LCOV - code coverage report
Current view: top level - src/backend/storage/sync - sync.c (source / functions) Hit Total Coverage
Test: PostgreSQL 17devel Lines: 95 124 76.6 %
Date: 2024-04-13 09:11:47 Functions: 6 6 100.0 %
Legend: Lines: hit not hit

          Line data    Source code
       1             : /*-------------------------------------------------------------------------
       2             :  *
       3             :  * sync.c
       4             :  *    File synchronization management code.
       5             :  *
       6             :  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
       7             :  * Portions Copyright (c) 1994, Regents of the University of California
       8             :  *
       9             :  *
      10             :  * IDENTIFICATION
      11             :  *    src/backend/storage/sync/sync.c
      12             :  *
      13             :  *-------------------------------------------------------------------------
      14             :  */
      15             : #include "postgres.h"
      16             : 
      17             : #include <unistd.h>
      18             : #include <fcntl.h>
      19             : #include <sys/file.h>
      20             : 
      21             : #include "access/clog.h"
      22             : #include "access/commit_ts.h"
      23             : #include "access/multixact.h"
      24             : #include "access/xlog.h"
      25             : #include "miscadmin.h"
      26             : #include "pgstat.h"
      27             : #include "portability/instr_time.h"
      28             : #include "postmaster/bgwriter.h"
      29             : #include "storage/fd.h"
      30             : #include "storage/latch.h"
      31             : #include "storage/md.h"
      32             : #include "utils/hsearch.h"
      33             : #include "utils/memutils.h"
      34             : 
      35             : /*
      36             :  * In some contexts (currently, standalone backends and the checkpointer)
      37             :  * we keep track of pending fsync operations: we need to remember all relation
      38             :  * segments that have been written since the last checkpoint, so that we can
      39             :  * fsync them down to disk before completing the next checkpoint.  This hash
      40             :  * table remembers the pending operations.  We use a hash table mostly as
      41             :  * a convenient way of merging duplicate requests.
      42             :  *
      43             :  * We use a similar mechanism to remember no-longer-needed files that can
      44             :  * be deleted after the next checkpoint, but we use a linked list instead of
      45             :  * a hash table, because we don't expect there to be any duplicate requests.
      46             :  *
      47             :  * These mechanisms are only used for non-temp relations; we never fsync
      48             :  * temp rels, nor do we need to postpone their deletion (see comments in
      49             :  * mdunlink).
      50             :  *
      51             :  * (Regular backends do not track pending operations locally, but forward
      52             :  * them to the checkpointer.)
      53             :  */
      54             : typedef uint16 CycleCtr;        /* can be any convenient integer size */
      55             : 
      56             : typedef struct
      57             : {
      58             :     FileTag     tag;            /* identifies handler and file */
      59             :     CycleCtr    cycle_ctr;      /* sync_cycle_ctr of oldest request */
      60             :     bool        canceled;       /* canceled is true if we canceled "recently" */
      61             : } PendingFsyncEntry;
      62             : 
      63             : typedef struct
      64             : {
      65             :     FileTag     tag;            /* identifies handler and file */
      66             :     CycleCtr    cycle_ctr;      /* checkpoint_cycle_ctr when request was made */
      67             :     bool        canceled;       /* true if request has been canceled */
      68             : } PendingUnlinkEntry;
      69             : 
      70             : static HTAB *pendingOps = NULL;
      71             : static List *pendingUnlinks = NIL;
      72             : static MemoryContext pendingOpsCxt; /* context for the above  */
      73             : 
      74             : static CycleCtr sync_cycle_ctr = 0;
      75             : static CycleCtr checkpoint_cycle_ctr = 0;
      76             : 
      77             : /* Intervals for calling AbsorbSyncRequests */
      78             : #define FSYNCS_PER_ABSORB       10
      79             : #define UNLINKS_PER_ABSORB      10
      80             : 
      81             : /*
      82             :  * Function pointers for handling sync and unlink requests.
      83             :  */
      84             : typedef struct SyncOps
      85             : {
      86             :     int         (*sync_syncfiletag) (const FileTag *ftag, char *path);
      87             :     int         (*sync_unlinkfiletag) (const FileTag *ftag, char *path);
      88             :     bool        (*sync_filetagmatches) (const FileTag *ftag,
      89             :                                         const FileTag *candidate);
      90             : } SyncOps;
      91             : 
      92             : /*
      93             :  * These indexes must correspond to the values of the SyncRequestHandler enum.
      94             :  */
      95             : static const SyncOps syncsw[] = {
      96             :     /* magnetic disk */
      97             :     [SYNC_HANDLER_MD] = {
      98             :         .sync_syncfiletag = mdsyncfiletag,
      99             :         .sync_unlinkfiletag = mdunlinkfiletag,
     100             :         .sync_filetagmatches = mdfiletagmatches
     101             :     },
     102             :     /* pg_xact */
     103             :     [SYNC_HANDLER_CLOG] = {
     104             :         .sync_syncfiletag = clogsyncfiletag
     105             :     },
     106             :     /* pg_commit_ts */
     107             :     [SYNC_HANDLER_COMMIT_TS] = {
     108             :         .sync_syncfiletag = committssyncfiletag
     109             :     },
     110             :     /* pg_multixact/offsets */
     111             :     [SYNC_HANDLER_MULTIXACT_OFFSET] = {
     112             :         .sync_syncfiletag = multixactoffsetssyncfiletag
     113             :     },
     114             :     /* pg_multixact/members */
     115             :     [SYNC_HANDLER_MULTIXACT_MEMBER] = {
     116             :         .sync_syncfiletag = multixactmemberssyncfiletag
     117             :     }
     118             : };
     119             : 
     120             : /*
     121             :  * Initialize data structures for the file sync tracking.
     122             :  */
     123             : void
     124       29856 : InitSync(void)
     125             : {
     126             :     /*
     127             :      * Create pending-operations hashtable if we need it.  Currently, we need
     128             :      * it if we are standalone (not under a postmaster) or if we are a
     129             :      * checkpointer auxiliary process.
     130             :      */
     131       29856 :     if (!IsUnderPostmaster || AmCheckpointerProcess())
     132             :     {
     133             :         HASHCTL     hash_ctl;
     134             : 
     135             :         /*
     136             :          * XXX: The checkpointer needs to add entries to the pending ops table
     137             :          * when absorbing fsync requests.  That is done within a critical
     138             :          * section, which isn't usually allowed, but we make an exception. It
     139             :          * means that there's a theoretical possibility that you run out of
     140             :          * memory while absorbing fsync requests, which leads to a PANIC.
     141             :          * Fortunately the hash table is small so that's unlikely to happen in
     142             :          * practice.
     143             :          */
     144        1006 :         pendingOpsCxt = AllocSetContextCreate(TopMemoryContext,
     145             :                                               "Pending ops context",
     146             :                                               ALLOCSET_DEFAULT_SIZES);
     147        1006 :         MemoryContextAllowInCriticalSection(pendingOpsCxt, true);
     148             : 
     149        1006 :         hash_ctl.keysize = sizeof(FileTag);
     150        1006 :         hash_ctl.entrysize = sizeof(PendingFsyncEntry);
     151        1006 :         hash_ctl.hcxt = pendingOpsCxt;
     152        1006 :         pendingOps = hash_create("Pending Ops Table",
     153             :                                  100L,
     154             :                                  &hash_ctl,
     155             :                                  HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
     156        1006 :         pendingUnlinks = NIL;
     157             :     }
     158       29856 : }
     159             : 
     160             : /*
     161             :  * SyncPreCheckpoint() -- Do pre-checkpoint work
     162             :  *
     163             :  * To distinguish unlink requests that arrived before this checkpoint
     164             :  * started from those that arrived during the checkpoint, we use a cycle
     165             :  * counter similar to the one we use for fsync requests. That cycle
     166             :  * counter is incremented here.
     167             :  *
     168             :  * This must be called *before* the checkpoint REDO point is determined.
     169             :  * That ensures that we won't delete files too soon.  Since this calls
     170             :  * AbsorbSyncRequests(), which performs memory allocations, it cannot be
     171             :  * called within a critical section.
     172             :  *
     173             :  * Note that we can't do anything here that depends on the assumption
     174             :  * that the checkpoint will be completed.
     175             :  */
     176             : void
     177        1628 : SyncPreCheckpoint(void)
     178             : {
     179             :     /*
     180             :      * Operations such as DROP TABLESPACE assume that the next checkpoint will
     181             :      * process all recently forwarded unlink requests, but if they aren't
     182             :      * absorbed prior to advancing the cycle counter, they won't be processed
     183             :      * until a future checkpoint.  The following absorb ensures that any
     184             :      * unlink requests forwarded before the checkpoint began will be processed
     185             :      * in the current checkpoint.
     186             :      */
     187        1628 :     AbsorbSyncRequests();
     188             : 
     189             :     /*
     190             :      * Any unlink requests arriving after this point will be assigned the next
     191             :      * cycle counter, and won't be unlinked until next checkpoint.
     192             :      */
     193        1628 :     checkpoint_cycle_ctr++;
     194        1628 : }
     195             : 
     196             : /*
     197             :  * SyncPostCheckpoint() -- Do post-checkpoint work
     198             :  *
     199             :  * Remove any lingering files that can now be safely removed.
     200             :  */
     201             : void
     202        1628 : SyncPostCheckpoint(void)
     203             : {
     204             :     int         absorb_counter;
     205             :     ListCell   *lc;
     206             : 
     207        1628 :     absorb_counter = UNLINKS_PER_ABSORB;
     208       63168 :     foreach(lc, pendingUnlinks)
     209             :     {
     210       61568 :         PendingUnlinkEntry *entry = (PendingUnlinkEntry *) lfirst(lc);
     211             :         char        path[MAXPGPATH];
     212             : 
     213             :         /* Skip over any canceled entries */
     214       61568 :         if (entry->canceled)
     215           2 :             continue;
     216             : 
     217             :         /*
     218             :          * New entries are appended to the end, so if the entry is new we've
     219             :          * reached the end of old entries.
     220             :          *
     221             :          * Note: if just the right number of consecutive checkpoints fail, we
     222             :          * could be fooled here by cycle_ctr wraparound.  However, the only
     223             :          * consequence is that we'd delay unlinking for one more checkpoint,
     224             :          * which is perfectly tolerable.
     225             :          */
     226       61566 :         if (entry->cycle_ctr == checkpoint_cycle_ctr)
     227          28 :             break;
     228             : 
     229             :         /* Unlink the file */
     230       61538 :         if (syncsw[entry->tag.handler].sync_unlinkfiletag(&entry->tag,
     231             :                                                           path) < 0)
     232             :         {
     233             :             /*
     234             :              * There's a race condition, when the database is dropped at the
     235             :              * same time that we process the pending unlink requests. If the
     236             :              * DROP DATABASE deletes the file before we do, we will get ENOENT
     237             :              * here. rmtree() also has to ignore ENOENT errors, to deal with
     238             :              * the possibility that we delete the file first.
     239             :              */
     240           6 :             if (errno != ENOENT)
     241           0 :                 ereport(WARNING,
     242             :                         (errcode_for_file_access(),
     243             :                          errmsg("could not remove file \"%s\": %m", path)));
     244             :         }
     245             : 
     246             :         /* Mark the list entry as canceled, just in case */
     247       61538 :         entry->canceled = true;
     248             : 
     249             :         /*
     250             :          * As in ProcessSyncRequests, we don't want to stop absorbing fsync
     251             :          * requests for a long time when there are many deletions to be done.
     252             :          * We can safely call AbsorbSyncRequests() at this point in the loop.
     253             :          */
     254       61538 :         if (--absorb_counter <= 0)
     255             :         {
     256        6048 :             AbsorbSyncRequests();
     257        6048 :             absorb_counter = UNLINKS_PER_ABSORB;
     258             :         }
     259             :     }
     260             : 
     261             :     /*
     262             :      * If we reached the end of the list, we can just remove the whole list
     263             :      * (remembering to pfree all the PendingUnlinkEntry objects).  Otherwise,
     264             :      * we must keep the entries at or after "lc".
     265             :      */
     266        1628 :     if (lc == NULL)
     267             :     {
     268        1600 :         list_free_deep(pendingUnlinks);
     269        1600 :         pendingUnlinks = NIL;
     270             :     }
     271             :     else
     272             :     {
     273          28 :         int         ntodelete = list_cell_number(pendingUnlinks, lc);
     274             : 
     275       28542 :         for (int i = 0; i < ntodelete; i++)
     276       28514 :             pfree(list_nth(pendingUnlinks, i));
     277             : 
     278          28 :         pendingUnlinks = list_delete_first_n(pendingUnlinks, ntodelete);
     279             :     }
     280        1628 : }
     281             : 
     282             : /*
     283             :  *  ProcessSyncRequests() -- Process queued fsync requests.
     284             :  */
     285             : void
     286        1704 : ProcessSyncRequests(void)
     287             : {
     288             :     static bool sync_in_progress = false;
     289             : 
     290             :     HASH_SEQ_STATUS hstat;
     291             :     PendingFsyncEntry *entry;
     292             :     int         absorb_counter;
     293             : 
     294             :     /* Statistics on sync times */
     295        1704 :     int         processed = 0;
     296             :     instr_time  sync_start,
     297             :                 sync_end,
     298             :                 sync_diff;
     299             :     uint64      elapsed;
     300        1704 :     uint64      longest = 0;
     301        1704 :     uint64      total_elapsed = 0;
     302             : 
     303             :     /*
     304             :      * This is only called during checkpoints, and checkpoints should only
     305             :      * occur in processes that have created a pendingOps.
     306             :      */
     307        1704 :     if (!pendingOps)
     308           0 :         elog(ERROR, "cannot sync without a pendingOps table");
     309             : 
     310             :     /*
     311             :      * If we are in the checkpointer, the sync had better include all fsync
     312             :      * requests that were queued by backends up to this point.  The tightest
     313             :      * race condition that could occur is that a buffer that must be written
     314             :      * and fsync'd for the checkpoint could have been dumped by a backend just
     315             :      * before it was visited by BufferSync().  We know the backend will have
     316             :      * queued an fsync request before clearing the buffer's dirtybit, so we
     317             :      * are safe as long as we do an Absorb after completing BufferSync().
     318             :      */
     319        1704 :     AbsorbSyncRequests();
     320             : 
     321             :     /*
     322             :      * To avoid excess fsync'ing (in the worst case, maybe a never-terminating
     323             :      * checkpoint), we want to ignore fsync requests that are entered into the
     324             :      * hashtable after this point --- they should be processed next time,
     325             :      * instead.  We use sync_cycle_ctr to tell old entries apart from new
     326             :      * ones: new ones will have cycle_ctr equal to the incremented value of
     327             :      * sync_cycle_ctr.
     328             :      *
     329             :      * In normal circumstances, all entries present in the table at this point
     330             :      * will have cycle_ctr exactly equal to the current (about to be old)
     331             :      * value of sync_cycle_ctr.  However, if we fail partway through the
     332             :      * fsync'ing loop, then older values of cycle_ctr might remain when we
     333             :      * come back here to try again.  Repeated checkpoint failures would
     334             :      * eventually wrap the counter around to the point where an old entry
     335             :      * might appear new, causing us to skip it, possibly allowing a checkpoint
     336             :      * to succeed that should not have.  To forestall wraparound, any time the
     337             :      * previous ProcessSyncRequests() failed to complete, run through the
     338             :      * table and forcibly set cycle_ctr = sync_cycle_ctr.
     339             :      *
     340             :      * Think not to merge this loop with the main loop, as the problem is
     341             :      * exactly that that loop may fail before having visited all the entries.
     342             :      * From a performance point of view it doesn't matter anyway, as this path
     343             :      * will never be taken in a system that's functioning normally.
     344             :      */
     345        1704 :     if (sync_in_progress)
     346             :     {
     347             :         /* prior try failed, so update any stale cycle_ctr values */
     348           0 :         hash_seq_init(&hstat, pendingOps);
     349           0 :         while ((entry = (PendingFsyncEntry *) hash_seq_search(&hstat)) != NULL)
     350             :         {
     351           0 :             entry->cycle_ctr = sync_cycle_ctr;
     352             :         }
     353             :     }
     354             : 
     355             :     /* Advance counter so that new hashtable entries are distinguishable */
     356        1704 :     sync_cycle_ctr++;
     357             : 
     358             :     /* Set flag to detect failure if we don't reach the end of the loop */
     359        1704 :     sync_in_progress = true;
     360             : 
     361             :     /* Now scan the hashtable for fsync requests to process */
     362        1704 :     absorb_counter = FSYNCS_PER_ABSORB;
     363        1704 :     hash_seq_init(&hstat, pendingOps);
     364      257152 :     while ((entry = (PendingFsyncEntry *) hash_seq_search(&hstat)) != NULL)
     365             :     {
     366             :         int         failures;
     367             : 
     368             :         /*
     369             :          * If the entry is new then don't process it this time; it is new.
     370             :          * Note "continue" bypasses the hash-remove call at the bottom of the
     371             :          * loop.
     372             :          */
     373      255448 :         if (entry->cycle_ctr == sync_cycle_ctr)
     374           0 :             continue;
     375             : 
     376             :         /* Else assert we haven't missed it */
     377             :         Assert((CycleCtr) (entry->cycle_ctr + 1) == sync_cycle_ctr);
     378             : 
     379             :         /*
     380             :          * If fsync is off then we don't have to bother opening the file at
     381             :          * all.  (We delay checking until this point so that changing fsync on
     382             :          * the fly behaves sensibly.)
     383             :          */
     384      255448 :         if (enableFsync)
     385             :         {
     386             :             /*
     387             :              * If in checkpointer, we want to absorb pending requests every so
     388             :              * often to prevent overflow of the fsync request queue.  It is
     389             :              * unspecified whether newly-added entries will be visited by
     390             :              * hash_seq_search, but we don't care since we don't need to
     391             :              * process them anyway.
     392             :              */
     393           0 :             if (--absorb_counter <= 0)
     394             :             {
     395           0 :                 AbsorbSyncRequests();
     396           0 :                 absorb_counter = FSYNCS_PER_ABSORB;
     397             :             }
     398             : 
     399             :             /*
     400             :              * The fsync table could contain requests to fsync segments that
     401             :              * have been deleted (unlinked) by the time we get to them. Rather
     402             :              * than just hoping an ENOENT (or EACCES on Windows) error can be
     403             :              * ignored, what we do on error is absorb pending requests and
     404             :              * then retry. Since mdunlink() queues a "cancel" message before
     405             :              * actually unlinking, the fsync request is guaranteed to be
     406             :              * marked canceled after the absorb if it really was this case.
     407             :              * DROP DATABASE likewise has to tell us to forget fsync requests
     408             :              * before it starts deletions.
     409             :              */
     410           0 :             for (failures = 0; !entry->canceled; failures++)
     411             :             {
     412             :                 char        path[MAXPGPATH];
     413             : 
     414           0 :                 INSTR_TIME_SET_CURRENT(sync_start);
     415           0 :                 if (syncsw[entry->tag.handler].sync_syncfiletag(&entry->tag,
     416             :                                                                 path) == 0)
     417             :                 {
     418             :                     /* Success; update statistics about sync timing */
     419           0 :                     INSTR_TIME_SET_CURRENT(sync_end);
     420           0 :                     sync_diff = sync_end;
     421           0 :                     INSTR_TIME_SUBTRACT(sync_diff, sync_start);
     422           0 :                     elapsed = INSTR_TIME_GET_MICROSEC(sync_diff);
     423           0 :                     if (elapsed > longest)
     424           0 :                         longest = elapsed;
     425           0 :                     total_elapsed += elapsed;
     426           0 :                     processed++;
     427             : 
     428           0 :                     if (log_checkpoints)
     429           0 :                         elog(DEBUG1, "checkpoint sync: number=%d file=%s time=%.3f ms",
     430             :                              processed,
     431             :                              path,
     432             :                              (double) elapsed / 1000);
     433             : 
     434           0 :                     break;      /* out of retry loop */
     435             :                 }
     436             : 
     437             :                 /*
     438             :                  * It is possible that the relation has been dropped or
     439             :                  * truncated since the fsync request was entered. Therefore,
     440             :                  * allow ENOENT, but only if we didn't fail already on this
     441             :                  * file.
     442             :                  */
     443           0 :                 if (!FILE_POSSIBLY_DELETED(errno) || failures > 0)
     444           0 :                     ereport(data_sync_elevel(ERROR),
     445             :                             (errcode_for_file_access(),
     446             :                              errmsg("could not fsync file \"%s\": %m",
     447             :                                     path)));
     448             :                 else
     449           0 :                     ereport(DEBUG1,
     450             :                             (errcode_for_file_access(),
     451             :                              errmsg_internal("could not fsync file \"%s\" but retrying: %m",
     452             :                                              path)));
     453             : 
     454             :                 /*
     455             :                  * Absorb incoming requests and check to see if a cancel
     456             :                  * arrived for this relation fork.
     457             :                  */
     458           0 :                 AbsorbSyncRequests();
     459           0 :                 absorb_counter = FSYNCS_PER_ABSORB; /* might as well... */
     460             :             }                   /* end retry loop */
     461             :         }
     462             : 
     463             :         /* We are done with this entry, remove it */
     464      255448 :         if (hash_search(pendingOps, &entry->tag, HASH_REMOVE, NULL) == NULL)
     465           0 :             elog(ERROR, "pendingOps corrupted");
     466             :     }                           /* end loop over hashtable entries */
     467             : 
     468             :     /* Return sync performance metrics for report at checkpoint end */
     469        1704 :     CheckpointStats.ckpt_sync_rels = processed;
     470        1704 :     CheckpointStats.ckpt_longest_sync = longest;
     471        1704 :     CheckpointStats.ckpt_agg_sync_time = total_elapsed;
     472             : 
     473             :     /* Flag successful completion of ProcessSyncRequests */
     474        1704 :     sync_in_progress = false;
     475        1704 : }
     476             : 
     477             : /*
     478             :  * RememberSyncRequest() -- callback from checkpointer side of sync request
     479             :  *
     480             :  * We stuff fsync requests into the local hash table for execution
     481             :  * during the checkpointer's next checkpoint.  UNLINK requests go into a
     482             :  * separate linked list, however, because they get processed separately.
     483             :  *
     484             :  * See sync.h for more information on the types of sync requests supported.
     485             :  */
     486             : void
     487     1718372 : RememberSyncRequest(const FileTag *ftag, SyncRequestType type)
     488             : {
     489             :     Assert(pendingOps);
     490             : 
     491     1718372 :     if (type == SYNC_FORGET_REQUEST)
     492             :     {
     493             :         PendingFsyncEntry *entry;
     494             : 
     495             :         /* Cancel previously entered request */
     496      240442 :         entry = (PendingFsyncEntry *) hash_search(pendingOps,
     497             :                                                   ftag,
     498             :                                                   HASH_FIND,
     499             :                                                   NULL);
     500      240442 :         if (entry != NULL)
     501       16520 :             entry->canceled = true;
     502             :     }
     503     1477930 :     else if (type == SYNC_FILTER_REQUEST)
     504             :     {
     505             :         HASH_SEQ_STATUS hstat;
     506             :         PendingFsyncEntry *pfe;
     507             :         ListCell   *cell;
     508             : 
     509             :         /* Cancel matching fsync requests */
     510          40 :         hash_seq_init(&hstat, pendingOps);
     511       13550 :         while ((pfe = (PendingFsyncEntry *) hash_seq_search(&hstat)) != NULL)
     512             :         {
     513       27012 :             if (pfe->tag.handler == ftag->handler &&
     514       13502 :                 syncsw[ftag->handler].sync_filetagmatches(ftag, &pfe->tag))
     515        7270 :                 pfe->canceled = true;
     516             :         }
     517             : 
     518             :         /* Cancel matching unlink requests */
     519          62 :         foreach(cell, pendingUnlinks)
     520             :         {
     521          22 :             PendingUnlinkEntry *pue = (PendingUnlinkEntry *) lfirst(cell);
     522             : 
     523          44 :             if (pue->tag.handler == ftag->handler &&
     524          22 :                 syncsw[ftag->handler].sync_filetagmatches(ftag, &pue->tag))
     525           2 :                 pue->canceled = true;
     526             :         }
     527             :     }
     528     1477890 :     else if (type == SYNC_UNLINK_REQUEST)
     529             :     {
     530             :         /* Unlink request: put it in the linked list */
     531       61540 :         MemoryContext oldcxt = MemoryContextSwitchTo(pendingOpsCxt);
     532             :         PendingUnlinkEntry *entry;
     533             : 
     534       61540 :         entry = palloc(sizeof(PendingUnlinkEntry));
     535       61540 :         entry->tag = *ftag;
     536       61540 :         entry->cycle_ctr = checkpoint_cycle_ctr;
     537       61540 :         entry->canceled = false;
     538             : 
     539       61540 :         pendingUnlinks = lappend(pendingUnlinks, entry);
     540             : 
     541       61540 :         MemoryContextSwitchTo(oldcxt);
     542             :     }
     543             :     else
     544             :     {
     545             :         /* Normal case: enter a request to fsync this segment */
     546     1416350 :         MemoryContext oldcxt = MemoryContextSwitchTo(pendingOpsCxt);
     547             :         PendingFsyncEntry *entry;
     548             :         bool        found;
     549             : 
     550             :         Assert(type == SYNC_REQUEST);
     551             : 
     552     1416350 :         entry = (PendingFsyncEntry *) hash_search(pendingOps,
     553             :                                                   ftag,
     554             :                                                   HASH_ENTER,
     555             :                                                   &found);
     556             :         /* if new entry, or was previously canceled, initialize it */
     557     1416350 :         if (!found || entry->canceled)
     558             :         {
     559      258794 :             entry->cycle_ctr = sync_cycle_ctr;
     560      258794 :             entry->canceled = false;
     561             :         }
     562             : 
     563             :         /*
     564             :          * NB: it's intentional that we don't change cycle_ctr if the entry
     565             :          * already exists.  The cycle_ctr must represent the oldest fsync
     566             :          * request that could be in the entry.
     567             :          */
     568             : 
     569     1416350 :         MemoryContextSwitchTo(oldcxt);
     570             :     }
     571     1718372 : }
     572             : 
     573             : /*
     574             :  * Register the sync request locally, or forward it to the checkpointer.
     575             :  *
     576             :  * If retryOnError is true, we'll keep trying if there is no space in the
     577             :  * queue.  Return true if we succeeded, or false if there wasn't space.
     578             :  */
     579             : bool
     580     1896022 : RegisterSyncRequest(const FileTag *ftag, SyncRequestType type,
     581             :                     bool retryOnError)
     582             : {
     583             :     bool        ret;
     584             : 
     585     1896022 :     if (pendingOps != NULL)
     586             :     {
     587             :         /* standalone backend or startup process: fsync state is local */
     588      527440 :         RememberSyncRequest(ftag, type);
     589      527440 :         return true;
     590             :     }
     591             : 
     592             :     for (;;)
     593             :     {
     594             :         /*
     595             :          * Notify the checkpointer about it.  If we fail to queue a message in
     596             :          * retryOnError mode, we have to sleep and try again ... ugly, but
     597             :          * hopefully won't happen often.
     598             :          *
     599             :          * XXX should we CHECK_FOR_INTERRUPTS in this loop?  Escaping with an
     600             :          * error in the case of SYNC_UNLINK_REQUEST would leave the
     601             :          * no-longer-used file still present on disk, which would be bad, so
     602             :          * I'm inclined to assume that the checkpointer will always empty the
     603             :          * queue soon.
     604             :          */
     605     1368590 :         ret = ForwardSyncRequest(ftag, type);
     606             : 
     607             :         /*
     608             :          * If we are successful in queueing the request, or we failed and were
     609             :          * instructed not to retry on error, break.
     610             :          */
     611     1368590 :         if (ret || (!ret && !retryOnError))
     612             :             break;
     613             : 
     614           8 :         WaitLatch(NULL, WL_EXIT_ON_PM_DEATH | WL_TIMEOUT, 10,
     615             :                   WAIT_EVENT_REGISTER_SYNC_REQUEST);
     616             :     }
     617             : 
     618     1368582 :     return ret;
     619             : }

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