md.c

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/*-------------------------------------------------------------------------
 *
 * 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) 1996-2017, 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 "miscadmin.h"
#include "access/xlog.h"
#include "catalog/catalog.h"
#include "pgstat.h"
#include "portability/instr_time.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"
#include "pg_trace.h"


/* intervals for calling AbsorbFsyncRequests in mdsync and mdpostckpt */
#define FSYNCS_PER_ABSORB       10
#define UNLINKS_PER_ABSORB      10

/*
 * Special values for the segno arg to RememberFsyncRequest.
 *
 * Note that CompactCheckpointerRequestQueue assumes that it's OK to remove an
 * fsync request from the queue if an identical, subsequent request is found.
 * See comments there before making changes here.
 */
#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 canceled ... 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 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 */


/*
 * In some contexts (currently, standalone backends and the checkpointer)
 * 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 merging 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.
 *
 * These mechanisms are only used for non-temp relations; we never fsync
 * temp rels, nor do we need to postpone their deletion (see comments in
 * mdunlink).
 *
 * (Regular backends do not track pending operations locally, but forward
 * them to the checkpointer.)
 */
typedef uint16 CycleCtr;        /* can be any convenient integer size */

typedef struct
{
    RelFileNode rnode;          /* hash table key (must be first!) */
    CycleCtr    cycle_ctr;      /* mdsync_cycle_ctr of oldest request */
    /* requests[f] has bit n set if we need to fsync segment n of fork f */
    Bitmapset  *requests[MAX_FORKNUM + 1];
    /* canceled[f] is true if we canceled fsyncs for fork "recently" */
    bool        canceled[MAX_FORKNUM + 1];
} 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 MemoryContext pendingOpsCxt; /* context for the above  */

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


/*** 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 is 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(RelFileNodeBackend rnode);
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);

    /*
     * 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 startup
     * or checkpointer auxiliary process.
     */
    if (!IsUnderPostmaster || AmStartupProcess() || AmCheckpointerProcess())
    {
        HASHCTL     hash_ctl;

        /*
         * XXX: The checkpointer needs to add entries to the pending ops table
         * when absorbing fsync requests.  That is done within a critical
         * section, which isn't usually allowed, but we make an exception. It
         * means that there's a theoretical possibility that you run out of
         * memory while absorbing fsync requests, which leads to a PANIC.
         * Fortunately the hash table is small so that's unlikely to happen in
         * practice.
         */
        pendingOpsCxt = AllocSetContextCreate(MdCxt,
                                              "Pending ops context",
                                              ALLOCSET_DEFAULT_SIZES);
        MemoryContextAllowInCriticalSection(pendingOpsCxt, true);

        MemSet(&hash_ctl, 0, sizeof(hash_ctl));
        hash_ctl.keysize = sizeof(RelFileNode);
        hash_ctl.entrysize = sizeof(PendingOperationEntry);
        hash_ctl.hcxt = pendingOpsCxt;
        pendingOpsTable = hash_create("Pending Ops Table",
                                      100L,
                                      &hash_ctl,
                                      HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
        pendingUnlinks = NIL;
    }
}

/*
 * In archive recovery, we rely on checkpointer to do fsyncs, but we will have
 * already created the pendingOpsTable during initialization of the startup
 * process.  Calling this function drops the local pendingOpsTable so that
 * subsequent requests will be forwarded to checkpointer.
 */
void
SetForwardFsyncRequests(void)
{
    /* Perform any pending fsyncs we may have queued up, then drop table */
    if (pendingOpsTable)
    {
        mdsync();
        hash_destroy(pendingOpsTable);
    }
    pendingOpsTable = NULL;

    /*
     * We should not have any pending unlink requests, since mdunlink doesn't
     * queue unlink requests when isRedo.
     */
    Assert(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)
{
    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, 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 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;
}

/*
 *  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)
{
    /*
     * We have to clean out any pending fsync requests for the doomed
     * relation, else the next mdsync() will fail.  There can't be any such
     * requests for a temp relation, though.  We can send just one request
     * even when deleting multiple forks, since the fsync queuing code accepts
     * the "InvalidForkNumber = all forks" convention.
     */
    if (!RelFileNodeBackendIsTemp(rnode))
        ForgetRelationFsyncRequests(rnode.node, forkNum);

    /* 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))
    {
        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, 0);
        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(rnode);
    }

    /*
     * 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++)
        {
            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);

    /*
     * 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 in file \"%s\": %m",
                        blocknum, FilePathName(v->mdfd_vfd))));

    if ((nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ, 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, 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 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);

    if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not seek to block %u in file \"%s\": %m",
                        blocknum, FilePathName(v->mdfd_vfd))));

    nbytes = FileRead(v->mdfd_vfd, buffer, BLCKSZ, 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);

    if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not seek to block %u in file \"%s\": %m",
                        blocknum, FilePathName(v->mdfd_vfd))));

    nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ, 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's 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)
        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(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not fsync file \"%s\": %m",
                            FilePathName(v->mdfd_vfd))));
        segno--;
    }
}

/*
 *  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;

    /* Statistics on sync times */
    int         processed = 0;
    instr_time  sync_start,
                sync_end,
                sync_diff;
    uint64      elapsed;
    uint64      longest = 0;
    uint64      total_elapsed = 0;

    /*
     * 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 checkpointer, 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)
    {
        ForkNumber  forknum;

        /*
         * If the entry is new then don't process it this time; it might
         * contain multiple fsync-request bits, but they are all new.  Note
         * "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);

        /*
         * Scan over the forks and segments represented by the entry.
         *
         * The bitmap manipulations are slightly tricky, because we can call
         * AbsorbFsyncRequests() inside the loop and that could result in
         * bms_add_member() modifying and even re-palloc'ing the bitmapsets.
         * This is okay because we unlink each bitmapset from the hashtable
         * entry before scanning it.  That means that any incoming fsync
         * requests will be processed now if they reach the table before we
         * begin to scan their fork.
         */
        for (forknum = 0; forknum <= MAX_FORKNUM; forknum++)
        {
            Bitmapset  *requests = entry->requests[forknum];
            int         segno;

            entry->requests[forknum] = NULL;
            entry->canceled[forknum] = false;

            while ((segno = bms_first_member(requests)) >= 0)
            {
                int         failures;

                /*
                 * 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.)
                 */
                if (!enableFsync)
                    continue;

                /*
                 * If in checkpointer, 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 "cancel" 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;
                    int         save_errno;

                    /*
                     * 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 checkpointer,
                     * 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-checkpointer
                     * cases we couldn't safely do that.)
                     */
                    reln = smgropen(entry->rnode, InvalidBackendId);

                    /* Attempt to open and fsync the target segment */
                    seg = _mdfd_getseg(reln, forknum,
                                       (BlockNumber) segno * (BlockNumber) RELSEG_SIZE,
                                       false,
                                       EXTENSION_RETURN_NULL
                                       | EXTENSION_DONT_CHECK_SIZE);

                    INSTR_TIME_SET_CURRENT(sync_start);

                    if (seg != NULL &&
                        FileSync(seg->mdfd_vfd, WAIT_EVENT_DATA_FILE_SYNC) >= 0)
                    {
                        /* Success; update statistics about sync timing */
                        INSTR_TIME_SET_CURRENT(sync_end);
                        sync_diff = sync_end;
                        INSTR_TIME_SUBTRACT(sync_diff, sync_start);
                        elapsed = INSTR_TIME_GET_MICROSEC(sync_diff);
                        if (elapsed > longest)
                            longest = elapsed;
                        total_elapsed += elapsed;
                        processed++;
                        if (log_checkpoints)
                            elog(DEBUG1, "checkpoint sync: number=%d file=%s time=%.3f msec",
                                 processed,
                                 FilePathName(seg->mdfd_vfd),
                                 (double) elapsed / 1000);

                        break;  /* out of retry loop */
                    }

                    /* Compute file name for use in message */
                    save_errno = errno;
                    path = _mdfd_segpath(reln, forknum, (BlockNumber) segno);
                    errno = save_errno;

                    /*
                     * 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 for
                     * _mdfd_getseg() and for FileSync, since fd.c might have
                     * closed the file behind our back.
                     *
                     * 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.
                     */
                    if (!FILE_POSSIBLY_DELETED(errno) ||
                        failures > 0)
                        ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\": %m",
                                        path)));
                    else
                        ereport(DEBUG1,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\" but retrying: %m",
                                        path)));
                    pfree(path);

                    /*
                     * Absorb incoming requests and check to see if a cancel
                     * arrived for this relation fork.
                     */
                    AbsorbFsyncRequests();
                    absorb_counter = FSYNCS_PER_ABSORB; /* might as well... */

                    if (entry->canceled[forknum])
                        break;
                }               /* end retry loop */
            }
            bms_free(requests);
        }

        /*
         * We've finished everything that was requested before we started to
         * scan the entry.  If no new requests have been inserted meanwhile,
         * remove the entry.  Otherwise, update its cycle counter, as all the
         * requests now in it must have arrived during this cycle.
         */
        for (forknum = 0; forknum <= MAX_FORKNUM; forknum++)
        {
            if (entry->requests[forknum] != NULL)
                break;
        }
        if (forknum <= MAX_FORKNUM)
            entry->cycle_ctr = mdsync_cycle_ctr;
        else
        {
            /* Okay to remove it */
            if (hash_search(pendingOpsTable, &entry->rnode,
                            HASH_REMOVE, NULL) == NULL)
                elog(ERROR, "pendingOpsTable corrupted");
        }
    }                           /* end loop over hashtable entries */

    /* Return sync performance metrics for report at checkpoint end */
    CheckpointStats.ckpt_sync_rels = processed;
    CheckpointStats.ckpt_longest_sync = longest;
    CheckpointStats.ckpt_agg_sync_time = total_elapsed;

    /* 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)
{
    /*
     * 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)
{
    int         absorb_counter;

    absorb_counter = UNLINKS_PER_ABSORB;
    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.
         *
         * Note: if just the right number of consecutive checkpoints fail, we
         * could be fooled here by cycle_ctr wraparound.  However, the only
         * consequence is that we'd delay unlinking for one more checkpoint,
         * which is perfectly tolerable.
         */
        if (entry->cycle_ctr == mdckpt_cycle_ctr)
            break;

        /* Unlink the file */
        path = relpathperm(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 file \"%s\": %m", path)));
        }
        pfree(path);

        /* And remove the list entry */
        pendingUnlinks = list_delete_first(pendingUnlinks);
        pfree(entry);

        /*
         * As in mdsync, we don't want to stop absorbing fsync requests for a
         * long time when there are many deletions to be done.  We can safely
         * call AbsorbFsyncRequests() at this point in the loop (note it might
         * try to delete list entries).
         */
        if (--absorb_counter <= 0)
        {
            AbsorbFsyncRequests();
            absorb_counter = UNLINKS_PER_ABSORB;
        }
    }
}

/*
 * 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 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)
{
    /* Temp relations should never be fsync'd */
    Assert(!SmgrIsTemp(reln));

    if (pendingOpsTable)
    {
        /* push it into local pending-ops table */
        RememberFsyncRequest(reln->smgr_rnode.node, forknum, seg->mdfd_segno);
    }
    else
    {
        if (ForwardFsyncRequest(reln->smgr_rnode.node, forknum, seg->mdfd_segno))
            return;             /* passed it off successfully */

        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(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not fsync file \"%s\": %m",
                            FilePathName(seg->mdfd_vfd))));
    }
}

/*
 * register_unlink() -- Schedule a file to be deleted after next checkpoint
 *
 * We don't bother passing in the fork number, because this is only used
 * with main forks.
 *
 * As with register_dirty_segment, this could involve either a local or
 * a remote pending-ops table.
 */
static void
register_unlink(RelFileNodeBackend rnode)
{
    /* Should never be used with temp relations */
    Assert(!RelFileNodeBackendIsTemp(rnode));

    if (pendingOpsTable)
    {
        /* push it into local pending-ops table */
        RememberFsyncRequest(rnode.node, MAIN_FORKNUM,
                             UNLINK_RELATION_REQUEST);
    }
    else
    {
        /*
         * Notify the checkpointer 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.node, MAIN_FORKNUM,
                                    UNLINK_RELATION_REQUEST))
            pg_usleep(10000L);  /* 10 msec seems a good number */
    }
}

/*
 * RememberFsyncRequest() -- callback from checkpointer side of fsync request
 *
 * We stuff fsync requests into the local hash table for execution
 * during the checkpointer'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,
 *   either for one fork, or all forks if forknum is InvalidForkNumber
 * - 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.
 * Note also that we're assuming real segment numbers don't exceed INT_MAX.
 *
 * (Handling FORGET_DATABASE_FSYNC requests is a tad slow because the hash
 * table has to be searched linearly, but dropping a database is a pretty
 * heavyweight operation anyhow, so we'll live with it.)
 */
void
RememberFsyncRequest(RelFileNode rnode, ForkNumber forknum, BlockNumber segno)
{
    Assert(pendingOpsTable);

    if (segno == FORGET_RELATION_FSYNC)
    {
        /* Remove any pending requests for the relation (one or all forks) */
        PendingOperationEntry *entry;

        entry = (PendingOperationEntry *) hash_search(pendingOpsTable,
                                                      &rnode,
                                                      HASH_FIND,
                                                      NULL);
        if (entry)
        {
            /*
             * We can't just delete the entry since mdsync could have an
             * active hashtable scan.  Instead we delete the bitmapsets; this
             * is safe because of the way mdsync is coded.  We also set the
             * "canceled" flags so that mdsync can tell that a cancel arrived
             * for the fork(s).
             */
            if (forknum == InvalidForkNumber)
            {
                /* remove requests for all forks */
                for (forknum = 0; forknum <= MAX_FORKNUM; forknum++)
                {
                    bms_free(entry->requests[forknum]);
                    entry->requests[forknum] = NULL;
                    entry->canceled[forknum] = true;
                }
            }
            else
            {
                /* remove requests for single fork */
                bms_free(entry->requests[forknum]);
                entry->requests[forknum] = NULL;
                entry->canceled[forknum] = 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->rnode.dbNode == rnode.dbNode)
            {
                /* remove requests for all forks */
                for (forknum = 0; forknum <= MAX_FORKNUM; forknum++)
                {
                    bms_free(entry->requests[forknum]);
                    entry->requests[forknum] = NULL;
                    entry->canceled[forknum] = 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(pendingOpsCxt);
        PendingUnlinkEntry *entry;

        /* PendingUnlinkEntry doesn't store forknum, since it's always MAIN */
        Assert(forknum == MAIN_FORKNUM);

        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 */
        MemoryContext oldcxt = MemoryContextSwitchTo(pendingOpsCxt);
        PendingOperationEntry *entry;
        bool        found;

        entry = (PendingOperationEntry *) hash_search(pendingOpsTable,
                                                      &rnode,
                                                      HASH_ENTER,
                                                      &found);
        /* if new entry, initialize it */
        if (!found)
        {
            entry->cycle_ctr = mdsync_cycle_ctr;
            MemSet(entry->requests, 0, sizeof(entry->requests));
            MemSet(entry->canceled, 0, sizeof(entry->canceled));
        }

        /*
         * NB: it's intentional that we don't change cycle_ctr if the entry
         * already exists.  The cycle_ctr must represent the oldest fsync
         * request that could be in the entry.
         */

        entry->requests[forknum] = bms_add_member(entry->requests[forknum],
                                                  (int) segno);

        MemoryContextSwitchTo(oldcxt);
    }
}

/*
 * ForgetRelationFsyncRequests -- forget any fsyncs for a relation fork
 *
 * forknum == InvalidForkNumber means all forks, although this code doesn't
 * actually know that, since it's just forwarding the request elsewhere.
 */
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 checkpointer about it.  If we fail to queue the cancel
         * 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 checkpointer
         * 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 checkpointer to actually absorb the
         * cancel 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_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, 0600);

    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 = FileSeek(seg->mdfd_vfd, 0L, SEEK_END);
    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);
}