postgresql/src/backend/storage/smgr/md.c

/*-------------------------------------------------------------------------
 *
 * md.c--
 *	  This code manages relations that reside on magnetic disk.
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/storage/smgr/md.c,v 1.32 1998/07/20 16:14:14 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include <unistd.h>
#include <stdio.h>				/* for sprintf() */
#include <string.h>
#include <fcntl.h>				/* for open() flags */
#include <sys/file.h>

#include "postgres.h"
#include "miscadmin.h"			/* for DataDir */

#include "catalog/catalog.h"
#include "storage/block.h"
#include "storage/fd.h"
#include "storage/smgr.h"		/* where the declarations go */
#include "utils/mcxt.h"
#include "utils/rel.h"

#undef DIAGNOSTIC

/*
 *	The magnetic disk storage manager keeps track of open file descriptors
 *	in its own descriptor pool.  This happens for two reasons.	First, at
 *	transaction boundaries, we walk the list of descriptors and flush
 *	anything that we've dirtied in the current transaction.  Second, we
 *	have to support relations of > 4GBytes.  In order to do this, we break
 *	relations up into chunks of < 2GBytes and store one chunk in each of
 *	several files that represent the relation.
 */

typedef struct _MdfdVec
{
	int			mdfd_vfd;		/* fd number in vfd pool */
	uint16		mdfd_flags;		/* clean, dirty, free */
	int			mdfd_lstbcnt;	/* most recent block count */
	int			mdfd_nextFree;	/* next free vector */
#ifndef LET_OS_MANAGE_FILESIZE
	struct _MdfdVec *mdfd_chain;/* for large relations */
#endif
} MdfdVec;

static int	Nfds = 100;
static MdfdVec *Md_fdvec = (MdfdVec *) NULL;
static int	Md_Free = -1;
static int	CurFd = 0;
static MemoryContext MdCxt;

#define MDFD_DIRTY		(uint16) 0x01
#define MDFD_FREE		(uint16) 0x02

/*
 * RELSEG_SIZE appears to be the number of segments that can
 * be in a disk file.  It was defined as 262144 based on 8k
 * blocks, but now that the block size can be changed, this
 * has to be calculated at compile time.  Otherwise, the file
 * size limit would not work out to 2-gig (2147483648).
 *
 * The number needs to be (2 ** 31) / BLCKSZ, but to be keep
 * the math under MAXINT, pre-divide by 256 and use ...
 *
 *			 (((2 ** 23) / BLCKSZ) * (2 ** 8))
 *
 * 07 Jan 98  darrenk
 *
 * Now possibly let the OS handle it...
 *
 * 19 Mar 98  darrenk
 *
 */

#ifndef LET_OS_MANAGE_FILESIZE
#define RELSEG_SIZE		((8388608 / BLCKSZ) * 256)
#endif

/* routines declared here */
static MdfdVec *_mdfd_openseg(Relation reln, int segno, int oflags);
static MdfdVec *_mdfd_getseg(Relation reln, int blkno, int oflag);
static int	_fdvec_alloc(void);
static void _fdvec_free(int);
static BlockNumber _mdnblocks(File file, Size blcksz);

/*
 *	mdinit() -- Initialize private state for magnetic disk storage manager.
 *
 *		We keep a private table of all file descriptors.  Whenever we do
 *		a write to one, we mark it dirty in our table.	Whenever we force
 *		changes to disk, we mark the file descriptor clean.  At transaction
 *		commit, we force changes to disk for all dirty file descriptors.
 *		This routine allocates and initializes the table.
 *
 *		Returns SM_SUCCESS or SM_FAIL with errno set as appropriate.
 */
int
mdinit()
{
	MemoryContext oldcxt;
	int			i;

	MdCxt = (MemoryContext) CreateGlobalMemory("MdSmgr");
	if (MdCxt == (MemoryContext) NULL)
		return (SM_FAIL);

	oldcxt = MemoryContextSwitchTo(MdCxt);
	Md_fdvec = (MdfdVec *) palloc(Nfds * sizeof(MdfdVec));
	MemoryContextSwitchTo(oldcxt);

	if (Md_fdvec == (MdfdVec *) NULL)
		return (SM_FAIL);

	MemSet(Md_fdvec, 0, Nfds * sizeof(MdfdVec));

	/* Set free list */
	for (i = 0; i < Nfds; i++)
	{
		Md_fdvec[i].mdfd_nextFree = i + 1;
		Md_fdvec[i].mdfd_flags = MDFD_FREE;
	}
	Md_Free = 0;
	Md_fdvec[Nfds - 1].mdfd_nextFree = -1;

	return (SM_SUCCESS);
}

int
mdcreate(Relation reln)
{
	int			fd,
				vfd;
	char	   *path;

	path = relpath(reln->rd_rel->relname.data);
	fd = FileNameOpenFile(path, O_RDWR | O_CREAT | O_EXCL, 0600);

	/*
	 * If the file already exists and is empty, we pretend that the create
	 * succeeded.  During bootstrap processing, we skip that check,
	 * because pg_time, pg_variable, and pg_log get created before their
	 * .bki file entries are processed.
	 *
	 * As the result of this pretence it was possible to have in pg_class > 1
	 * records with the same relname. Actually, it should be fixed in
	 * upper levels, too, but... -	vadim 05/06/97
	 */

	if (fd < 0)
	{
		if (!IsBootstrapProcessingMode())
			return (-1);
		fd = FileNameOpenFile(path, O_RDWR, 0600);		/* Bootstrap */
		if (fd < 0)
			return (-1);
	}

	vfd = _fdvec_alloc();
	if (vfd < 0)
		return (-1);

	Md_fdvec[vfd].mdfd_vfd = fd;
	Md_fdvec[vfd].mdfd_flags = (uint16) 0;
#ifndef LET_OS_MANAGE_FILESIZE
	Md_fdvec[vfd].mdfd_chain = (MdfdVec *) NULL;
#endif
	Md_fdvec[vfd].mdfd_lstbcnt = 0;

	return (vfd);
}

/*
 *	mdunlink() -- Unlink a relation.
 */
int
mdunlink(Relation reln)
{
	int			fd;
	int			i;
	MdfdVec    *v,
			   *ov;
	MemoryContext oldcxt;
	char		fname[NAMEDATALEN];
	char		tname[NAMEDATALEN + 10];		/* leave room for overflow
												 * suffixes */

	/*
	 * On Windows NT you can't unlink a file if it is open so we have * to
	 * do this.
	 */

	StrNCpy(fname, RelationGetRelationName(reln)->data, NAMEDATALEN);

	if (FileNameUnlink(fname) < 0)
		return (SM_FAIL);

	/* unlink all the overflow files for large relations */
	for (i = 1;; i++)
	{
		sprintf(tname, "%s.%d", fname, i);
		if (FileNameUnlink(tname) < 0)
			break;
	}

	/* finally, clean out the mdfd vector */
	fd = RelationGetFile(reln);
	Md_fdvec[fd].mdfd_flags = (uint16) 0;

	oldcxt = MemoryContextSwitchTo(MdCxt);
#ifndef LET_OS_MANAGE_FILESIZE
	for (v = &Md_fdvec[fd]; v != (MdfdVec *) NULL;)
	{
		FileUnlink(v->mdfd_vfd);
		ov = v;
		v = v->mdfd_chain;
		if (ov != &Md_fdvec[fd])
			pfree(ov);
	}
	Md_fdvec[fd].mdfd_chain = (MdfdVec *) NULL;
#else
	v = &Md_fdvec[fd];
	if (v != (MdfdVec *) NULL)
		FileUnlink(v->mdfd_vfd);
#endif
	MemoryContextSwitchTo(oldcxt);

	_fdvec_free(fd);

	return (SM_SUCCESS);
}

/*
 *	mdextend() -- Add a block to the specified relation.
 *
 *		This routine returns SM_FAIL or SM_SUCCESS, with errno set as
 *		appropriate.
 */
int
mdextend(Relation reln, char *buffer)
{
	long		pos;
	int			nblocks;
	MdfdVec    *v;

	nblocks = mdnblocks(reln);
	v = _mdfd_getseg(reln, nblocks, O_CREAT);

	if ((pos = FileSeek(v->mdfd_vfd, 0L, SEEK_END)) < 0)
		return (SM_FAIL);

	if (FileWrite(v->mdfd_vfd, buffer, BLCKSZ) != BLCKSZ)
		return (SM_FAIL);

	/* remember that we did a write, so we can sync at xact commit */
	v->mdfd_flags |= MDFD_DIRTY;

	/* try to keep the last block count current, though it's just a hint */
#ifndef LET_OS_MANAGE_FILESIZE
	if ((v->mdfd_lstbcnt = (++nblocks % RELSEG_SIZE)) == 0)
		v->mdfd_lstbcnt = RELSEG_SIZE;

#ifdef DIAGNOSTIC
	if (_mdnblocks(v->mdfd_vfd, BLCKSZ) > RELSEG_SIZE
		|| v->mdfd_lstbcnt > RELSEG_SIZE)
		elog(FATAL, "segment too big!");
#endif
#else
	v->mdfd_lstbcnt = ++nblocks;
#endif

	return (SM_SUCCESS);
}

/*
 *	mdopen() -- Open the specified relation.
 */
int
mdopen(Relation reln)
{
	char	   *path;
	int			fd;
	int			vfd;

	path = relpath(reln->rd_rel->relname.data);

	fd = FileNameOpenFile(path, O_RDWR, 0600);

	/* this should only happen during bootstrap processing */
	if (fd < 0)
		fd = FileNameOpenFile(path, O_RDWR | O_CREAT | O_EXCL, 0600);

	vfd = _fdvec_alloc();
	if (vfd < 0)
		return (-1);

	Md_fdvec[vfd].mdfd_vfd = fd;
	Md_fdvec[vfd].mdfd_flags = (uint16) 0;
	Md_fdvec[vfd].mdfd_lstbcnt = _mdnblocks(fd, BLCKSZ);
#ifndef LET_OS_MANAGE_FILESIZE
	Md_fdvec[vfd].mdfd_chain = (MdfdVec *) NULL;

#ifdef DIAGNOSTIC
	if (Md_fdvec[vfd].mdfd_lstbcnt > RELSEG_SIZE)
		elog(FATAL, "segment too big on relopen!");
#endif
#endif

	return (vfd);
}

/*
 *	mdclose() -- Close the specified relation
 *
 *		AND FREE fd vector! It may be re-used for other relation!
 *		reln should be flushed from cache after closing !..
 *
 *		Returns SM_SUCCESS or SM_FAIL with errno set as appropriate.
 */
int
mdclose(Relation reln)
{
	int			fd;
	MdfdVec    *v,
			   *ov;
	MemoryContext oldcxt;

	fd = RelationGetFile(reln);

	oldcxt = MemoryContextSwitchTo(MdCxt);
#ifndef LET_OS_MANAGE_FILESIZE
	for (v = &Md_fdvec[fd]; v != (MdfdVec *) NULL;)
	{
		/* if not closed already */
		if (v->mdfd_vfd >= 0)
		{

			/*
			 * We sync the file descriptor so that we don't need to reopen
			 * it at transaction commit to force changes to disk.
			 */

			FileSync(v->mdfd_vfd);
			FileClose(v->mdfd_vfd);

			/* mark this file descriptor as clean in our private table */
			v->mdfd_flags &= ~MDFD_DIRTY;
		}
		/* Now free vector */
		ov = v;
		v = v->mdfd_chain;
		if (ov != &Md_fdvec[fd])
			pfree(ov);
	}

	Md_fdvec[fd].mdfd_chain = (MdfdVec *) NULL;
#else
	v = &Md_fdvec[fd];
	if (v != (MdfdVec *) NULL)
	{
		if (v->mdfd_vfd >= 0)
		{

			/*
			 * We sync the file descriptor so that we don't need to reopen
			 * it at transaction commit to force changes to disk.
			 */

			FileSync(v->mdfd_vfd);
			FileClose(v->mdfd_vfd);

			/* mark this file descriptor as clean in our private table */
			v->mdfd_flags &= ~MDFD_DIRTY;
		}
	}
#endif
	MemoryContextSwitchTo(oldcxt);

	_fdvec_free(fd);

	return (SM_SUCCESS);
}

/*
 *	mdread() -- Read the specified block from a relation.
 *
 *		Returns SM_SUCCESS or SM_FAIL.
 */
int
mdread(Relation reln, BlockNumber blocknum, char *buffer)
{
	int			status;
	long		seekpos;
	int			nbytes;
	MdfdVec    *v;

	v = _mdfd_getseg(reln, blocknum, 0);

#ifndef LET_OS_MANAGE_FILESIZE
	seekpos = (long) (BLCKSZ * (blocknum % RELSEG_SIZE));

#ifdef DIAGNOSTIC
	if (seekpos >= BLCKSZ * RELSEG_SIZE)
		elog(FATAL, "seekpos too big!");
#endif
#else
	seekpos = (long) (BLCKSZ * (blocknum));
#endif

	if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)
		return (SM_FAIL);

	status = SM_SUCCESS;
	if ((nbytes = FileRead(v->mdfd_vfd, buffer, BLCKSZ)) != BLCKSZ)
	{
		if (nbytes == 0)
			MemSet(buffer, 0, BLCKSZ);
		else
			status = SM_FAIL;
	}

	return (status);
}

/*
 *	mdwrite() -- Write the supplied block at the appropriate location.
 *
 *		Returns SM_SUCCESS or SM_FAIL.
 */
int
mdwrite(Relation reln, BlockNumber blocknum, char *buffer)
{
	int			status;
	long		seekpos;
	MdfdVec    *v;

	v = _mdfd_getseg(reln, blocknum, 0);

#ifndef LET_OS_MANAGE_FILESIZE
	seekpos = (long) (BLCKSZ * (blocknum % RELSEG_SIZE));
#ifdef DIAGNOSTIC
	if (seekpos >= BLCKSZ * RELSEG_SIZE)
		elog(FATAL, "seekpos too big!");
#endif
#else
	seekpos = (long) (BLCKSZ * (blocknum));
#endif

	if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)
		return (SM_FAIL);

	status = SM_SUCCESS;
	if (FileWrite(v->mdfd_vfd, buffer, BLCKSZ) != BLCKSZ)
		status = SM_FAIL;

	v->mdfd_flags |= MDFD_DIRTY;

	return (status);
}

/*
 *	mdflush() -- Synchronously write a block to disk.
 *
 *		This is exactly like mdwrite(), but doesn't return until the file
 *		system buffer cache has been flushed.
 */
int
mdflush(Relation reln, BlockNumber blocknum, char *buffer)
{
	int			status;
	long		seekpos;
	MdfdVec    *v;

	v = _mdfd_getseg(reln, blocknum, 0);

#ifndef LET_OS_MANAGE_FILESIZE
	seekpos = (long) (BLCKSZ * (blocknum % RELSEG_SIZE));
#ifdef DIAGNOSTIC
	if (seekpos >= BLCKSZ * RELSEG_SIZE)
		elog(FATAL, "seekpos too big!");
#endif
#else
	seekpos = (long) (BLCKSZ * (blocknum));
#endif

	if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)
		return (SM_FAIL);

	/* write and sync the block */
	status = SM_SUCCESS;
	if (FileWrite(v->mdfd_vfd, buffer, BLCKSZ) != BLCKSZ
		|| FileSync(v->mdfd_vfd) < 0)
		status = SM_FAIL;

	/*
	 * By here, the block is written and changes have been forced to
	 * stable storage.	Mark the descriptor as clean until the next write,
	 * so we don't sync it again unnecessarily at transaction commit.
	 */

	v->mdfd_flags &= ~MDFD_DIRTY;

	return (status);
}

/*
 *	mdblindwrt() -- Write a block to disk blind.
 *
 *		We have to be able to do this using only the name and OID of
 *		the database and relation in which the block belongs.  This
 *		is a synchronous write.
 */
int
mdblindwrt(char *dbstr,
		   char *relstr,
		   Oid dbid,
		   Oid relid,
		   BlockNumber blkno,
		   char *buffer)
{
	int			fd;
	int			segno;
	long		seekpos;
	int			status;
	char	   *path;

#ifndef LET_OS_MANAGE_FILESIZE
	int			nchars;

	/* be sure we have enough space for the '.segno', if any */
	segno = blkno / RELSEG_SIZE;
	if (segno > 0)
		nchars = 10;
	else
		nchars = 0;

	/* construct the path to the file and open it */
	/* system table? then put in system area... */
	if (dbid == (Oid) 0)
	{
		path = (char *) palloc(strlen(DataDir) + sizeof(NameData) + 2 + nchars);
		if (segno == 0)
			sprintf(path, "%s/%s", DataDir, relstr);
		else
			sprintf(path, "%s/%s.%d", DataDir, relstr, segno);
	}
	/* user table? then put in user database area... */
	else if (dbid == MyDatabaseId)
	{
		extern char *DatabasePath;

		path = (char *) palloc(strlen(DatabasePath) + 2 * sizeof(NameData) + 2 + nchars);
		if (segno == 0)
			sprintf(path, "%s%c%s", DatabasePath, SEP_CHAR, relstr);
		else
			sprintf(path, "%s%c%s.%d", DatabasePath, SEP_CHAR, relstr, segno);
	}
	else
/* this is work arround only !!! */
	{
		char		dbpath[MAXPGPATH + 1];
		Oid			owner,
					id;
		char	   *tmpPath;

		GetRawDatabaseInfo(dbstr, &owner, &id, dbpath);

		if (id != dbid)
			elog(FATAL, "mdblindwrt: oid of db %s is not %u", dbstr, dbid);
		tmpPath = ExpandDatabasePath(dbpath);
		if (tmpPath == NULL)
			elog(FATAL, "mdblindwrt: can't expand path for db %s", dbstr);
		path = (char *) palloc(strlen(tmpPath) + 2 * sizeof(NameData) + 2 + nchars);
		if (segno == 0)
			sprintf(path, "%s%c%s", tmpPath, SEP_CHAR, relstr);
		else
			sprintf(path, "%s%c%s.%d", tmpPath, SEP_CHAR, relstr, segno);
		pfree(tmpPath);
	}
#else
	/* construct the path to the file and open it */
	/* system table? then put in system area... */
	if (dbid == (Oid) 0)
	{
		path = (char *) palloc(strlen(DataDir) + sizeof(NameData) + 2);
		sprintf(path, "%s/%s", DataDir, relstr);
	}
	/* user table? then put in user database area... */
	else if (dbid == MyDatabaseId)
	{
		extern char *DatabasePath;

		path = (char *) palloc(strlen(DatabasePath) + 2 * sizeof(NameData) + 2);
		sprintf(path, "%s%c%s", DatabasePath, SEP_CHAR, relstr);
	}
	else
/* this is work arround only !!! */
	{
		char		dbpath[MAXPGPATH + 1];
		Oid			owner,
					id;
		char	   *tmpPath;

		GetRawDatabaseInfo(dbstr, &owner, &id, dbpath);

		if (id != dbid)
			elog(FATAL, "mdblindwrt: oid of db %s is not %u", dbstr, dbid);
		tmpPath = ExpandDatabasePath(dbpath);
		if (tmpPath == NULL)
			elog(FATAL, "mdblindwrt: can't expand path for db %s", dbstr);
		path = (char *) palloc(strlen(tmpPath) + 2 * sizeof(NameData) + 2);
		sprintf(path, "%s%c%s", tmpPath, SEP_CHAR, relstr);
		pfree(tmpPath);
	}
#endif

	if ((fd = open(path, O_RDWR, 0600)) < 0)
		return (SM_FAIL);

	/* seek to the right spot */
#ifndef LET_OS_MANAGE_FILESIZE
	seekpos = (long) (BLCKSZ * (blkno % RELSEG_SIZE));
#else
	seekpos = (long) (BLCKSZ * (blkno));
#endif

	if (lseek(fd, seekpos, SEEK_SET) != seekpos)
	{
		close(fd);
		return (SM_FAIL);
	}

	status = SM_SUCCESS;

	/* write and sync the block */
	if (write(fd, buffer, BLCKSZ) != BLCKSZ || (pg_fsync(fd) < 0))
		status = SM_FAIL;

	if (close(fd) < 0)
		status = SM_FAIL;

	pfree(path);

	return (status);
}

/*
 *	mdnblocks() -- Get the number of blocks stored in a relation.
 *
 *		Returns # of blocks or -1 on error.
 */
int
mdnblocks(Relation reln)
{
	int			fd;
	MdfdVec    *v;
	int			nblocks;
	int			segno;

	fd = RelationGetFile(reln);
	v = &Md_fdvec[fd];

#ifndef LET_OS_MANAGE_FILESIZE
#ifdef DIAGNOSTIC
	if (_mdnblocks(v->mdfd_vfd, BLCKSZ) > RELSEG_SIZE)
		elog(FATAL, "segment too big in getseg!");
#endif

	segno = 0;
	for (;;)
	{
		if (v->mdfd_lstbcnt == RELSEG_SIZE
			|| (nblocks = _mdnblocks(v->mdfd_vfd, BLCKSZ)) == RELSEG_SIZE)
		{

			v->mdfd_lstbcnt = RELSEG_SIZE;
			segno++;

			if (v->mdfd_chain == (MdfdVec *) NULL)
			{
				v->mdfd_chain = _mdfd_openseg(reln, segno, O_CREAT);
				if (v->mdfd_chain == (MdfdVec *) NULL)
					elog(ERROR, "cannot count blocks for %.16s -- open failed",
						 RelationGetRelationName(reln));
			}

			v = v->mdfd_chain;
		}
		else
			return ((segno * RELSEG_SIZE) + nblocks);
	}
#else
	return (_mdnblocks(v->mdfd_vfd, BLCKSZ));
#endif
}

/*
 *	mdtruncate() -- Truncate relation to specified number of blocks.
 *
 *		Returns # of blocks or -1 on error.
 */
int
mdtruncate(Relation reln, int nblocks)
{
	int			fd;
	MdfdVec    *v;

#ifndef LET_OS_MANAGE_FILESIZE
	int			curnblk;

	curnblk = mdnblocks(reln);
	if (curnblk / RELSEG_SIZE > 0)
	{
		elog(NOTICE, "Can't truncate multi-segments relation %s",
			 reln->rd_rel->relname.data);
		return (curnblk);
	}
#endif

	fd = RelationGetFile(reln);
	v = &Md_fdvec[fd];

	if (FileTruncate(v->mdfd_vfd, nblocks * BLCKSZ) < 0)
		return (-1);

	return (nblocks);

}	/* mdtruncate */

/*
 *	mdcommit() -- Commit a transaction.
 *
 *		All changes to magnetic disk relations must be forced to stable
 *		storage.  This routine makes a pass over the private table of
 *		file descriptors.  Any descriptors to which we have done writes,
 *		but not synced, are synced here.
 *
 *		Returns SM_SUCCESS or SM_FAIL with errno set as appropriate.
 */
int
mdcommit()
{
	int			i;
	MdfdVec    *v;

	for (i = 0; i < CurFd; i++)
	{
#ifndef LET_OS_MANAGE_FILESIZE
		for (v = &Md_fdvec[i]; v != (MdfdVec *) NULL; v = v->mdfd_chain)
#else
		v = &Md_fdvec[i];
		if (v != (MdfdVec *) NULL)
#endif
		{
			if (v->mdfd_flags & MDFD_DIRTY)
			{
				if (FileSync(v->mdfd_vfd) < 0)
					return (SM_FAIL);

				v->mdfd_flags &= ~MDFD_DIRTY;
			}
		}
	}

	return (SM_SUCCESS);
}

/*
 *	mdabort() -- Abort a transaction.
 *
 *		Changes need not be forced to disk at transaction abort.  We mark
 *		all file descriptors as clean here.  Always returns SM_SUCCESS.
 */
int
mdabort()
{
	int			i;
	MdfdVec    *v;

	for (i = 0; i < CurFd; i++)
	{
#ifndef LET_OS_MANAGE_FILESIZE
		for (v = &Md_fdvec[i]; v != (MdfdVec *) NULL; v = v->mdfd_chain)
#else
		v = &Md_fdvec[i];
		if (v != (MdfdVec *) NULL)
#endif
			v->mdfd_flags &= ~MDFD_DIRTY;
	}

	return (SM_SUCCESS);
}

/*
 *	_fdvec_alloc () -- grab a free (or new) md file descriptor vector.
 *
 */
static
int
_fdvec_alloc()
{
	MdfdVec    *nvec;
	int			fdvec,
				i;
	MemoryContext oldcxt;

	if (Md_Free >= 0)			/* get from free list */
	{
		fdvec = Md_Free;
		Md_Free = Md_fdvec[fdvec].mdfd_nextFree;
		Assert(Md_fdvec[fdvec].mdfd_flags == MDFD_FREE);
		Md_fdvec[fdvec].mdfd_flags = 0;
		if (fdvec >= CurFd)
		{
			Assert(fdvec == CurFd);
			CurFd++;
		}
		return (fdvec);
	}

	/* Must allocate more room */

	if (Nfds != CurFd)
		elog(FATAL, "_fdvec_alloc error");

	Nfds *= 2;

	oldcxt = MemoryContextSwitchTo(MdCxt);

	nvec = (MdfdVec *) palloc(Nfds * sizeof(MdfdVec));
	MemSet(nvec, 0, Nfds * sizeof(MdfdVec));
	memmove(nvec, (char *) Md_fdvec, CurFd * sizeof(MdfdVec));
	pfree(Md_fdvec);

	MemoryContextSwitchTo(oldcxt);

	Md_fdvec = nvec;

	/* Set new free list */
	for (i = CurFd; i < Nfds; i++)
	{
		Md_fdvec[i].mdfd_nextFree = i + 1;
		Md_fdvec[i].mdfd_flags = MDFD_FREE;
	}
	Md_fdvec[Nfds - 1].mdfd_nextFree = -1;
	Md_Free = CurFd + 1;

	fdvec = CurFd;
	CurFd++;
	Md_fdvec[fdvec].mdfd_flags = 0;

	return (fdvec);
}

/*
 *	_fdvec_free () -- free md file descriptor vector.
 *
 */
static
void
_fdvec_free(int fdvec)
{

	Assert(Md_Free < 0 || Md_fdvec[Md_Free].mdfd_flags == MDFD_FREE);
	Md_fdvec[fdvec].mdfd_nextFree = Md_Free;
	Md_fdvec[fdvec].mdfd_flags = MDFD_FREE;
	Md_Free = fdvec;

}

static MdfdVec *
_mdfd_openseg(Relation reln, int segno, int oflags)
{
	MemoryContext oldcxt;
	MdfdVec    *v;
	int			fd;
	bool		dofree;
	char	   *path,
			   *fullpath;

	/* be sure we have enough space for the '.segno', if any */
	path = relpath(RelationGetRelationName(reln)->data);

	dofree = false;
	if (segno > 0)
	{
		dofree = true;
		fullpath = (char *) palloc(strlen(path) + 12);
		sprintf(fullpath, "%s.%d", path, segno);
	}
	else
		fullpath = path;

	/* open the file */
	fd = PathNameOpenFile(fullpath, O_RDWR | oflags, 0600);

	if (dofree)
		pfree(fullpath);

	if (fd < 0)
		return ((MdfdVec *) NULL);

	/* allocate an mdfdvec entry for it */
	oldcxt = MemoryContextSwitchTo(MdCxt);
	v = (MdfdVec *) palloc(sizeof(MdfdVec));
	MemoryContextSwitchTo(oldcxt);

	/* fill the entry */
	v->mdfd_vfd = fd;
	v->mdfd_flags = (uint16) 0;
	v->mdfd_lstbcnt = _mdnblocks(fd, BLCKSZ);
#ifndef LET_OS_MANAGE_FILESIZE
	v->mdfd_chain = (MdfdVec *) NULL;

#ifdef DIAGNOSTIC
	if (v->mdfd_lstbcnt > RELSEG_SIZE)
		elog(FATAL, "segment too big on open!");
#endif
#endif

	/* all done */
	return (v);
}

static MdfdVec *
_mdfd_getseg(Relation reln, int blkno, int oflag)
{
	MdfdVec    *v;
	int			segno;
	int			fd;
	int			i;

	fd = RelationGetFile(reln);
	if (fd < 0)
	{
		if ((fd = mdopen(reln)) < 0)
			elog(ERROR, "cannot open relation %.16s",
				 RelationGetRelationName(reln));
		reln->rd_fd = fd;
	}

#ifndef LET_OS_MANAGE_FILESIZE
	for (v = &Md_fdvec[fd], segno = blkno / RELSEG_SIZE, i = 1;
		 segno > 0;
		 i++, segno--)
	{

		if (v->mdfd_chain == (MdfdVec *) NULL)
		{
			v->mdfd_chain = _mdfd_openseg(reln, i, oflag);

			if (v->mdfd_chain == (MdfdVec *) NULL)
				elog(ERROR, "cannot open segment %d of relation %.16s",
					 i, RelationGetRelationName(reln));
		}
		v = v->mdfd_chain;
	}
#else
	v = &Md_fdvec[fd];
#endif

	return (v);
}

static BlockNumber
_mdnblocks(File file, Size blcksz)
{
	long		len;

	len = FileSeek(file, 0L, SEEK_END) - 1;
	return ((BlockNumber) ((len < 0) ? 0 : 1 + len / blcksz));
}