Fix oversight in previous error-reporting patch; mustn't pfree path string

[PostgreSQL.git] / src / backend / storage / smgr / md.c

/*-------------------------------------------------------------------------
 *
 * md.c
 *        This code manages relations that reside on magnetic disk.
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL$
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>
#include <fcntl.h>
#include <sys/file.h>

#include "catalog/catalog.h"
#include "miscadmin.h"
#include "postmaster/bgwriter.h"
#include "storage/fd.h"
#include "storage/bufmgr.h"
#include "storage/relfilenode.h"
#include "storage/smgr.h"
#include "utils/hsearch.h"
#include "utils/memutils.h"


/* interval for calling AbsorbFsyncRequests in mdsync */
#define FSYNCS_PER_ABSORB               10

/* special values for the segno arg to RememberFsyncRequest */
#define FORGET_RELATION_FSYNC   (InvalidBlockNumber)
#define FORGET_DATABASE_FSYNC   (InvalidBlockNumber-1)
#define UNLINK_RELATION_REQUEST (InvalidBlockNumber-2)

/*
 * On Windows, we have to interpret EACCES as possibly meaning the same as
 * ENOENT, because if a file is unlinked-but-not-yet-gone on that platform,
 * that's what you get.  Ugh.  This code is designed so that we don't
 * actually believe these cases are okay without further evidence (namely,
 * a pending fsync request getting revoked ... see mdsync).
 */
#ifndef WIN32
#define FILE_POSSIBLY_DELETED(err)      ((err) == ENOENT)
#else
#define FILE_POSSIBLY_DELETED(err)      ((err) == ENOENT || (err) == EACCES)
#endif

/*
 *      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 bgwriter 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.
 *
 *      The file descriptor pointer (md_fd field) stored in the SMgrRelation
 *      cache is, therefore, just the head of a list of MdfdVec objects, one
 *      per segment.  But note the md_fd pointer can be NULL, indicating
 *      relation not open.
 *
 *      Also note that mdfd_chain == NULL does not necessarily mean the relation
 *      doesn't have another segment after this one; we may just not have
 *      opened the next segment yet.  (We could not have "all segments are
 *      in the chain" as an invariant anyway, since another backend could
 *      extend the relation when we weren't looking.)  We do not make chain
 *      entries for inactive segments, however; as soon as we find a partial
 *      segment, we assume that any subsequent segments are inactive.
 *
 *      All MdfdVec objects are 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 */
        struct _MdfdVec *mdfd_chain;    /* next segment, or NULL */
} MdfdVec;

static MemoryContext MdCxt;             /* context for all md.c allocations */


/*
 * In some contexts (currently, standalone backends and the bgwriter process)
 * we keep track of pending fsync operations: we need to remember all relation
 * segments that have been written since the last checkpoint, so that we can
 * fsync them down to disk before completing the next checkpoint.  This hash
 * table remembers the pending operations.      We use a hash table mostly as
 * a convenient way of eliminating duplicate requests.
 *
 * We use a similar mechanism to remember no-longer-needed files that can
 * be deleted after the next checkpoint, but we use a linked list instead of
 * a hash table, because we don't expect there to be any duplicate requests.
 *
 * (Regular backends do not track pending operations locally, but forward
 * them to the bgwriter.)
 */
typedef struct
{
        RelFileNode rnode;                      /* the targeted relation */
        ForkNumber forknum;
        BlockNumber segno;                      /* which segment */
} PendingOperationTag;

typedef uint16 CycleCtr;                /* can be any convenient integer size */

typedef struct
{
        PendingOperationTag tag;        /* hash table key (must be first!) */
        bool            canceled;               /* T => request canceled, not yet removed */
        CycleCtr        cycle_ctr;              /* mdsync_cycle_ctr when request was made */
} PendingOperationEntry;

typedef struct
{
        RelFileNode rnode;                      /* the dead relation to delete */
        CycleCtr        cycle_ctr;              /* mdckpt_cycle_ctr when request was made */
} PendingUnlinkEntry;

static HTAB *pendingOpsTable = NULL;
static List *pendingUnlinks = NIL;

static CycleCtr mdsync_cycle_ctr = 0;
static CycleCtr mdckpt_cycle_ctr = 0;


typedef enum                                    /* behavior for mdopen & _mdfd_getseg */
{
        EXTENSION_FAIL,                         /* ereport if segment not present */
        EXTENSION_RETURN_NULL,          /* return NULL if not present */
        EXTENSION_CREATE                        /* create new segments as needed */
} ExtensionBehavior;

/* local routines */
static MdfdVec *mdopen(SMgrRelation reln, ForkNumber forknum, 
                                           ExtensionBehavior behavior);
static void register_dirty_segment(SMgrRelation reln, ForkNumber forknum,
                                                                   MdfdVec *seg);
static void register_unlink(RelFileNode rnode);
static MdfdVec *_fdvec_alloc(void);
static MdfdVec *_mdfd_openseg(SMgrRelation reln, ForkNumber forkno,
                                                          BlockNumber segno, int oflags);
static MdfdVec *_mdfd_getseg(SMgrRelation reln, ForkNumber forkno,
                         BlockNumber blkno, bool isTemp, ExtensionBehavior 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_MINSIZE,
                                                                  ALLOCSET_DEFAULT_INITSIZE,
                                                                  ALLOCSET_DEFAULT_MAXSIZE);

        /*
         * Create pending-operations hashtable if we need it.  Currently, we need
         * it if we are standalone (not under a postmaster) OR if we are a
         * bootstrap-mode subprocess of a postmaster (that is, a startup or
         * bgwriter process).
         */
        if (!IsUnderPostmaster || IsBootstrapProcessingMode())
        {
                HASHCTL         hash_ctl;

                MemSet(&hash_ctl, 0, sizeof(hash_ctl));
                hash_ctl.keysize = sizeof(PendingOperationTag);
                hash_ctl.entrysize = sizeof(PendingOperationEntry);
                hash_ctl.hash = tag_hash;
                hash_ctl.hcxt = MdCxt;
                pendingOpsTable = hash_create("Pending Ops Table",
                                                                          100L,
                                                                          &hash_ctl,
                                                                   HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);
                pendingUnlinks = NIL;
        }
}

/*
 *  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)
{
        char       *path;
        File            fd;

        if (isRedo && reln->md_fd[forkNum] != NULL)
                return;                                 /* created and opened already... */

        Assert(reln->md_fd[forkNum] == NULL);

        path = relpath(reln->smgr_rnode, forkNum);

        fd = PathNameOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY, 0600);

        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, 0600);
                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)));
                }
        }

        pfree(path);

        reln->md_fd[forkNum] = _fdvec_alloc();

        reln->md_fd[forkNum]->mdfd_vfd = fd;
        reln->md_fd[forkNum]->mdfd_segno = 0;
        reln->md_fd[forkNum]->mdfd_chain = NULL;
}

/*
 *      mdunlink() -- Unlink a relation.
 *
 * Note that we're passed a RelFileNode --- by the time this is called,
 * there won't be an SMgrRelation hashtable entry anymore.
 *
 * Actually, we don't unlink the first segment file of the relation, 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.
 *
 * If isRedo is true, it's okay 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(RelFileNode rnode, ForkNumber forkNum, bool isRedo)
{
        char       *path;
        int                     ret;

        /*
         * We have to clean out any pending fsync requests for the doomed
         * relation, else the next mdsync() will fail.
         */
        ForgetRelationFsyncRequests(rnode, forkNum);

        path = relpath(rnode, forkNum);

        /*
         * Delete or truncate the first segment.
         */
        if (isRedo || forkNum != MAIN_FORKNUM)
                ret = unlink(path);
        else
        {
                /* truncate(2) would be easier here, but Windows hasn't got it */
                int                     fd;

                fd = BasicOpenFile(path, O_RDWR | PG_BINARY, 0);
                if (fd >= 0)
                {
                        int                     save_errno;

                        ret = ftruncate(fd, 0);
                        save_errno = errno;
                        close(fd);
                        errno = save_errno;
                }
                else
                        ret = -1;
        }
        if (ret < 0)
        {
                if (!isRedo || errno != ENOENT)
                        ereport(WARNING,
                                        (errcode_for_file_access(),
                                         errmsg("could not remove relation %s: %m", path)));
        }

        /*
         * Delete any additional segments.
         */
        else
        {
                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++)
                {
                        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 segment %u of relation %s: %m",
                                                                        segno, path)));
                                break;
                        }
                }
                pfree(segpath);
        }

        pfree(path);

        /* Register request to unlink first segment later */
        if (!isRedo && forkNum == MAIN_FORKNUM)
                register_unlink(rnode);
}

/*
 *      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 isTemp)
{
        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 relation %s beyond %u blocks",
                                                relpath(reln->smgr_rnode, forknum),
                                                InvalidBlockNumber)));

        v = _mdfd_getseg(reln, forknum, blocknum, isTemp, EXTENSION_CREATE);

        seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
        Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);

        /*
         * Note: because caller usually obtained blocknum by calling mdnblocks,
         * which did a seek(SEEK_END), this seek is often redundant and will be
         * optimized away by fd.c.      It's not redundant, however, if there is a
         * partial page at the end of the file. In that case we want to try to
         * overwrite the partial page with a full page.  It's also not redundant
         * if bufmgr.c had to dump another buffer of the same file to make room
         * for the new page's buffer.
         */
        if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not seek to block %u of relation %s: %m",
                                                blocknum,
                                                relpath(reln->smgr_rnode, forknum))));

        if ((nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ)) != BLCKSZ)
        {
                if (nbytes < 0)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not extend relation %s: %m",
                                                        relpath(reln->smgr_rnode, forknum)),
                                         errhint("Check free disk space.")));
                /* short write: complain appropriately */
                ereport(ERROR,
                                (errcode(ERRCODE_DISK_FULL),
                                 errmsg("could not extend relation %s: wrote only %d of %d bytes at block %u",
                                                relpath(reln->smgr_rnode, forknum),
                                                nbytes, BLCKSZ, blocknum),
                                 errhint("Check free disk space.")));
        }

        if (!isTemp)
                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, ExtensionBehavior behavior)
{
        MdfdVec    *mdfd;
        char       *path;
        File            fd;

        /* No work if already open */
        if (reln->md_fd[forknum])
                return reln->md_fd[forknum];

        path = relpath(reln->smgr_rnode, forknum);

        fd = PathNameOpenFile(path, O_RDWR | PG_BINARY, 0600);

        if (fd < 0)
        {
                /*
                 * During bootstrap, there are cases where a system relation will be
                 * accessed (by internal backend processes) before the bootstrap
                 * script nominally creates it.  Therefore, accept mdopen() as a
                 * substitute for mdcreate() in bootstrap mode only. (See mdcreate)
                 */
                if (IsBootstrapProcessingMode())
                        fd = PathNameOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY, 0600);
                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 relation %s: %m", path)));
                }
        }

        pfree(path);

        reln->md_fd[forknum] = mdfd = _fdvec_alloc();

        mdfd->mdfd_vfd = fd;
        mdfd->mdfd_segno = 0;
        mdfd->mdfd_chain = NULL;
        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)
{
        MdfdVec    *v = reln->md_fd[forknum];

        /* No work if already closed */
        if (v == NULL)
                return;

        reln->md_fd[forknum] = NULL;                    /* prevent dangling pointer after error */

        while (v != NULL)
        {
                MdfdVec    *ov = v;

                /* if not closed already */
                if (v->mdfd_vfd >= 0)
                        FileClose(v->mdfd_vfd);
                /* Now free vector */
                v = v->mdfd_chain;
                pfree(ov);
        }
}

/*
 *      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;

        v = _mdfd_getseg(reln, forknum, blocknum, false, EXTENSION_FAIL);

        seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
        Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);

        if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not seek to block %u of relation %s: %m",
                                                blocknum, relpath(reln->smgr_rnode, forknum))));

        if ((nbytes = FileRead(v->mdfd_vfd, buffer, BLCKSZ)) != BLCKSZ)
        {
                if (nbytes < 0)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                   errmsg("could not read block %u of relation %s: %m",
                                                  blocknum, relpath(reln->smgr_rnode, forknum))));

                /*
                 * 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 of relation %s: read only %d of %d bytes",
                                                        blocknum, relpath(reln->smgr_rnode, forknum),
                                                        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 isTemp)
{
        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

        v = _mdfd_getseg(reln, forknum, blocknum, isTemp, EXTENSION_FAIL);

        seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
        Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);

        if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not seek to block %u of relation %s: %m",
                                                blocknum, relpath(reln->smgr_rnode, forknum))));

        if ((nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ)) != BLCKSZ)
        {
                if (nbytes < 0)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                  errmsg("could not write block %u of relation %s: %m",
                                                 blocknum, relpath(reln->smgr_rnode, forknum))));
                /* short write: complain appropriately */
                ereport(ERROR,
                                (errcode(ERRCODE_DISK_FULL),
                                 errmsg("could not write block %u of relation %s: wrote only %d of %d bytes",
                                                blocknum,
                                                relpath(reln->smgr_rnode, forknum),
                                                nbytes, BLCKSZ),
                                 errhint("Check free disk space.")));
        }

        if (!isTemp)
                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_chain list.  If this routine has not been
 *              called, then only segments up to the last one actually touched
 *              are present in the chain.
 */
BlockNumber
mdnblocks(SMgrRelation reln, ForkNumber forknum)
{
        MdfdVec    *v = mdopen(reln, forknum, EXTENSION_FAIL);
        BlockNumber nblocks;
        BlockNumber segno = 0;

        /*
         * Skip through any segments that aren't the last one, to avoid redundant
         * seeks on them.  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 bgwriter doesn't participate in relcache
         * flush, it could have segment chain entries for inactive segments;
         * that's OK because the bgwriter never needs to compute relation size.)
         */
        while (v->mdfd_chain != NULL)
        {
                segno++;
                v = v->mdfd_chain;
        }

        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++;

                if (v->mdfd_chain == NULL)
                {
                        /*
                         * Because we pass O_CREAT, we will create the next segment (with
                         * zero length) immediately, if the last segment is of length
                         * RELSEG_SIZE.  While perhaps not strictly necessary, this keeps
                         * the logic simple.
                         */
                        v->mdfd_chain = _mdfd_openseg(reln, forknum, segno, O_CREAT);
                        if (v->mdfd_chain == NULL)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                 errmsg("could not open segment %u of relation %s: %m",
                                                segno,
                                                relpath(reln->smgr_rnode, forknum))));
                }

                v = v->mdfd_chain;
        }
}

/*
 *      mdtruncate() -- Truncate relation to specified number of blocks.
 */
void
mdtruncate(SMgrRelation reln, ForkNumber forknum, BlockNumber nblocks,
                   bool isTemp)
{
        MdfdVec    *v;
        BlockNumber curnblk;
        BlockNumber priorblocks;

        /*
         * 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 relation %s to %u blocks: it's only %u blocks now",
                                                relpath(reln->smgr_rnode, forknum),
                                                nblocks, curnblk)));
        }
        if (nblocks == curnblk)
                return;                                 /* no work */

        v = mdopen(reln, forknum, EXTENSION_FAIL);

        priorblocks = 0;
        while (v != NULL)
        {
                MdfdVec    *ov = v;

                if (priorblocks > nblocks)
                {
                        /*
                         * This segment is no longer active (and has already been unlinked
                         * from the mdfd_chain). We truncate the file, but do not delete
                         * it, for reasons explained in the header comments.
                         */
                        if (FileTruncate(v->mdfd_vfd, 0) < 0)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not truncate relation %s to %u blocks: %m",
                                                                relpath(reln->smgr_rnode, forknum),
                                                                nblocks)));
                        if (!isTemp)
                                register_dirty_segment(reln, forknum, v);
                        v = v->mdfd_chain;
                        Assert(ov != reln->md_fd[forknum]);     /* we never drop the 1st segment */
                        pfree(ov);
                }
                else if (priorblocks + ((BlockNumber) RELSEG_SIZE) > nblocks)
                {
                        /*
                         * This is the last segment we want to keep. Truncate the file to
                         * the right length, and clear chain link that points to any
                         * remaining segments (which we shall zap). 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) < 0)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not truncate relation %s to %u blocks: %m",
                                                                relpath(reln->smgr_rnode, forknum),
                                                                nblocks)));
                        if (!isTemp)
                                register_dirty_segment(reln, forknum, v);
                        v = v->mdfd_chain;
                        ov->mdfd_chain = NULL;
                }
                else
                {
                        /*
                         * We still need this segment and 0 or more blocks beyond it, so
                         * nothing to do here.
                         */
                        v = v->mdfd_chain;
                }
                priorblocks += RELSEG_SIZE;
        }
}

/*
 *      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)
{
        MdfdVec    *v;
        BlockNumber curnblk;

        /*
         * NOTE: mdnblocks makes sure we have opened all active segments, so that
         * fsync loop will get them all!
         */
        curnblk = mdnblocks(reln, forknum);

        v = mdopen(reln, forknum, EXTENSION_FAIL);

        while (v != NULL)
        {
                if (FileSync(v->mdfd_vfd) < 0)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not fsync segment %u of relation %s: %m",
                                                        v->mdfd_segno,
                                                        relpath(reln->smgr_rnode, forknum))));
                v = v->mdfd_chain;
        }
}

/*
 *      mdsync() -- Sync previous writes to stable storage.
 */
void
mdsync(void)
{
        static bool mdsync_in_progress = false;

        HASH_SEQ_STATUS hstat;
        PendingOperationEntry *entry;
        int                     absorb_counter;

        /*
         * This is only called during checkpoints, and checkpoints should only
         * occur in processes that have created a pendingOpsTable.
         */
        if (!pendingOpsTable)
                elog(ERROR, "cannot sync without a pendingOpsTable");

        /*
         * If we are in the bgwriter, the sync had better include all fsync
         * requests that were queued by backends up to this point.      The tightest
         * race condition that could occur is that a buffer that must be written
         * and fsync'd for the checkpoint could have been dumped by a backend just
         * before it was visited by BufferSync().  We know the backend will have
         * queued an fsync request before clearing the buffer's dirtybit, so we
         * are safe as long as we do an Absorb after completing BufferSync().
         */
        AbsorbFsyncRequests();

        /*
         * To avoid excess fsync'ing (in the worst case, maybe a never-terminating
         * checkpoint), we want to ignore fsync requests that are entered into the
         * hashtable after this point --- they should be processed next time,
         * instead.  We use mdsync_cycle_ctr to tell old entries apart from new
         * ones: new ones will have cycle_ctr equal to the incremented value of
         * mdsync_cycle_ctr.
         *
         * In normal circumstances, all entries present in the table at this point
         * will have cycle_ctr exactly equal to the current (about to be old)
         * value of mdsync_cycle_ctr.  However, if we fail partway through the
         * fsync'ing loop, then older values of cycle_ctr might remain when we
         * come back here to try again.  Repeated checkpoint failures would
         * eventually wrap the counter around to the point where an old entry
         * might appear new, causing us to skip it, possibly allowing a checkpoint
         * to succeed that should not have.  To forestall wraparound, any time the
         * previous mdsync() failed to complete, run through the table and
         * forcibly set cycle_ctr = mdsync_cycle_ctr.
         *
         * Think not to merge this loop with the main loop, as the problem is
         * exactly that that loop may fail before having visited all the entries.
         * From a performance point of view it doesn't matter anyway, as this path
         * will never be taken in a system that's functioning normally.
         */
        if (mdsync_in_progress)
        {
                /* prior try failed, so update any stale cycle_ctr values */
                hash_seq_init(&hstat, pendingOpsTable);
                while ((entry = (PendingOperationEntry *) hash_seq_search(&hstat)) != NULL)
                {
                        entry->cycle_ctr = mdsync_cycle_ctr;
                }
        }

        /* Advance counter so that new hashtable entries are distinguishable */
        mdsync_cycle_ctr++;

        /* Set flag to detect failure if we don't reach the end of the loop */
        mdsync_in_progress = true;

        /* Now scan the hashtable for fsync requests to process */
        absorb_counter = FSYNCS_PER_ABSORB;
        hash_seq_init(&hstat, pendingOpsTable);
        while ((entry = (PendingOperationEntry *) hash_seq_search(&hstat)) != NULL)
        {
                /*
                 * If the entry is new then don't process it this time.  Note that
                 * "continue" bypasses the hash-remove call at the bottom of the loop.
                 */
                if (entry->cycle_ctr == mdsync_cycle_ctr)
                        continue;

                /* Else assert we haven't missed it */
                Assert((CycleCtr) (entry->cycle_ctr + 1) == mdsync_cycle_ctr);

                /*
                 * If fsync is off then we don't have to bother opening the file at
                 * all.  (We delay checking until this point so that changing fsync on
                 * the fly behaves sensibly.)  Also, if the entry is marked canceled,
                 * fall through to delete it.
                 */
                if (enableFsync && !entry->canceled)
                {
                        int                     failures;

                        /*
                         * If in bgwriter, we want to absorb pending requests every so
                         * often to prevent overflow of the fsync request queue.  It is
                         * unspecified whether newly-added entries will be visited by
                         * hash_seq_search, but we don't care since we don't need to
                         * process them anyway.
                         */
                        if (--absorb_counter <= 0)
                        {
                                AbsorbFsyncRequests();
                                absorb_counter = FSYNCS_PER_ABSORB;
                        }

                        /*
                         * The fsync table could contain requests to fsync segments that
                         * have been deleted (unlinked) by the time we get to them. Rather
                         * than just hoping an ENOENT (or EACCES on Windows) error can be
                         * ignored, what we do on error is absorb pending requests and
                         * then retry.  Since mdunlink() queues a "revoke" message before
                         * actually unlinking, the fsync request is guaranteed to be
                         * marked canceled after the absorb if it really was this case.
                         * DROP DATABASE likewise has to tell us to forget fsync requests
                         * before it starts deletions.
                         */
                        for (failures = 0;; failures++)         /* loop exits at "break" */
                        {
                                SMgrRelation reln;
                                MdfdVec    *seg;
                                char       *path;

                                /*
                                 * Find or create an smgr hash entry for this relation. This
                                 * may seem a bit unclean -- md calling smgr?  But it's really
                                 * the best solution.  It ensures that the open file reference
                                 * isn't permanently leaked if we get an error here. (You may
                                 * say "but an unreferenced SMgrRelation is still a leak!" Not
                                 * really, because the only case in which a checkpoint is done
                                 * by a process that isn't about to shut down is in the
                                 * bgwriter, and it will periodically do smgrcloseall(). This
                                 * fact justifies our not closing the reln in the success path
                                 * either, which is a good thing since in non-bgwriter cases
                                 * we couldn't safely do that.)  Furthermore, in many cases
                                 * the relation will have been dirtied through this same smgr
                                 * relation, and so we can save a file open/close cycle.
                                 */
                                reln = smgropen(entry->tag.rnode);

                                /*
                                 * It is possible that the relation has been dropped or
                                 * truncated since the fsync request was entered.  Therefore,
                                 * allow ENOENT, but only if we didn't fail already on this
                                 * file.  This applies both during _mdfd_getseg() and during
                                 * FileSync, since fd.c might have closed the file behind our
                                 * back.
                                 */
                                seg = _mdfd_getseg(reln, entry->tag.forknum,
                                                          entry->tag.segno * ((BlockNumber) RELSEG_SIZE),
                                                                   false, EXTENSION_RETURN_NULL);
                                if (seg != NULL &&
                                        FileSync(seg->mdfd_vfd) >= 0)
                                        break;          /* success; break out of retry loop */

                                /*
                                 * XXX is there any point in allowing more than one retry?
                                 * Don't see one at the moment, but easy to change the test
                                 * here if so.
                                 */
                                path = relpath(entry->tag.rnode, entry->tag.forknum);
                                if (!FILE_POSSIBLY_DELETED(errno) ||
                                        failures > 0)
                                        ereport(ERROR,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not fsync segment %u of relation %s: %m",
                                                                        entry->tag.segno, path)));
                                else
                                        ereport(DEBUG1,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not fsync segment %u of relation %s but retrying: %m",
                                                                        entry->tag.segno, path)));
                                pfree(path);

                                /*
                                 * Absorb incoming requests and check to see if canceled.
                                 */
                                AbsorbFsyncRequests();
                                absorb_counter = FSYNCS_PER_ABSORB;             /* might as well... */

                                if (entry->canceled)
                                        break;
                        }                                       /* end retry loop */
                }

                /*
                 * If we get here, either we fsync'd successfully, or we don't have to
                 * because enableFsync is off, or the entry is (now) marked canceled.
                 * Okay to delete it.
                 */
                if (hash_search(pendingOpsTable, &entry->tag,
                                                HASH_REMOVE, NULL) == NULL)
                        elog(ERROR, "pendingOpsTable corrupted");
        }                                                       /* end loop over hashtable entries */

        /* Flag successful completion of mdsync */
        mdsync_in_progress = false;
}

/*
 * mdpreckpt() -- Do pre-checkpoint work
 *
 * To distinguish unlink requests that arrived before this checkpoint
 * started from those that arrived during the checkpoint, we use a cycle
 * counter similar to the one we use for fsync requests. That cycle
 * counter is incremented here.
 *
 * This must be called *before* the checkpoint REDO point is determined.
 * That ensures that we won't delete files too soon.
 *
 * Note that we can't do anything here that depends on the assumption
 * that the checkpoint will be completed.
 */
void
mdpreckpt(void)
{
        ListCell   *cell;

        /*
         * In case the prior checkpoint wasn't completed, stamp all entries in the
         * list with the current cycle counter.  Anything that's in the list at
         * the start of checkpoint can surely be deleted after the checkpoint is
         * finished, regardless of when the request was made.
         */
        foreach(cell, pendingUnlinks)
        {
                PendingUnlinkEntry *entry = (PendingUnlinkEntry *) lfirst(cell);

                entry->cycle_ctr = mdckpt_cycle_ctr;
        }

        /*
         * Any unlink requests arriving after this point will be assigned the next
         * cycle counter, and won't be unlinked until next checkpoint.
         */
        mdckpt_cycle_ctr++;
}

/*
 * mdpostckpt() -- Do post-checkpoint work
 *
 * Remove any lingering files that can now be safely removed.
 */
void
mdpostckpt(void)
{
        while (pendingUnlinks != NIL)
        {
                PendingUnlinkEntry *entry = (PendingUnlinkEntry *) linitial(pendingUnlinks);
                char       *path;

                /*
                 * New entries are appended to the end, so if the entry is new we've
                 * reached the end of old entries.
                 */
                if (entry->cycle_ctr == mdckpt_cycle_ctr)
                        break;

                /* Else assert we haven't missed it */
                Assert((CycleCtr) (entry->cycle_ctr + 1) == mdckpt_cycle_ctr);

                /* Unlink the file */
                path = relpath(entry->rnode, MAIN_FORKNUM);
                if (unlink(path) < 0)
                {
                        /*
                         * There's a race condition, when the database is dropped at the
                         * same time that we process the pending unlink requests. If the
                         * DROP DATABASE deletes the file before we do, we will get ENOENT
                         * here. rmtree() also has to ignore ENOENT errors, to deal with
                         * the possibility that we delete the file first.
                         */
                        if (errno != ENOENT)
                                ereport(WARNING,
                                                (errcode_for_file_access(),
                                                 errmsg("could not remove relation %s: %m", path)));
                }
                pfree(path);

                pendingUnlinks = list_delete_first(pendingUnlinks);
                pfree(entry);
        }
}

/*
 * 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
 * mdsync to process later.  Otherwise, try to pass off the fsync request
 * to the background writer process.  If that fails, just do the fsync
 * locally before returning (we expect this will not happen often enough
 * to be a performance problem).
 */
static void
register_dirty_segment(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
{
        if (pendingOpsTable)
        {
                /* push it into local pending-ops table */
                RememberFsyncRequest(reln->smgr_rnode, forknum, seg->mdfd_segno);
        }
        else
        {
                if (ForwardFsyncRequest(reln->smgr_rnode, forknum, seg->mdfd_segno))
                        return;                         /* passed it off successfully */

                if (FileSync(seg->mdfd_vfd) < 0)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not fsync segment %u of relation %s: %m",
                                                        seg->mdfd_segno,
                                                        relpath(reln->smgr_rnode, forknum))));
        }
}

/*
 * register_unlink() -- Schedule a file to be deleted after next checkpoint
 *
 * As with register_dirty_segment, this could involve either a local or
 * a remote pending-ops table.
 */
static void
register_unlink(RelFileNode rnode)
{
        if (pendingOpsTable)
        {
                /* push it into local pending-ops table */
                RememberFsyncRequest(rnode, MAIN_FORKNUM, UNLINK_RELATION_REQUEST);
        }
        else
        {
                /*
                 * Notify the bgwriter about it.  If we fail to queue the request
                 * message, we have to sleep and try again, because we can't simply
                 * delete the file now.  Ugly, but hopefully won't happen often.
                 *
                 * XXX should we just leave the file orphaned instead?
                 */
                Assert(IsUnderPostmaster);
                while (!ForwardFsyncRequest(rnode, MAIN_FORKNUM,
                                                                        UNLINK_RELATION_REQUEST))
                        pg_usleep(10000L);      /* 10 msec seems a good number */
        }
}

/*
 * RememberFsyncRequest() -- callback from bgwriter side of fsync request
 *
 * We stuff most fsync requests into the local hash table for execution
 * during the bgwriter's next checkpoint.  UNLINK requests go into a
 * separate linked list, however, because they get processed separately.
 *
 * The range of possible segment numbers is way less than the range of
 * BlockNumber, so we can reserve high values of segno for special purposes.
 * We define three:
 * - FORGET_RELATION_FSYNC means to cancel pending fsyncs for a relation
 * - FORGET_DATABASE_FSYNC means to cancel pending fsyncs for a whole database
 * - UNLINK_RELATION_REQUEST is a request to delete the file after the next
 *       checkpoint.
 *
 * (Handling the FORGET_* requests is a tad slow because the hash table has
 * to be searched linearly, but it doesn't seem worth rethinking the table
 * structure for them.)
 */
void
RememberFsyncRequest(RelFileNode rnode, ForkNumber forknum, BlockNumber segno)
{
        Assert(pendingOpsTable);

        if (segno == FORGET_RELATION_FSYNC)
        {
                /* Remove any pending requests for the entire relation */
                HASH_SEQ_STATUS hstat;
                PendingOperationEntry *entry;

                hash_seq_init(&hstat, pendingOpsTable);
                while ((entry = (PendingOperationEntry *) hash_seq_search(&hstat)) != NULL)
                {
                        if (RelFileNodeEquals(entry->tag.rnode, rnode) && 
                                entry->tag.forknum == forknum)
                        {
                                /* Okay, cancel this entry */
                                entry->canceled = true;
                        }
                }
        }
        else if (segno == FORGET_DATABASE_FSYNC)
        {
                /* Remove any pending requests for the entire database */
                HASH_SEQ_STATUS hstat;
                PendingOperationEntry *entry;
                ListCell   *cell, 
                                   *prev,
                                   *next;

                /* Remove fsync requests */
                hash_seq_init(&hstat, pendingOpsTable);
                while ((entry = (PendingOperationEntry *) hash_seq_search(&hstat)) != NULL)
                {
                        if (entry->tag.rnode.dbNode == rnode.dbNode)
                        {
                                /* Okay, cancel this entry */
                                entry->canceled = true;
                        }
                }
        
                /* Remove unlink requests */
                prev = NULL;
                for (cell = list_head(pendingUnlinks); cell; cell = next)
                {
                        PendingUnlinkEntry *entry = (PendingUnlinkEntry *) lfirst(cell);

                        next = lnext(cell);
                        if (entry->rnode.dbNode == rnode.dbNode) 
                        {
                                pendingUnlinks = list_delete_cell(pendingUnlinks, cell, prev);
                                pfree(entry);
                        }
                        else
                                prev = cell;
                }
        }
        else if (segno == UNLINK_RELATION_REQUEST)
        {
                /* Unlink request: put it in the linked list */
                MemoryContext oldcxt = MemoryContextSwitchTo(MdCxt);
                PendingUnlinkEntry *entry;

                entry = palloc(sizeof(PendingUnlinkEntry));
                entry->rnode = rnode;
                entry->cycle_ctr = mdckpt_cycle_ctr;

                pendingUnlinks = lappend(pendingUnlinks, entry);

                MemoryContextSwitchTo(oldcxt);
        }
        else
        {
                /* Normal case: enter a request to fsync this segment */
                PendingOperationTag key;
                PendingOperationEntry *entry;
                bool            found;

                /* ensure any pad bytes in the hash key are zeroed */
                MemSet(&key, 0, sizeof(key));
                key.rnode = rnode;
                key.forknum = forknum;
                key.segno = segno;

                entry = (PendingOperationEntry *) hash_search(pendingOpsTable,
                                                                                                          &key,
                                                                                                          HASH_ENTER,
                                                                                                          &found);
                /* if new or previously canceled entry, initialize it */
                if (!found || entry->canceled)
                {
                        entry->canceled = false;
                        entry->cycle_ctr = mdsync_cycle_ctr;
                }

                /*
                 * NB: it's intentional that we don't change cycle_ctr if the entry
                 * already exists.      The fsync request must be treated as old, even
                 * though the new request will be satisfied too by any subsequent
                 * fsync.
                 *
                 * However, if the entry is present but is marked canceled, we should
                 * act just as though it wasn't there.  The only case where this could
                 * happen would be if a file had been deleted, we received but did not
                 * yet act on the cancel request, and the same relfilenode was then
                 * assigned to a new file.      We mustn't lose the new request, but it
                 * should be considered new not old.
                 */
        }
}

/*
 * ForgetRelationFsyncRequests -- forget any fsyncs for a rel
 */
void
ForgetRelationFsyncRequests(RelFileNode rnode, ForkNumber forknum)
{
        if (pendingOpsTable)
        {
                /* standalone backend or startup process: fsync state is local */
                RememberFsyncRequest(rnode, forknum, FORGET_RELATION_FSYNC);
        }
        else if (IsUnderPostmaster)
        {
                /*
                 * Notify the bgwriter about it.  If we fail to queue the revoke
                 * message, we have to sleep and try again ... ugly, but hopefully
                 * won't happen often.
                 *
                 * XXX should we CHECK_FOR_INTERRUPTS in this loop?  Escaping with an
                 * error would leave the no-longer-used file still present on disk,
                 * which would be bad, so I'm inclined to assume that the bgwriter
                 * will always empty the queue soon.
                 */
                while (!ForwardFsyncRequest(rnode, forknum, FORGET_RELATION_FSYNC))
                        pg_usleep(10000L);      /* 10 msec seems a good number */

                /*
                 * Note we don't wait for the bgwriter to actually absorb the revoke
                 * message; see mdsync() for the implications.
                 */
        }
}

/*
 * ForgetDatabaseFsyncRequests -- forget any fsyncs and unlinks for a DB
 */
void
ForgetDatabaseFsyncRequests(Oid dbid)
{
        RelFileNode rnode;

        rnode.dbNode = dbid;
        rnode.spcNode = 0;
        rnode.relNode = 0;

        if (pendingOpsTable)
        {
                /* standalone backend or startup process: fsync state is local */
                RememberFsyncRequest(rnode, InvalidForkNumber, FORGET_DATABASE_FSYNC);
        }
        else if (IsUnderPostmaster)
        {
                /* see notes in ForgetRelationFsyncRequests */
                while (!ForwardFsyncRequest(rnode, InvalidForkNumber,
                                                                        FORGET_DATABASE_FSYNC))
                        pg_usleep(10000L);      /* 10 msec seems a good number */
        }
}


/*
 *      _fdvec_alloc() -- Make a MdfdVec object.
 */
static MdfdVec *
_fdvec_alloc(void)
{
        return (MdfdVec *) MemoryContextAlloc(MdCxt, sizeof(MdfdVec));
}

/*
 * 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       *path,
                           *fullpath;

        path = relpath(reln->smgr_rnode, forknum);

        if (segno > 0)
        {
                /* be sure we have enough space for the '.segno' */
                fullpath = (char *) palloc(strlen(path) + 12);
                sprintf(fullpath, "%s.%u", path, segno);
                pfree(path);
        }
        else
                fullpath = path;

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

        pfree(fullpath);

        if (fd < 0)
                return NULL;

        /* allocate an mdfdvec entry for it */
        v = _fdvec_alloc();

        /* fill the entry */
        v->mdfd_vfd = fd;
        v->mdfd_segno = segno;
        v->mdfd_chain = NULL;
        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: isTemp need only be correct
 * in the EXTENSION_CREATE case.
 */
static MdfdVec *
_mdfd_getseg(SMgrRelation reln, ForkNumber forknum, BlockNumber blkno,
                         bool isTemp, ExtensionBehavior behavior)
{
        MdfdVec    *v = mdopen(reln, forknum, behavior);
        BlockNumber targetseg;
        BlockNumber nextsegno;

        if (!v)
                return NULL;                    /* only possible if EXTENSION_RETURN_NULL */

        targetseg = blkno / ((BlockNumber) RELSEG_SIZE);
        for (nextsegno = 1; nextsegno <= targetseg; nextsegno++)
        {
                Assert(nextsegno == v->mdfd_segno + 1);

                if (v->mdfd_chain == NULL)
                {
                        /*
                         * 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.
                         *
                         * We have to maintain the invariant that segments before the last
                         * active segment are of size RELSEG_SIZE; therefore, pad them out
                         * with zeroes if needed.  (This only matters if caller is
                         * extending the relation discontiguously, but that can happen in
                         * hash indexes.)
                         */
                        if (behavior == EXTENSION_CREATE || InRecovery)
                        {
                                if (_mdnblocks(reln, forknum, v) < RELSEG_SIZE)
                                {
                                        char       *zerobuf = palloc0(BLCKSZ);

                                        mdextend(reln, forknum,
                                                         nextsegno * ((BlockNumber) RELSEG_SIZE) - 1,
                                                         zerobuf, isTemp);
                                        pfree(zerobuf);
                                }
                                v->mdfd_chain = _mdfd_openseg(reln, forknum, +nextsegno, O_CREAT);
                        }
                        else
                        {
                                /* We won't create segment if not existent */
                                v->mdfd_chain = _mdfd_openseg(reln, forknum, nextsegno, 0);
                        }
                        if (v->mdfd_chain == NULL)
                        {
                                if (behavior == EXTENSION_RETURN_NULL &&
                                        FILE_POSSIBLY_DELETED(errno))
                                        return NULL;
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not open segment %u of relation %s (target block %u): %m",
                                                                nextsegno,
                                                                relpath(reln->smgr_rnode, forknum),
                                                                blkno)));
                        }
                }
                v = v->mdfd_chain;
        }
        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 = FileSeek(seg->mdfd_vfd, 0L, SEEK_END);
        if (len < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not seek to end of segment %u of relation %s: %m",
                                                seg->mdfd_segno, relpath(reln->smgr_rnode, forknum))));
        /* note that this calculation will ignore any partial block at EOF */
        return (BlockNumber) (len / BLCKSZ);
}