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slot.c
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1 /*-------------------------------------------------------------------------
2  *
3  * slot.c
4  * Replication slot management.
5  *
6  *
7  * Copyright (c) 2012-2017, PostgreSQL Global Development Group
8  *
9  *
10  * IDENTIFICATION
11  * src/backend/replication/slot.c
12  *
13  * NOTES
14  *
15  * Replication slots are used to keep state about replication streams
16  * originating from this cluster. Their primary purpose is to prevent the
17  * premature removal of WAL or of old tuple versions in a manner that would
18  * interfere with replication; they are also useful for monitoring purposes.
19  * Slots need to be permanent (to allow restarts), crash-safe, and allocatable
20  * on standbys (to support cascading setups). The requirement that slots be
21  * usable on standbys precludes storing them in the system catalogs.
22  *
23  * Each replication slot gets its own directory inside the $PGDATA/pg_replslot
24  * directory. Inside that directory the state file will contain the slot's
25  * own data. Additional data can be stored alongside that file if required.
26  * While the server is running, the state data is also cached in memory for
27  * efficiency.
28  *
29  * ReplicationSlotAllocationLock must be taken in exclusive mode to allocate
30  * or free a slot. ReplicationSlotControlLock must be taken in shared mode
31  * to iterate over the slots, and in exclusive mode to change the in_use flag
32  * of a slot. The remaining data in each slot is protected by its mutex.
33  *
34  *-------------------------------------------------------------------------
35  */
36 
37 #include "postgres.h"
38 
39 #include <unistd.h>
40 #include <sys/stat.h>
41 
42 #include "access/transam.h"
43 #include "access/xlog_internal.h"
44 #include "common/string.h"
45 #include "miscadmin.h"
46 #include "pgstat.h"
47 #include "replication/slot.h"
48 #include "storage/fd.h"
49 #include "storage/proc.h"
50 #include "storage/procarray.h"
51 #include "utils/builtins.h"
52 
53 /*
54  * Replication slot on-disk data structure.
55  */
56 typedef struct ReplicationSlotOnDisk
57 {
58  /* first part of this struct needs to be version independent */
59 
60  /* data not covered by checksum */
63 
64  /* data covered by checksum */
67 
68  /*
69  * The actual data in the slot that follows can differ based on the above
70  * 'version'.
71  */
72 
75 
76 /* size of version independent data */
77 #define ReplicationSlotOnDiskConstantSize \
78  offsetof(ReplicationSlotOnDisk, slotdata)
79 /* size of the part of the slot not covered by the checksum */
80 #define SnapBuildOnDiskNotChecksummedSize \
81  offsetof(ReplicationSlotOnDisk, version)
82 /* size of the part covered by the checksum */
83 #define SnapBuildOnDiskChecksummedSize \
84  sizeof(ReplicationSlotOnDisk) - SnapBuildOnDiskNotChecksummedSize
85 /* size of the slot data that is version dependent */
86 #define ReplicationSlotOnDiskV2Size \
87  sizeof(ReplicationSlotOnDisk) - ReplicationSlotOnDiskConstantSize
88 
89 #define SLOT_MAGIC 0x1051CA1 /* format identifier */
90 #define SLOT_VERSION 2 /* version for new files */
91 
92 /* Control array for replication slot management */
94 
95 /* My backend's replication slot in the shared memory array */
97 
98 /* GUCs */
99 int max_replication_slots = 0; /* the maximum number of replication
100  * slots */
101 
102 static void ReplicationSlotDropAcquired(void);
103 static void ReplicationSlotDropPtr(ReplicationSlot *slot);
104 
105 /* internal persistency functions */
106 static void RestoreSlotFromDisk(const char *name);
107 static void CreateSlotOnDisk(ReplicationSlot *slot);
108 static void SaveSlotToPath(ReplicationSlot *slot, const char *path, int elevel);
109 
110 /*
111  * Report shared-memory space needed by ReplicationSlotShmemInit.
112  */
113 Size
115 {
116  Size size = 0;
117 
118  if (max_replication_slots == 0)
119  return size;
120 
121  size = offsetof(ReplicationSlotCtlData, replication_slots);
122  size = add_size(size,
124 
125  return size;
126 }
127 
128 /*
129  * Allocate and initialize walsender-related shared memory.
130  */
131 void
133 {
134  bool found;
135 
136  if (max_replication_slots == 0)
137  return;
138 
139  ReplicationSlotCtl = (ReplicationSlotCtlData *)
140  ShmemInitStruct("ReplicationSlot Ctl", ReplicationSlotsShmemSize(),
141  &found);
142 
144  "replication_slot_io");
145 
146  if (!found)
147  {
148  int i;
149 
150  /* First time through, so initialize */
151  MemSet(ReplicationSlotCtl, 0, ReplicationSlotsShmemSize());
152 
153  for (i = 0; i < max_replication_slots; i++)
154  {
155  ReplicationSlot *slot = &ReplicationSlotCtl->replication_slots[i];
156 
157  /* everything else is zeroed by the memset above */
158  SpinLockInit(&slot->mutex);
160  }
161  }
162 }
163 
164 /*
165  * Check whether the passed slot name is valid and report errors at elevel.
166  *
167  * Slot names may consist out of [a-z0-9_]{1,NAMEDATALEN-1} which should allow
168  * the name to be used as a directory name on every supported OS.
169  *
170  * Returns whether the directory name is valid or not if elevel < ERROR.
171  */
172 bool
174 {
175  const char *cp;
176 
177  if (strlen(name) == 0)
178  {
179  ereport(elevel,
180  (errcode(ERRCODE_INVALID_NAME),
181  errmsg("replication slot name \"%s\" is too short",
182  name)));
183  return false;
184  }
185 
186  if (strlen(name) >= NAMEDATALEN)
187  {
188  ereport(elevel,
189  (errcode(ERRCODE_NAME_TOO_LONG),
190  errmsg("replication slot name \"%s\" is too long",
191  name)));
192  return false;
193  }
194 
195  for (cp = name; *cp; cp++)
196  {
197  if (!((*cp >= 'a' && *cp <= 'z')
198  || (*cp >= '0' && *cp <= '9')
199  || (*cp == '_')))
200  {
201  ereport(elevel,
202  (errcode(ERRCODE_INVALID_NAME),
203  errmsg("replication slot name \"%s\" contains invalid character",
204  name),
205  errhint("Replication slot names may only contain lower case letters, numbers, and the underscore character.")));
206  return false;
207  }
208  }
209  return true;
210 }
211 
212 /*
213  * Create a new replication slot and mark it as used by this backend.
214  *
215  * name: Name of the slot
216  * db_specific: logical decoding is db specific; if the slot is going to
217  * be used for that pass true, otherwise false.
218  */
219 void
220 ReplicationSlotCreate(const char *name, bool db_specific,
221  ReplicationSlotPersistency persistency)
222 {
223  ReplicationSlot *slot = NULL;
224  int i;
225 
226  Assert(MyReplicationSlot == NULL);
227 
229 
230  /*
231  * If some other backend ran this code concurrently with us, we'd likely
232  * both allocate the same slot, and that would be bad. We'd also be at
233  * risk of missing a name collision. Also, we don't want to try to create
234  * a new slot while somebody's busy cleaning up an old one, because we
235  * might both be monkeying with the same directory.
236  */
237  LWLockAcquire(ReplicationSlotAllocationLock, LW_EXCLUSIVE);
238 
239  /*
240  * Check for name collision, and identify an allocatable slot. We need to
241  * hold ReplicationSlotControlLock in shared mode for this, so that nobody
242  * else can change the in_use flags while we're looking at them.
243  */
244  LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
245  for (i = 0; i < max_replication_slots; i++)
246  {
247  ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
248 
249  if (s->in_use && strcmp(name, NameStr(s->data.name)) == 0)
250  ereport(ERROR,
252  errmsg("replication slot \"%s\" already exists", name)));
253  if (!s->in_use && slot == NULL)
254  slot = s;
255  }
256  LWLockRelease(ReplicationSlotControlLock);
257 
258  /* If all slots are in use, we're out of luck. */
259  if (slot == NULL)
260  ereport(ERROR,
261  (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
262  errmsg("all replication slots are in use"),
263  errhint("Free one or increase max_replication_slots.")));
264 
265  /*
266  * Since this slot is not in use, nobody should be looking at any part of
267  * it other than the in_use field unless they're trying to allocate it.
268  * And since we hold ReplicationSlotAllocationLock, nobody except us can
269  * be doing that. So it's safe to initialize the slot.
270  */
271  Assert(!slot->in_use);
272  Assert(slot->active_pid == 0);
273 
274  /* first initialize persistent data */
275  memset(&slot->data, 0, sizeof(ReplicationSlotPersistentData));
276  StrNCpy(NameStr(slot->data.name), name, NAMEDATALEN);
277  slot->data.database = db_specific ? MyDatabaseId : InvalidOid;
278  slot->data.persistency = persistency;
279 
280  /* and then data only present in shared memory */
281  slot->just_dirtied = false;
282  slot->dirty = false;
289 
290  /*
291  * Create the slot on disk. We haven't actually marked the slot allocated
292  * yet, so no special cleanup is required if this errors out.
293  */
294  CreateSlotOnDisk(slot);
295 
296  /*
297  * We need to briefly prevent any other backend from iterating over the
298  * slots while we flip the in_use flag. We also need to set the active
299  * flag while holding the ControlLock as otherwise a concurrent
300  * SlotAcquire() could acquire the slot as well.
301  */
302  LWLockAcquire(ReplicationSlotControlLock, LW_EXCLUSIVE);
303 
304  slot->in_use = true;
305 
306  /* We can now mark the slot active, and that makes it our slot. */
307  SpinLockAcquire(&slot->mutex);
308  Assert(slot->active_pid == 0);
309  slot->active_pid = MyProcPid;
310  SpinLockRelease(&slot->mutex);
311  MyReplicationSlot = slot;
312 
313  LWLockRelease(ReplicationSlotControlLock);
314 
315  /*
316  * Now that the slot has been marked as in_use and in_active, it's safe to
317  * let somebody else try to allocate a slot.
318  */
319  LWLockRelease(ReplicationSlotAllocationLock);
320 }
321 
322 /*
323  * Find a previously created slot and mark it as used by this backend.
324  */
325 void
327 {
328  ReplicationSlot *slot = NULL;
329  int i;
330  int active_pid = 0; /* Keep compiler quiet */
331 
332  Assert(MyReplicationSlot == NULL);
333 
334  /* Search for the named slot and mark it active if we find it. */
335  LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
336  for (i = 0; i < max_replication_slots; i++)
337  {
338  ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
339 
340  if (s->in_use && strcmp(name, NameStr(s->data.name)) == 0)
341  {
342  SpinLockAcquire(&s->mutex);
343  active_pid = s->active_pid;
344  if (active_pid == 0)
345  active_pid = s->active_pid = MyProcPid;
346  SpinLockRelease(&s->mutex);
347  slot = s;
348  break;
349  }
350  }
351  LWLockRelease(ReplicationSlotControlLock);
352 
353  /* If we did not find the slot or it was already active, error out. */
354  if (slot == NULL)
355  ereport(ERROR,
356  (errcode(ERRCODE_UNDEFINED_OBJECT),
357  errmsg("replication slot \"%s\" does not exist", name)));
358  if (active_pid != MyProcPid)
359  ereport(ERROR,
360  (errcode(ERRCODE_OBJECT_IN_USE),
361  errmsg("replication slot \"%s\" is active for PID %d",
362  name, active_pid)));
363 
364  /* We made this slot active, so it's ours now. */
365  MyReplicationSlot = slot;
366 }
367 
368 /*
369  * Release a replication slot, this or another backend can ReAcquire it
370  * later. Resources this slot requires will be preserved.
371  */
372 void
374 {
376 
377  Assert(slot != NULL && slot->active_pid != 0);
378 
379  if (slot->data.persistency == RS_EPHEMERAL)
380  {
381  /*
382  * Delete the slot. There is no !PANIC case where this is allowed to
383  * fail, all that may happen is an incomplete cleanup of the on-disk
384  * data.
385  */
387  }
388  else if (slot->data.persistency == RS_PERSISTENT)
389  {
390  /*
391  * Mark persistent slot inactive. We're not freeing it, just
392  * disconnecting.
393  */
394  SpinLockAcquire(&slot->mutex);
395  slot->active_pid = 0;
396  SpinLockRelease(&slot->mutex);
397  }
398 
399 
400  /*
401  * If slot needed to temporarily restrain both data and catalog xmin to
402  * create the catalog snapshot, remove that temporary constraint.
403  * Snapshots can only be exported while the initial snapshot is still
404  * acquired.
405  */
406  if (!TransactionIdIsValid(slot->data.xmin) &&
408  {
409  SpinLockAcquire(&slot->mutex);
411  SpinLockRelease(&slot->mutex);
413  }
414 
415  MyReplicationSlot = NULL;
416 
417  /* might not have been set when we've been a plain slot */
418  LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
420  LWLockRelease(ProcArrayLock);
421 }
422 
423 /*
424  * Cleanup all temporary slots created in current session.
425  */
426 void
428 {
429  int i;
430 
431  Assert(MyReplicationSlot == NULL);
432 
433  /*
434  * No need for locking as we are only interested in slots active in
435  * current process and those are not touched by other processes.
436  */
437  for (i = 0; i < max_replication_slots; i++)
438  {
439  ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
440 
441  if (s->active_pid == MyProcPid)
442  {
444 
446  }
447  }
448 }
449 
450 /*
451  * Permanently drop replication slot identified by the passed in name.
452  */
453 void
455 {
456  Assert(MyReplicationSlot == NULL);
457 
459 
461 }
462 
463 /*
464  * Permanently drop the currently acquired replication slot.
465  */
466 static void
468 {
470 
471  Assert(MyReplicationSlot != NULL);
472 
473  /* slot isn't acquired anymore */
474  MyReplicationSlot = NULL;
475 
477 }
478 
479 /*
480  * Permanently drop the replication slot which will be released by the point
481  * this function returns.
482  */
483 static void
485 {
486  char path[MAXPGPATH];
487  char tmppath[MAXPGPATH];
488 
489  /*
490  * If some other backend ran this code concurrently with us, we might try
491  * to delete a slot with a certain name while someone else was trying to
492  * create a slot with the same name.
493  */
494  LWLockAcquire(ReplicationSlotAllocationLock, LW_EXCLUSIVE);
495 
496  /* Generate pathnames. */
497  sprintf(path, "pg_replslot/%s", NameStr(slot->data.name));
498  sprintf(tmppath, "pg_replslot/%s.tmp", NameStr(slot->data.name));
499 
500  /*
501  * Rename the slot directory on disk, so that we'll no longer recognize
502  * this as a valid slot. Note that if this fails, we've got to mark the
503  * slot inactive before bailing out. If we're dropping an ephemeral or a
504  * temporary slot, we better never fail hard as the caller won't expect
505  * the slot to survive and this might get called during error handling.
506  */
507  if (rename(path, tmppath) == 0)
508  {
509  /*
510  * We need to fsync() the directory we just renamed and its parent to
511  * make sure that our changes are on disk in a crash-safe fashion. If
512  * fsync() fails, we can't be sure whether the changes are on disk or
513  * not. For now, we handle that by panicking;
514  * StartupReplicationSlots() will try to straighten it out after
515  * restart.
516  */
518  fsync_fname(tmppath, true);
519  fsync_fname("pg_replslot", true);
521  }
522  else
523  {
524  bool fail_softly = slot->data.persistency != RS_PERSISTENT;
525 
526  SpinLockAcquire(&slot->mutex);
527  slot->active_pid = 0;
528  SpinLockRelease(&slot->mutex);
529 
530  ereport(fail_softly ? WARNING : ERROR,
532  errmsg("could not rename file \"%s\" to \"%s\": %m",
533  path, tmppath)));
534  }
535 
536  /*
537  * The slot is definitely gone. Lock out concurrent scans of the array
538  * long enough to kill it. It's OK to clear the active flag here without
539  * grabbing the mutex because nobody else can be scanning the array here,
540  * and nobody can be attached to this slot and thus access it without
541  * scanning the array.
542  */
543  LWLockAcquire(ReplicationSlotControlLock, LW_EXCLUSIVE);
544  slot->active_pid = 0;
545  slot->in_use = false;
546  LWLockRelease(ReplicationSlotControlLock);
547 
548  /*
549  * Slot is dead and doesn't prevent resource removal anymore, recompute
550  * limits.
551  */
554 
555  /*
556  * If removing the directory fails, the worst thing that will happen is
557  * that the user won't be able to create a new slot with the same name
558  * until the next server restart. We warn about it, but that's all.
559  */
560  if (!rmtree(tmppath, true))
563  errmsg("could not remove directory \"%s\"", tmppath)));
564 
565  /*
566  * We release this at the very end, so that nobody starts trying to create
567  * a slot while we're still cleaning up the detritus of the old one.
568  */
569  LWLockRelease(ReplicationSlotAllocationLock);
570 }
571 
572 /*
573  * Serialize the currently acquired slot's state from memory to disk, thereby
574  * guaranteeing the current state will survive a crash.
575  */
576 void
578 {
579  char path[MAXPGPATH];
580 
581  Assert(MyReplicationSlot != NULL);
582 
583  sprintf(path, "pg_replslot/%s", NameStr(MyReplicationSlot->data.name));
584  SaveSlotToPath(MyReplicationSlot, path, ERROR);
585 }
586 
587 /*
588  * Signal that it would be useful if the currently acquired slot would be
589  * flushed out to disk.
590  *
591  * Note that the actual flush to disk can be delayed for a long time, if
592  * required for correctness explicitly do a ReplicationSlotSave().
593  */
594 void
596 {
598 
599  Assert(MyReplicationSlot != NULL);
600 
601  SpinLockAcquire(&slot->mutex);
602  MyReplicationSlot->just_dirtied = true;
603  MyReplicationSlot->dirty = true;
604  SpinLockRelease(&slot->mutex);
605 }
606 
607 /*
608  * Convert a slot that's marked as RS_EPHEMERAL to a RS_PERSISTENT slot,
609  * guaranteeing it will be there after an eventual crash.
610  */
611 void
613 {
615 
616  Assert(slot != NULL);
618 
619  SpinLockAcquire(&slot->mutex);
621  SpinLockRelease(&slot->mutex);
622 
625 }
626 
627 /*
628  * Compute the oldest xmin across all slots and store it in the ProcArray.
629  *
630  * If already_locked is true, ProcArrayLock has already been acquired
631  * exclusively.
632  */
633 void
635 {
636  int i;
638  TransactionId agg_catalog_xmin = InvalidTransactionId;
639 
640  Assert(ReplicationSlotCtl != NULL);
641 
642  LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
643 
644  for (i = 0; i < max_replication_slots; i++)
645  {
646  ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
647  TransactionId effective_xmin;
648  TransactionId effective_catalog_xmin;
649 
650  if (!s->in_use)
651  continue;
652 
653  SpinLockAcquire(&s->mutex);
654  effective_xmin = s->effective_xmin;
655  effective_catalog_xmin = s->effective_catalog_xmin;
656  SpinLockRelease(&s->mutex);
657 
658  /* check the data xmin */
659  if (TransactionIdIsValid(effective_xmin) &&
660  (!TransactionIdIsValid(agg_xmin) ||
661  TransactionIdPrecedes(effective_xmin, agg_xmin)))
662  agg_xmin = effective_xmin;
663 
664  /* check the catalog xmin */
665  if (TransactionIdIsValid(effective_catalog_xmin) &&
666  (!TransactionIdIsValid(agg_catalog_xmin) ||
667  TransactionIdPrecedes(effective_catalog_xmin, agg_catalog_xmin)))
668  agg_catalog_xmin = effective_catalog_xmin;
669  }
670 
671  LWLockRelease(ReplicationSlotControlLock);
672 
673  ProcArraySetReplicationSlotXmin(agg_xmin, agg_catalog_xmin, already_locked);
674 }
675 
676 /*
677  * Compute the oldest restart LSN across all slots and inform xlog module.
678  */
679 void
681 {
682  int i;
683  XLogRecPtr min_required = InvalidXLogRecPtr;
684 
685  Assert(ReplicationSlotCtl != NULL);
686 
687  LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
688  for (i = 0; i < max_replication_slots; i++)
689  {
690  ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
691  XLogRecPtr restart_lsn;
692 
693  if (!s->in_use)
694  continue;
695 
696  SpinLockAcquire(&s->mutex);
697  restart_lsn = s->data.restart_lsn;
698  SpinLockRelease(&s->mutex);
699 
700  if (restart_lsn != InvalidXLogRecPtr &&
701  (min_required == InvalidXLogRecPtr ||
702  restart_lsn < min_required))
703  min_required = restart_lsn;
704  }
705  LWLockRelease(ReplicationSlotControlLock);
706 
707  XLogSetReplicationSlotMinimumLSN(min_required);
708 }
709 
710 /*
711  * Compute the oldest WAL LSN required by *logical* decoding slots..
712  *
713  * Returns InvalidXLogRecPtr if logical decoding is disabled or no logical
714  * slots exist.
715  *
716  * NB: this returns a value >= ReplicationSlotsComputeRequiredLSN(), since it
717  * ignores physical replication slots.
718  *
719  * The results aren't required frequently, so we don't maintain a precomputed
720  * value like we do for ComputeRequiredLSN() and ComputeRequiredXmin().
721  */
724 {
726  int i;
727 
728  if (max_replication_slots <= 0)
729  return InvalidXLogRecPtr;
730 
731  LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
732 
733  for (i = 0; i < max_replication_slots; i++)
734  {
735  ReplicationSlot *s;
736  XLogRecPtr restart_lsn;
737 
738  s = &ReplicationSlotCtl->replication_slots[i];
739 
740  /* cannot change while ReplicationSlotCtlLock is held */
741  if (!s->in_use)
742  continue;
743 
744  /* we're only interested in logical slots */
745  if (!SlotIsLogical(s))
746  continue;
747 
748  /* read once, it's ok if it increases while we're checking */
749  SpinLockAcquire(&s->mutex);
750  restart_lsn = s->data.restart_lsn;
751  SpinLockRelease(&s->mutex);
752 
753  if (result == InvalidXLogRecPtr ||
754  restart_lsn < result)
755  result = restart_lsn;
756  }
757 
758  LWLockRelease(ReplicationSlotControlLock);
759 
760  return result;
761 }
762 
763 /*
764  * ReplicationSlotsCountDBSlots -- count the number of slots that refer to the
765  * passed database oid.
766  *
767  * Returns true if there are any slots referencing the database. *nslots will
768  * be set to the absolute number of slots in the database, *nactive to ones
769  * currently active.
770  */
771 bool
772 ReplicationSlotsCountDBSlots(Oid dboid, int *nslots, int *nactive)
773 {
774  int i;
775 
776  *nslots = *nactive = 0;
777 
778  if (max_replication_slots <= 0)
779  return false;
780 
781  LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
782  for (i = 0; i < max_replication_slots; i++)
783  {
784  ReplicationSlot *s;
785 
786  s = &ReplicationSlotCtl->replication_slots[i];
787 
788  /* cannot change while ReplicationSlotCtlLock is held */
789  if (!s->in_use)
790  continue;
791 
792  /* only logical slots are database specific, skip */
793  if (!SlotIsLogical(s))
794  continue;
795 
796  /* not our database, skip */
797  if (s->data.database != dboid)
798  continue;
799 
800  /* count slots with spinlock held */
801  SpinLockAcquire(&s->mutex);
802  (*nslots)++;
803  if (s->active_pid != 0)
804  (*nactive)++;
805  SpinLockRelease(&s->mutex);
806  }
807  LWLockRelease(ReplicationSlotControlLock);
808 
809  if (*nslots > 0)
810  return true;
811  return false;
812 }
813 
814 /*
815  * ReplicationSlotsDropDBSlots -- Drop all db-specific slots relating to the
816  * passed database oid. The caller should hold an exclusive lock on the
817  * pg_database oid for the database to prevent creation of new slots on the db
818  * or replay from existing slots.
819  *
820  * Another session that concurrently acquires an existing slot on the target DB
821  * (most likely to drop it) may cause this function to ERROR. If that happens
822  * it may have dropped some but not all slots.
823  *
824  * This routine isn't as efficient as it could be - but we don't drop
825  * databases often, especially databases with lots of slots.
826  */
827 void
829 {
830  int i;
831 
832  if (max_replication_slots <= 0)
833  return;
834 
835 restart:
836  LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
837  for (i = 0; i < max_replication_slots; i++)
838  {
839  ReplicationSlot *s;
840  char *slotname;
841  int active_pid;
842 
843  s = &ReplicationSlotCtl->replication_slots[i];
844 
845  /* cannot change while ReplicationSlotCtlLock is held */
846  if (!s->in_use)
847  continue;
848 
849  /* only logical slots are database specific, skip */
850  if (!SlotIsLogical(s))
851  continue;
852 
853  /* not our database, skip */
854  if (s->data.database != dboid)
855  continue;
856 
857  /* acquire slot, so ReplicationSlotDropAcquired can be reused */
858  SpinLockAcquire(&s->mutex);
859  /* can't change while ReplicationSlotControlLock is held */
860  slotname = NameStr(s->data.name);
861  active_pid = s->active_pid;
862  if (active_pid == 0)
863  {
864  MyReplicationSlot = s;
865  s->active_pid = MyProcPid;
866  }
867  SpinLockRelease(&s->mutex);
868 
869  /*
870  * Even though we hold an exclusive lock on the database object a
871  * logical slot for that DB can still be active, e.g. if it's
872  * concurrently being dropped by a backend connected to another DB.
873  *
874  * That's fairly unlikely in practice, so we'll just bail out.
875  */
876  if (active_pid)
877  ereport(ERROR,
878  (errcode(ERRCODE_OBJECT_IN_USE),
879  errmsg("replication slot \"%s\" is active for PID %d",
880  slotname, active_pid)));
881 
882  /*
883  * To avoid duplicating ReplicationSlotDropAcquired() and to avoid
884  * holding ReplicationSlotControlLock over filesystem operations,
885  * release ReplicationSlotControlLock and use
886  * ReplicationSlotDropAcquired.
887  *
888  * As that means the set of slots could change, restart scan from the
889  * beginning each time we release the lock.
890  */
891  LWLockRelease(ReplicationSlotControlLock);
893  goto restart;
894  }
895  LWLockRelease(ReplicationSlotControlLock);
896 }
897 
898 
899 /*
900  * Check whether the server's configuration supports using replication
901  * slots.
902  */
903 void
905 {
906  if (max_replication_slots == 0)
907  ereport(ERROR,
908  (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
909  (errmsg("replication slots can only be used if max_replication_slots > 0"))));
910 
912  ereport(ERROR,
913  (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
914  errmsg("replication slots can only be used if wal_level >= replica")));
915 }
916 
917 /*
918  * Reserve WAL for the currently active slot.
919  *
920  * Compute and set restart_lsn in a manner that's appropriate for the type of
921  * the slot and concurrency safe.
922  */
923 void
925 {
927 
928  Assert(slot != NULL);
930 
931  /*
932  * The replication slot mechanism is used to prevent removal of required
933  * WAL. As there is no interlock between this routine and checkpoints, WAL
934  * segments could concurrently be removed when a now stale return value of
935  * ReplicationSlotsComputeRequiredLSN() is used. In the unlikely case that
936  * this happens we'll just retry.
937  */
938  while (true)
939  {
940  XLogSegNo segno;
941 
942  /*
943  * For logical slots log a standby snapshot and start logical decoding
944  * at exactly that position. That allows the slot to start up more
945  * quickly.
946  *
947  * That's not needed (or indeed helpful) for physical slots as they'll
948  * start replay at the last logged checkpoint anyway. Instead return
949  * the location of the last redo LSN. While that slightly increases
950  * the chance that we have to retry, it's where a base backup has to
951  * start replay at.
952  */
953  if (!RecoveryInProgress() && SlotIsLogical(slot))
954  {
955  XLogRecPtr flushptr;
956 
957  /* start at current insert position */
959 
960  /* make sure we have enough information to start */
961  flushptr = LogStandbySnapshot();
962 
963  /* and make sure it's fsynced to disk */
964  XLogFlush(flushptr);
965  }
966  else
967  {
968  slot->data.restart_lsn = GetRedoRecPtr();
969  }
970 
971  /* prevent WAL removal as fast as possible */
973 
974  /*
975  * If all required WAL is still there, great, otherwise retry. The
976  * slot should prevent further removal of WAL, unless there's a
977  * concurrent ReplicationSlotsComputeRequiredLSN() after we've written
978  * the new restart_lsn above, so normally we should never need to loop
979  * more than twice.
980  */
981  XLByteToSeg(slot->data.restart_lsn, segno);
982  if (XLogGetLastRemovedSegno() < segno)
983  break;
984  }
985 }
986 
987 /*
988  * Flush all replication slots to disk.
989  *
990  * This needn't actually be part of a checkpoint, but it's a convenient
991  * location.
992  */
993 void
995 {
996  int i;
997 
998  elog(DEBUG1, "performing replication slot checkpoint");
999 
1000  /*
1001  * Prevent any slot from being created/dropped while we're active. As we
1002  * explicitly do *not* want to block iterating over replication_slots or
1003  * acquiring a slot we cannot take the control lock - but that's OK,
1004  * because holding ReplicationSlotAllocationLock is strictly stronger, and
1005  * enough to guarantee that nobody can change the in_use bits on us.
1006  */
1007  LWLockAcquire(ReplicationSlotAllocationLock, LW_SHARED);
1008 
1009  for (i = 0; i < max_replication_slots; i++)
1010  {
1011  ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
1012  char path[MAXPGPATH];
1013 
1014  if (!s->in_use)
1015  continue;
1016 
1017  /* save the slot to disk, locking is handled in SaveSlotToPath() */
1018  sprintf(path, "pg_replslot/%s", NameStr(s->data.name));
1019  SaveSlotToPath(s, path, LOG);
1020  }
1021  LWLockRelease(ReplicationSlotAllocationLock);
1022 }
1023 
1024 /*
1025  * Load all replication slots from disk into memory at server startup. This
1026  * needs to be run before we start crash recovery.
1027  */
1028 void
1030 {
1031  DIR *replication_dir;
1032  struct dirent *replication_de;
1033 
1034  elog(DEBUG1, "starting up replication slots");
1035 
1036  /* restore all slots by iterating over all on-disk entries */
1037  replication_dir = AllocateDir("pg_replslot");
1038  while ((replication_de = ReadDir(replication_dir, "pg_replslot")) != NULL)
1039  {
1040  struct stat statbuf;
1041  char path[MAXPGPATH + 12];
1042 
1043  if (strcmp(replication_de->d_name, ".") == 0 ||
1044  strcmp(replication_de->d_name, "..") == 0)
1045  continue;
1046 
1047  snprintf(path, sizeof(path), "pg_replslot/%s", replication_de->d_name);
1048 
1049  /* we're only creating directories here, skip if it's not our's */
1050  if (lstat(path, &statbuf) == 0 && !S_ISDIR(statbuf.st_mode))
1051  continue;
1052 
1053  /* we crashed while a slot was being setup or deleted, clean up */
1054  if (pg_str_endswith(replication_de->d_name, ".tmp"))
1055  {
1056  if (!rmtree(path, true))
1057  {
1058  ereport(WARNING,
1060  errmsg("could not remove directory \"%s\"", path)));
1061  continue;
1062  }
1063  fsync_fname("pg_replslot", true);
1064  continue;
1065  }
1066 
1067  /* looks like a slot in a normal state, restore */
1068  RestoreSlotFromDisk(replication_de->d_name);
1069  }
1070  FreeDir(replication_dir);
1071 
1072  /* currently no slots exist, we're done. */
1073  if (max_replication_slots <= 0)
1074  return;
1075 
1076  /* Now that we have recovered all the data, compute replication xmin */
1079 }
1080 
1081 /* ----
1082  * Manipulation of on-disk state of replication slots
1083  *
1084  * NB: none of the routines below should take any notice whether a slot is the
1085  * current one or not, that's all handled a layer above.
1086  * ----
1087  */
1088 static void
1090 {
1091  char tmppath[MAXPGPATH];
1092  char path[MAXPGPATH];
1093  struct stat st;
1094 
1095  /*
1096  * No need to take out the io_in_progress_lock, nobody else can see this
1097  * slot yet, so nobody else will write. We're reusing SaveSlotToPath which
1098  * takes out the lock, if we'd take the lock here, we'd deadlock.
1099  */
1100 
1101  sprintf(path, "pg_replslot/%s", NameStr(slot->data.name));
1102  sprintf(tmppath, "pg_replslot/%s.tmp", NameStr(slot->data.name));
1103 
1104  /*
1105  * It's just barely possible that some previous effort to create or drop a
1106  * slot with this name left a temp directory lying around. If that seems
1107  * to be the case, try to remove it. If the rmtree() fails, we'll error
1108  * out at the mkdir() below, so we don't bother checking success.
1109  */
1110  if (stat(tmppath, &st) == 0 && S_ISDIR(st.st_mode))
1111  rmtree(tmppath, true);
1112 
1113  /* Create and fsync the temporary slot directory. */
1114  if (mkdir(tmppath, S_IRWXU) < 0)
1115  ereport(ERROR,
1117  errmsg("could not create directory \"%s\": %m",
1118  tmppath)));
1119  fsync_fname(tmppath, true);
1120 
1121  /* Write the actual state file. */
1122  slot->dirty = true; /* signal that we really need to write */
1123  SaveSlotToPath(slot, tmppath, ERROR);
1124 
1125  /* Rename the directory into place. */
1126  if (rename(tmppath, path) != 0)
1127  ereport(ERROR,
1129  errmsg("could not rename file \"%s\" to \"%s\": %m",
1130  tmppath, path)));
1131 
1132  /*
1133  * If we'd now fail - really unlikely - we wouldn't know whether this slot
1134  * would persist after an OS crash or not - so, force a restart. The
1135  * restart would try to fsync this again till it works.
1136  */
1138 
1139  fsync_fname(path, true);
1140  fsync_fname("pg_replslot", true);
1141 
1142  END_CRIT_SECTION();
1143 }
1144 
1145 /*
1146  * Shared functionality between saving and creating a replication slot.
1147  */
1148 static void
1149 SaveSlotToPath(ReplicationSlot *slot, const char *dir, int elevel)
1150 {
1151  char tmppath[MAXPGPATH];
1152  char path[MAXPGPATH];
1153  int fd;
1155  bool was_dirty;
1156 
1157  /* first check whether there's something to write out */
1158  SpinLockAcquire(&slot->mutex);
1159  was_dirty = slot->dirty;
1160  slot->just_dirtied = false;
1161  SpinLockRelease(&slot->mutex);
1162 
1163  /* and don't do anything if there's nothing to write */
1164  if (!was_dirty)
1165  return;
1166 
1168 
1169  /* silence valgrind :( */
1170  memset(&cp, 0, sizeof(ReplicationSlotOnDisk));
1171 
1172  sprintf(tmppath, "%s/state.tmp", dir);
1173  sprintf(path, "%s/state", dir);
1174 
1175  fd = OpenTransientFile(tmppath,
1176  O_CREAT | O_EXCL | O_WRONLY | PG_BINARY,
1177  S_IRUSR | S_IWUSR);
1178  if (fd < 0)
1179  {
1180  ereport(elevel,
1182  errmsg("could not create file \"%s\": %m",
1183  tmppath)));
1184  return;
1185  }
1186 
1187  cp.magic = SLOT_MAGIC;
1188  INIT_CRC32C(cp.checksum);
1189  cp.version = SLOT_VERSION;
1191 
1192  SpinLockAcquire(&slot->mutex);
1193 
1194  memcpy(&cp.slotdata, &slot->data, sizeof(ReplicationSlotPersistentData));
1195 
1196  SpinLockRelease(&slot->mutex);
1197 
1198  COMP_CRC32C(cp.checksum,
1199  (char *) (&cp) + SnapBuildOnDiskNotChecksummedSize,
1201  FIN_CRC32C(cp.checksum);
1202 
1204  if ((write(fd, &cp, sizeof(cp))) != sizeof(cp))
1205  {
1206  int save_errno = errno;
1207 
1209  CloseTransientFile(fd);
1210  errno = save_errno;
1211  ereport(elevel,
1213  errmsg("could not write to file \"%s\": %m",
1214  tmppath)));
1215  return;
1216  }
1218 
1219  /* fsync the temporary file */
1221  if (pg_fsync(fd) != 0)
1222  {
1223  int save_errno = errno;
1224 
1226  CloseTransientFile(fd);
1227  errno = save_errno;
1228  ereport(elevel,
1230  errmsg("could not fsync file \"%s\": %m",
1231  tmppath)));
1232  return;
1233  }
1235 
1236  CloseTransientFile(fd);
1237 
1238  /* rename to permanent file, fsync file and directory */
1239  if (rename(tmppath, path) != 0)
1240  {
1241  ereport(elevel,
1243  errmsg("could not rename file \"%s\" to \"%s\": %m",
1244  tmppath, path)));
1245  return;
1246  }
1247 
1248  /* Check CreateSlot() for the reasoning of using a crit. section. */
1250 
1251  fsync_fname(path, false);
1252  fsync_fname(dir, true);
1253  fsync_fname("pg_replslot", true);
1254 
1255  END_CRIT_SECTION();
1256 
1257  /*
1258  * Successfully wrote, unset dirty bit, unless somebody dirtied again
1259  * already.
1260  */
1261  SpinLockAcquire(&slot->mutex);
1262  if (!slot->just_dirtied)
1263  slot->dirty = false;
1264  SpinLockRelease(&slot->mutex);
1265 
1267 }
1268 
1269 /*
1270  * Load a single slot from disk into memory.
1271  */
1272 static void
1274 {
1276  int i;
1277  char path[MAXPGPATH + 22];
1278  int fd;
1279  bool restored = false;
1280  int readBytes;
1281  pg_crc32c checksum;
1282 
1283  /* no need to lock here, no concurrent access allowed yet */
1284 
1285  /* delete temp file if it exists */
1286  sprintf(path, "pg_replslot/%s/state.tmp", name);
1287  if (unlink(path) < 0 && errno != ENOENT)
1288  ereport(PANIC,
1290  errmsg("could not remove file \"%s\": %m", path)));
1291 
1292  sprintf(path, "pg_replslot/%s/state", name);
1293 
1294  elog(DEBUG1, "restoring replication slot from \"%s\"", path);
1295 
1296  fd = OpenTransientFile(path, O_RDWR | PG_BINARY, 0);
1297 
1298  /*
1299  * We do not need to handle this as we are rename()ing the directory into
1300  * place only after we fsync()ed the state file.
1301  */
1302  if (fd < 0)
1303  ereport(PANIC,
1305  errmsg("could not open file \"%s\": %m", path)));
1306 
1307  /*
1308  * Sync state file before we're reading from it. We might have crashed
1309  * while it wasn't synced yet and we shouldn't continue on that basis.
1310  */
1312  if (pg_fsync(fd) != 0)
1313  {
1314  CloseTransientFile(fd);
1315  ereport(PANIC,
1317  errmsg("could not fsync file \"%s\": %m",
1318  path)));
1319  }
1321 
1322  /* Also sync the parent directory */
1324  fsync_fname(path, true);
1325  END_CRIT_SECTION();
1326 
1327  /* read part of statefile that's guaranteed to be version independent */
1329  readBytes = read(fd, &cp, ReplicationSlotOnDiskConstantSize);
1331  if (readBytes != ReplicationSlotOnDiskConstantSize)
1332  {
1333  int saved_errno = errno;
1334 
1335  CloseTransientFile(fd);
1336  errno = saved_errno;
1337  ereport(PANIC,
1339  errmsg("could not read file \"%s\", read %d of %u: %m",
1340  path, readBytes,
1342  }
1343 
1344  /* verify magic */
1345  if (cp.magic != SLOT_MAGIC)
1346  ereport(PANIC,
1348  errmsg("replication slot file \"%s\" has wrong magic number: %u instead of %u",
1349  path, cp.magic, SLOT_MAGIC)));
1350 
1351  /* verify version */
1352  if (cp.version != SLOT_VERSION)
1353  ereport(PANIC,
1355  errmsg("replication slot file \"%s\" has unsupported version %u",
1356  path, cp.version)));
1357 
1358  /* boundary check on length */
1360  ereport(PANIC,
1362  errmsg("replication slot file \"%s\" has corrupted length %u",
1363  path, cp.length)));
1364 
1365  /* Now that we know the size, read the entire file */
1367  readBytes = read(fd,
1368  (char *) &cp + ReplicationSlotOnDiskConstantSize,
1369  cp.length);
1371  if (readBytes != cp.length)
1372  {
1373  int saved_errno = errno;
1374 
1375  CloseTransientFile(fd);
1376  errno = saved_errno;
1377  ereport(PANIC,
1379  errmsg("could not read file \"%s\", read %d of %u: %m",
1380  path, readBytes, cp.length)));
1381  }
1382 
1383  CloseTransientFile(fd);
1384 
1385  /* now verify the CRC */
1386  INIT_CRC32C(checksum);
1387  COMP_CRC32C(checksum,
1388  (char *) &cp + SnapBuildOnDiskNotChecksummedSize,
1390  FIN_CRC32C(checksum);
1391 
1392  if (!EQ_CRC32C(checksum, cp.checksum))
1393  ereport(PANIC,
1394  (errmsg("checksum mismatch for replication slot file \"%s\": is %u, should be %u",
1395  path, checksum, cp.checksum)));
1396 
1397  /*
1398  * If we crashed with an ephemeral slot active, don't restore but delete
1399  * it.
1400  */
1402  {
1403  sprintf(path, "pg_replslot/%s", name);
1404 
1405  if (!rmtree(path, true))
1406  {
1407  ereport(WARNING,
1409  errmsg("could not remove directory \"%s\"", path)));
1410  }
1411  fsync_fname("pg_replslot", true);
1412  return;
1413  }
1414 
1415  /* nothing can be active yet, don't lock anything */
1416  for (i = 0; i < max_replication_slots; i++)
1417  {
1418  ReplicationSlot *slot;
1419 
1420  slot = &ReplicationSlotCtl->replication_slots[i];
1421 
1422  if (slot->in_use)
1423  continue;
1424 
1425  /* restore the entire set of persistent data */
1426  memcpy(&slot->data, &cp.slotdata,
1428 
1429  /* initialize in memory state */
1430  slot->effective_xmin = cp.slotdata.xmin;
1432 
1437 
1438  slot->in_use = true;
1439  slot->active_pid = 0;
1440 
1441  restored = true;
1442  break;
1443  }
1444 
1445  if (!restored)
1446  ereport(PANIC,
1447  (errmsg("too many replication slots active before shutdown"),
1448  errhint("Increase max_replication_slots and try again.")));
1449 }
#define INIT_CRC32C(crc)
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uint64 XLogSegNo
Definition: xlogdefs.h:34
int OpenTransientFile(FileName fileName, int fileFlags, int fileMode)
Definition: fd.c:2144
int errcode_for_file_access(void)
Definition: elog.c:598
XLogRecPtr ReplicationSlotsComputeLogicalRestartLSN(void)
Definition: slot.c:723
TransactionId catalog_xmin
Definition: slot.h:65
#define InvalidTransactionId
Definition: transam.h:31
unsigned int uint32
Definition: c.h:268
DIR * AllocateDir(const char *dirname)
Definition: fd.c:2335
void ReplicationSlotRelease(void)
Definition: slot.c:373
static void pgstat_report_wait_end(void)
Definition: pgstat.h:1232
TransactionId xmin
Definition: slot.h:57
#define EQ_CRC32C(c1, c2)
Definition: pg_crc32c.h:42
#define SlotIsLogical(slot)
Definition: slot.h:134
int unlink(const char *filename)
#define ereport(elevel, rest)
Definition: elog.h:122
bool TransactionIdPrecedes(TransactionId id1, TransactionId id2)
Definition: transam.c:300
pg_crc32c checksum
Definition: slot.c:62
struct ReplicationSlotOnDisk ReplicationSlotOnDisk
void LWLockInitialize(LWLock *lock, int tranche_id)
Definition: lwlock.c:667
int CloseTransientFile(int fd)
Definition: fd.c:2305
#define WARNING
Definition: elog.h:40
bool rmtree(const char *path, bool rmtopdir)
Definition: rmtree.c:36
bool in_use
Definition: slot.h:91
static int elevel
Definition: vacuumlazy.c:136
#define SpinLockRelease(lock)
Definition: spin.h:64
Size mul_size(Size s1, Size s2)
Definition: shmem.c:492
bool just_dirtied
Definition: slot.h:97
Size add_size(Size s1, Size s2)
Definition: shmem.c:475
TransactionId effective_catalog_xmin
Definition: slot.h:112
Oid MyDatabaseId
Definition: globals.c:77
#define SLOT_VERSION
Definition: slot.c:90
void ReplicationSlotDrop(const char *name)
Definition: slot.c:454
#define InvalidOid
Definition: postgres_ext.h:36
ReplicationSlot * MyReplicationSlot
Definition: slot.c:96
#define XLByteToSeg(xlrp, logSegNo)
int max_replication_slots
Definition: slot.c:99
#define NULL
Definition: c.h:229
#define ReplicationSlotOnDiskV2Size
Definition: slot.c:86
uint64 XLogRecPtr
Definition: xlogdefs.h:21
#define Assert(condition)
Definition: c.h:675
#define StrNCpy(dst, src, len)
Definition: c.h:830
struct dirent * ReadDir(DIR *dir, const char *dirname)
Definition: fd.c:2401
XLogRecPtr restart_lsn
Definition: slot.h:68
void ReplicationSlotAcquire(const char *name)
Definition: slot.c:326
#define ReplicationSlotOnDiskConstantSize
Definition: slot.c:77
size_t Size
Definition: c.h:356
uint32 version
Definition: slot.c:65
static void pgstat_report_wait_start(uint32 wait_event_info)
Definition: pgstat.h:1208
#define SnapBuildOnDiskChecksummedSize
Definition: slot.c:83
bool LWLockAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1111
void LWLockRegisterTranche(int tranche_id, char *tranche_name)
Definition: lwlock.c:592
XLogRecPtr GetRedoRecPtr(void)
Definition: xlog.c:8168
const char * name
Definition: encode.c:521
bool ReplicationSlotsCountDBSlots(Oid dboid, int *nslots, int *nactive)
Definition: slot.c:772
XLogRecPtr candidate_xmin_lsn
Definition: slot.h:128
ReplicationSlotPersistency
Definition: slot.h:28
int errmsg(const char *fmt,...)
Definition: elog.c:797
pid_t active_pid
Definition: slot.h:94
int i
#define NameStr(name)
Definition: c.h:499
void ProcArraySetReplicationSlotXmin(TransactionId xmin, TransactionId catalog_xmin, bool already_locked)
Definition: procarray.c:2963
void ReplicationSlotCleanup(void)
Definition: slot.c:427
int pg_fsync(int fd)
Definition: fd.c:333
ReplicationSlot replication_slots[1]
Definition: slot.h:145
char d_name[MAX_PATH]
Definition: dirent.h:14
#define elog
Definition: elog.h:219
slock_t mutex
Definition: slot.h:88
#define TransactionIdIsValid(xid)
Definition: transam.h:41
#define COMP_CRC32C(crc, data, len)
Definition: pg_crc32c.h:73
#define ERRCODE_DUPLICATE_OBJECT
Definition: streamutil.c:32
#define lstat(path, sb)
Definition: win32.h:262
void CheckPointReplicationSlots(void)
Definition: slot.c:994
#define FIN_CRC32C(crc)
Definition: pg_crc32c.h:78
void ReplicationSlotsDropDBSlots(Oid dboid)
Definition: slot.c:828
void ReplicationSlotsComputeRequiredXmin(bool already_locked)
Definition: slot.c:634
bool dirty
Definition: slot.h:98
#define read(a, b, c)
Definition: win32.h:13
int FreeDir(DIR *dir)
Definition: fd.c:2444
XLogRecPtr candidate_restart_lsn
Definition: slot.h:130
#define offsetof(type, field)
Definition: c.h:555
void ReplicationSlotMarkDirty(void)
Definition: slot.c:595
LWLock io_in_progress_lock
Definition: slot.h:118