETURN_NULL));
  
  	targetseg = blkno / ((BlockNumber) RELSEG_SIZE);
  
  	/* if an existing and opened segment, we're done */
  	if (targetseg < reln->md_num_open_segs[forknum])
  	{
  		v = &reln->md_seg_fds[forknum][targetseg];
  		return v;
  	}
  
  	/*
  	 * The target segment is not yet open. Iterate over all the segments
  	 * between the last opened and the target segment. This way missing
  	 * segments either raise an error, or get created (according to
  	 * 'behavior'). Start with either the last opened, or the first segment if
  	 * none was opened before.
  	 */
  	if (reln->md_num_open_segs[forknum] > 0)
  		v = &reln->md_seg_fds[forknum][reln->md_num_open_segs[forknum] - 1];
  	else
  	{
  		v = mdopen(reln, forknum, behavior);
  		if (!v)
  			return NULL;		/* if behavior & EXTENSION_RETURN_NULL */
  	}
  
  	for (nextsegno = reln->md_num_open_segs[forknum];
  		 nextsegno <= targetseg; nextsegno++)
  	{
  		BlockNumber nblocks = _mdnblocks(reln, forknum, v);
  		int			flags = 0;
  
  		Assert(nextsegno == v->mdfd_segno + 1);
  
  		if (nblocks > ((BlockNumber) RELSEG_SIZE))
  			elog(FATAL, "segment too big");
  
  		if ((behavior & EXTENSION_CREATE) ||
  			(InRecovery && (behavior & EXTENSION_CREATE_RECOVERY)))
  		{
  			/*
  			 * Normally we will create new segments only if authorized by the
  			 * caller (i.e., we are doing mdextend()).  But when doing WAL
  			 * recovery, create segments anyway; this allows cases such as
  			 * replaying WAL data that has a write into a high-numbered
  			 * segment of a relation that was later deleted. We want to go
  			 * ahead and create the segments so we can finish out the replay.
  			 * However if the caller has specified
  			 * EXTENSION_REALLY_RETURN_NULL, then extension is not desired
  			 * even in recovery; we won't reach this point in that case.
  			 *
  			 * We have to maintain the invariant that segments before the last
  			 * active segment are of size RELSEG_SIZE; therefore, if
  			 * extending, pad them out with zeroes if needed.  (This only
  			 * matters if in recovery, or if the caller is extending the
  			 * relation discontiguously, but that can happen in hash indexes.)
  			 */
  			if (nblocks < ((BlockNumber) RELSEG_SIZE))
  			{
  				char	   *zerobuf = palloc0(BLCKSZ);
  
  				mdextend(reln, forknum,
  						 nextsegno * ((BlockNumber) RELSEG_SIZE) - 1,
  						 zerobuf, skipFsync);
  				pfree(zerobuf);
  			}
  			flags = O_CREAT;
  		}
  		else if (!(behavior & EXTENSION_DONT_CHECK_SIZE) &&
  				 nblocks < ((BlockNumber) RELSEG_SIZE))
  		{
  			/*
  			 * When not extending (or explicitly including truncated
  			 * segments), only open the next segment if the current one is
  			 * exactly RELSEG_SIZE.  If not (this branch), either return NULL
  			 * or fail.
  			 */
  			if (behavior & EXTENSION_RETURN_NULL)
  			{
  				/*
  				 * Some callers discern between reasons for _mdfd_getseg()
  				 * returning NULL based on errno. As there's no failing
  				 * syscall involved in this case, explicitly set errno to
  				 * ENOENT, as that seems the closest interpretation.
  				 */
  				errno = ENOENT;
  				return NULL;
  			}
  
  			ereport(ERROR,
  					(errcode_for_file_access(),
  					 errmsg("could not open file \"%s\" (target block %u): previous segment is only %u blocks",
  							_mdfd_segpath(reln, forknum, nextsegno),
  							blkno, nblocks)));
  		}
  
  		v = _mdfd_openseg(reln, forknum, nextsegno, flags);
  
  		if (v == NULL)
  		{
  			if ((behavior & EXTENSION_RETURN_NULL) &&
  				FILE_POSSIBLY_DELETED(errno))
  				return NULL;
  			ereport(ERROR,
  					(errcode_for_file_access(),
  					 errmsg("could not open file \"%s\" (target block %u): %m",
  							_mdfd_segpath(reln, forknum, nextsegno),
  							blkno)));
  		}
  	}
  
  	return v;
  }
  
  /*
   * Get number of blocks present in a single disk file
   */
  static BlockNumber
  _mdnblocks(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
  {
  	off_t		len;
  
  	len = FileSize(seg->mdfd_vfd);
  	if (len < 0)
  		ereport(ERROR,
  				(errcode_for_file_access(),
  				 errmsg("could not seek to end of file \"%s\": %m",
  						FilePathName(seg->mdfd_vfd))));
  	/* note that this calculation will ignore any partial block at EOF */
  	return (BlockNumber) (len / BLCKSZ);
  }
  
  /*
   * Sync a file to disk, given a file tag.  Write the path into an output
   * buffer so the caller can use it in error messages.
   *
   * Return 0 on success, -1 on failure, with errno set.
   */
  int
  mdsyncfiletag(const FileTag *ftag, char *path)
  {
  	SMgrRelation reln = smgropen(ftag->rnode, InvalidBackendId, 0);
  	MdfdVec    *v;
  	char	   *p;
  
  	/* Provide the path for informational messages. */
  	p = _mdfd_segpath(reln, ftag->forknum, ftag->segno);
  	strlcpy(path, p, MAXPGPATH);
  	pfree(p);
  
  	/* Try to open the requested segment. */
  	v = _mdfd_getseg(reln,
  					 ftag->forknum,
  					 ftag->segno * (BlockNumber) RELSEG_SIZE,
  					 false,
  					 EXTENSION_RETURN_NULL | EXTENSION_DONT_CHECK_SIZE);
  	if (v == NULL)
  		return -1;
  
  	/* Try to fsync the file. */
  	return FileSync(v->mdfd_vfd, WAIT_EVENT_DATA_FILE_SYNC);
  }
  
  
  int
  aosyncfiletag(const FileTag *ftag, char *path)
  {
  	SMgrRelation reln = smgropen(ftag->rnode, InvalidBackendId, 1);
  	char	   *p;
  
  	/* Provide the path for informational messages. */
  	p = _mdfd_segpath(reln, ftag->forknum, ftag->segno);
  	strlcpy(path, p, MAXPGPATH);
  	pfree(p);
  
  	File fd = PathNameOpenFile(path, O_RDWR);
  	if (fd <= 0)
  		elog(ERROR, "could not open file %s: %m", path);
  
  	/* Try to fsync the file. */
  	return FileSync(fd, WAIT_EVENT_DATA_FILE_SYNC);
  }
  
  /*
   * Unlink a file, given a file tag.  Write the path into an output
   * buffer so the caller can use it in error messages.
   *
   * Return 0 on success, -1 on failure, with errno set.
   */
  int
  mdunlinkfiletag(const FileTag *ftag, char *path)
  {
  	char	   *p;
  
  	/* Compute the path. */
  	p = relpathperm(ftag->rnode, MAIN_FORKNUM);
  	strlcpy(path, p, MAXPGPATH);
  	pfree(p);
  
  	/* Try to unlink the file. */
  	return unlink(path);
  }
  
  /*
   * Check if a given candidate request matches a given tag, when processing
   * a SYNC_FILTER_REQUEST request.  This will be called for all pending
   * requests to find out whether to forget them.
   */
  bool
  mdfiletagmatches(const FileTag *ftag, const FileTag *candidate)
  {
  	/*
  	 * For now we only use filter requests as a way to drop all scheduled
  	 * callbacks relating to a given database, when dropping the database.
  	 * We'll return true for all candidates that have the same database OID as
  	 * the ftag from the SYNC_FILTER_REQUEST request, so they're forgotten.
  	 */
  	return ftag->rnode.dbNode == candidate->rnode.dbNode;
  }
  