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xlogutils.c
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1 /*-------------------------------------------------------------------------
2  *
3  * xlogutils.c
4  *
5  * PostgreSQL transaction log manager utility routines
6  *
7  * This file contains support routines that are used by XLOG replay functions.
8  * None of this code is used during normal system operation.
9  *
10  *
11  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
12  * Portions Copyright (c) 1994, Regents of the University of California
13  *
14  * src/backend/access/transam/xlogutils.c
15  *
16  *-------------------------------------------------------------------------
17  */
18 #include "postgres.h"
19 
20 #include <unistd.h>
21 
22 #include "access/timeline.h"
23 #include "access/xlog.h"
24 #include "access/xlog_internal.h"
25 #include "access/xlogutils.h"
26 #include "catalog/catalog.h"
27 #include "miscadmin.h"
28 #include "pgstat.h"
29 #include "storage/smgr.h"
30 #include "utils/guc.h"
31 #include "utils/hsearch.h"
32 #include "utils/rel.h"
33 
34 
35 /*
36  * During XLOG replay, we may see XLOG records for incremental updates of
37  * pages that no longer exist, because their relation was later dropped or
38  * truncated. (Note: this is only possible when full_page_writes = OFF,
39  * since when it's ON, the first reference we see to a page should always
40  * be a full-page rewrite not an incremental update.) Rather than simply
41  * ignoring such records, we make a note of the referenced page, and then
42  * complain if we don't actually see a drop or truncate covering the page
43  * later in replay.
44  */
45 typedef struct xl_invalid_page_key
46 {
47  RelFileNode node; /* the relation */
48  ForkNumber forkno; /* the fork number */
49  BlockNumber blkno; /* the page */
51 
52 typedef struct xl_invalid_page
53 {
54  xl_invalid_page_key key; /* hash key ... must be first */
55  bool present; /* page existed but contained zeroes */
57 
59 
60 
61 /* Report a reference to an invalid page */
62 static void
64  BlockNumber blkno, bool present)
65 {
66  char *path = relpathperm(node, forkno);
67 
68  if (present)
69  elog(elevel, "page %u of relation %s is uninitialized",
70  blkno, path);
71  else
72  elog(elevel, "page %u of relation %s does not exist",
73  blkno, path);
74  pfree(path);
75 }
76 
77 /* Log a reference to an invalid page */
78 static void
80  bool present)
81 {
83  xl_invalid_page *hentry;
84  bool found;
85 
86  /*
87  * Once recovery has reached a consistent state, the invalid-page table
88  * should be empty and remain so. If a reference to an invalid page is
89  * found after consistency is reached, PANIC immediately. This might seem
90  * aggressive, but it's better than letting the invalid reference linger
91  * in the hash table until the end of recovery and PANIC there, which
92  * might come only much later if this is a standby server.
93  */
95  {
96  report_invalid_page(WARNING, node, forkno, blkno, present);
97  elog(PANIC, "WAL contains references to invalid pages");
98  }
99 
100  /*
101  * Log references to invalid pages at DEBUG1 level. This allows some
102  * tracing of the cause (note the elog context mechanism will tell us
103  * something about the XLOG record that generated the reference).
104  */
106  report_invalid_page(DEBUG1, node, forkno, blkno, present);
107 
108  if (invalid_page_tab == NULL)
109  {
110  /* create hash table when first needed */
111  HASHCTL ctl;
112 
113  memset(&ctl, 0, sizeof(ctl));
114  ctl.keysize = sizeof(xl_invalid_page_key);
115  ctl.entrysize = sizeof(xl_invalid_page);
116 
117  invalid_page_tab = hash_create("XLOG invalid-page table",
118  100,
119  &ctl,
121  }
122 
123  /* we currently assume xl_invalid_page_key contains no padding */
124  key.node = node;
125  key.forkno = forkno;
126  key.blkno = blkno;
127  hentry = (xl_invalid_page *)
128  hash_search(invalid_page_tab, (void *) &key, HASH_ENTER, &found);
129 
130  if (!found)
131  {
132  /* hash_search already filled in the key */
133  hentry->present = present;
134  }
135  else
136  {
137  /* repeat reference ... leave "present" as it was */
138  }
139 }
140 
141 /* Forget any invalid pages >= minblkno, because they've been dropped */
142 static void
144 {
146  xl_invalid_page *hentry;
147 
148  if (invalid_page_tab == NULL)
149  return; /* nothing to do */
150 
151  hash_seq_init(&status, invalid_page_tab);
152 
153  while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
154  {
155  if (RelFileNodeEquals(hentry->key.node, node) &&
156  hentry->key.forkno == forkno &&
157  hentry->key.blkno >= minblkno)
158  {
160  {
161  char *path = relpathperm(hentry->key.node, forkno);
162 
163  elog(DEBUG2, "page %u of relation %s has been dropped",
164  hentry->key.blkno, path);
165  pfree(path);
166  }
167 
168  if (hash_search(invalid_page_tab,
169  (void *) &hentry->key,
170  HASH_REMOVE, NULL) == NULL)
171  elog(ERROR, "hash table corrupted");
172  }
173  }
174 }
175 
176 /* Forget any invalid pages in a whole database */
177 static void
179 {
181  xl_invalid_page *hentry;
182 
183  if (invalid_page_tab == NULL)
184  return; /* nothing to do */
185 
186  hash_seq_init(&status, invalid_page_tab);
187 
188  while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
189  {
190  if (hentry->key.node.dbNode == dbid)
191  {
193  {
194  char *path = relpathperm(hentry->key.node, hentry->key.forkno);
195 
196  elog(DEBUG2, "page %u of relation %s has been dropped",
197  hentry->key.blkno, path);
198  pfree(path);
199  }
200 
201  if (hash_search(invalid_page_tab,
202  (void *) &hentry->key,
203  HASH_REMOVE, NULL) == NULL)
204  elog(ERROR, "hash table corrupted");
205  }
206  }
207 }
208 
209 /* Are there any unresolved references to invalid pages? */
210 bool
212 {
213  if (invalid_page_tab != NULL &&
214  hash_get_num_entries(invalid_page_tab) > 0)
215  return true;
216  return false;
217 }
218 
219 /* Complain about any remaining invalid-page entries */
220 void
222 {
224  xl_invalid_page *hentry;
225  bool foundone = false;
226 
227  if (invalid_page_tab == NULL)
228  return; /* nothing to do */
229 
230  hash_seq_init(&status, invalid_page_tab);
231 
232  /*
233  * Our strategy is to emit WARNING messages for all remaining entries and
234  * only PANIC after we've dumped all the available info.
235  */
236  while ((hentry = (xl_invalid_page *) hash_seq_search(&status)) != NULL)
237  {
238  report_invalid_page(WARNING, hentry->key.node, hentry->key.forkno,
239  hentry->key.blkno, hentry->present);
240  foundone = true;
241  }
242 
243  if (foundone)
244  elog(PANIC, "WAL contains references to invalid pages");
245 
246  hash_destroy(invalid_page_tab);
247  invalid_page_tab = NULL;
248 }
249 
250 
251 /*
252  * XLogReadBufferForRedo
253  * Read a page during XLOG replay
254  *
255  * Reads a block referenced by a WAL record into shared buffer cache, and
256  * determines what needs to be done to redo the changes to it. If the WAL
257  * record includes a full-page image of the page, it is restored.
258  *
259  * 'lsn' is the LSN of the record being replayed. It is compared with the
260  * page's LSN to determine if the record has already been replayed.
261  * 'block_id' is the ID number the block was registered with, when the WAL
262  * record was created.
263  *
264  * Returns one of the following:
265  *
266  * BLK_NEEDS_REDO - changes from the WAL record need to be applied
267  * BLK_DONE - block doesn't need replaying
268  * BLK_RESTORED - block was restored from a full-page image included in
269  * the record
270  * BLK_NOTFOUND - block was not found (because it was truncated away by
271  * an operation later in the WAL stream)
272  *
273  * On return, the buffer is locked in exclusive-mode, and returned in *buf.
274  * Note that the buffer is locked and returned even if it doesn't need
275  * replaying. (Getting the buffer lock is not really necessary during
276  * single-process crash recovery, but some subroutines such as MarkBufferDirty
277  * will complain if we don't have the lock. In hot standby mode it's
278  * definitely necessary.)
279  *
280  * Note: when a backup block is available in XLOG with the BKPIMAGE_APPLY flag
281  * set, we restore it, even if the page in the database appears newer. This
282  * is to protect ourselves against database pages that were partially or
283  * incorrectly written during a crash. We assume that the XLOG data must be
284  * good because it has passed a CRC check, while the database page might not
285  * be. This will force us to replay all subsequent modifications of the page
286  * that appear in XLOG, rather than possibly ignoring them as already
287  * applied, but that's not a huge drawback.
288  */
291  Buffer *buf)
292 {
293  return XLogReadBufferForRedoExtended(record, block_id, RBM_NORMAL,
294  false, buf);
295 }
296 
297 /*
298  * Pin and lock a buffer referenced by a WAL record, for the purpose of
299  * re-initializing it.
300  */
301 Buffer
303 {
304  Buffer buf;
305 
306  XLogReadBufferForRedoExtended(record, block_id, RBM_ZERO_AND_LOCK, false,
307  &buf);
308  return buf;
309 }
310 
311 /*
312  * XLogReadBufferForRedoExtended
313  * Like XLogReadBufferForRedo, but with extra options.
314  *
315  * In RBM_ZERO_* modes, if the page doesn't exist, the relation is extended
316  * with all-zeroes pages up to the referenced block number. In
317  * RBM_ZERO_AND_LOCK and RBM_ZERO_AND_CLEANUP_LOCK modes, the return value
318  * is always BLK_NEEDS_REDO.
319  *
320  * (The RBM_ZERO_AND_CLEANUP_LOCK mode is redundant with the get_cleanup_lock
321  * parameter. Do not use an inconsistent combination!)
322  *
323  * If 'get_cleanup_lock' is true, a "cleanup lock" is acquired on the buffer
324  * using LockBufferForCleanup(), instead of a regular exclusive lock.
325  */
328  uint8 block_id,
329  ReadBufferMode mode, bool get_cleanup_lock,
330  Buffer *buf)
331 {
332  XLogRecPtr lsn = record->EndRecPtr;
333  RelFileNode rnode;
334  ForkNumber forknum;
335  BlockNumber blkno;
336  Page page;
337  bool zeromode;
338  bool willinit;
339 
340  if (!XLogRecGetBlockTag(record, block_id, &rnode, &forknum, &blkno))
341  {
342  /* Caller specified a bogus block_id */
343  elog(PANIC, "failed to locate backup block with ID %d", block_id);
344  }
345 
346  /*
347  * Make sure that if the block is marked with WILL_INIT, the caller is
348  * going to initialize it. And vice versa.
349  */
350  zeromode = (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK);
351  willinit = (record->blocks[block_id].flags & BKPBLOCK_WILL_INIT) != 0;
352  if (willinit && !zeromode)
353  elog(PANIC, "block with WILL_INIT flag in WAL record must be zeroed by redo routine");
354  if (!willinit && zeromode)
355  elog(PANIC, "block to be initialized in redo routine must be marked with WILL_INIT flag in the WAL record");
356 
357  /* If it has a full-page image and it should be restored, do it. */
358  if (XLogRecBlockImageApply(record, block_id))
359  {
360  Assert(XLogRecHasBlockImage(record, block_id));
361  *buf = XLogReadBufferExtended(rnode, forknum, blkno,
362  get_cleanup_lock ? RBM_ZERO_AND_CLEANUP_LOCK : RBM_ZERO_AND_LOCK);
363  page = BufferGetPage(*buf);
364  if (!RestoreBlockImage(record, block_id, page))
365  elog(ERROR, "failed to restore block image");
366 
367  /*
368  * The page may be uninitialized. If so, we can't set the LSN because
369  * that would corrupt the page.
370  */
371  if (!PageIsNew(page))
372  {
373  PageSetLSN(page, lsn);
374  }
375 
376  MarkBufferDirty(*buf);
377 
378  /*
379  * At the end of crash recovery the init forks of unlogged relations
380  * are copied, without going through shared buffers. So we need to
381  * force the on-disk state of init forks to always be in sync with the
382  * state in shared buffers.
383  */
384  if (forknum == INIT_FORKNUM)
385  FlushOneBuffer(*buf);
386 
387  return BLK_RESTORED;
388  }
389  else
390  {
391  *buf = XLogReadBufferExtended(rnode, forknum, blkno, mode);
392  if (BufferIsValid(*buf))
393  {
394  if (mode != RBM_ZERO_AND_LOCK && mode != RBM_ZERO_AND_CLEANUP_LOCK)
395  {
396  if (get_cleanup_lock)
397  LockBufferForCleanup(*buf);
398  else
400  }
401  if (lsn <= PageGetLSN(BufferGetPage(*buf)))
402  return BLK_DONE;
403  else
404  return BLK_NEEDS_REDO;
405  }
406  else
407  return BLK_NOTFOUND;
408  }
409 }
410 
411 /*
412  * XLogReadBufferExtended
413  * Read a page during XLOG replay
414  *
415  * This is functionally comparable to ReadBufferExtended. There's some
416  * differences in the behavior wrt. the "mode" argument:
417  *
418  * In RBM_NORMAL mode, if the page doesn't exist, or contains all-zeroes, we
419  * return InvalidBuffer. In this case the caller should silently skip the
420  * update on this page. (In this situation, we expect that the page was later
421  * dropped or truncated. If we don't see evidence of that later in the WAL
422  * sequence, we'll complain at the end of WAL replay.)
423  *
424  * In RBM_ZERO_* modes, if the page doesn't exist, the relation is extended
425  * with all-zeroes pages up to the given block number.
426  *
427  * In RBM_NORMAL_NO_LOG mode, we return InvalidBuffer if the page doesn't
428  * exist, and we don't check for all-zeroes. Thus, no log entry is made
429  * to imply that the page should be dropped or truncated later.
430  *
431  * NB: A redo function should normally not call this directly. To get a page
432  * to modify, use XLogReadBufferForRedoExtended instead. It is important that
433  * all pages modified by a WAL record are registered in the WAL records, or
434  * they will be invisible to tools that that need to know which pages are
435  * modified.
436  */
437 Buffer
439  BlockNumber blkno, ReadBufferMode mode)
440 {
441  BlockNumber lastblock;
442  Buffer buffer;
443  SMgrRelation smgr;
444 
445  Assert(blkno != P_NEW);
446 
447  /* Open the relation at smgr level */
448  smgr = smgropen(rnode, InvalidBackendId);
449 
450  /*
451  * Create the target file if it doesn't already exist. This lets us cope
452  * if the replay sequence contains writes to a relation that is later
453  * deleted. (The original coding of this routine would instead suppress
454  * the writes, but that seems like it risks losing valuable data if the
455  * filesystem loses an inode during a crash. Better to write the data
456  * until we are actually told to delete the file.)
457  */
458  smgrcreate(smgr, forknum, true);
459 
460  lastblock = smgrnblocks(smgr, forknum);
461 
462  if (blkno < lastblock)
463  {
464  /* page exists in file */
465  buffer = ReadBufferWithoutRelcache(rnode, forknum, blkno,
466  mode, NULL);
467  }
468  else
469  {
470  /* hm, page doesn't exist in file */
471  if (mode == RBM_NORMAL)
472  {
473  log_invalid_page(rnode, forknum, blkno, false);
474  return InvalidBuffer;
475  }
476  if (mode == RBM_NORMAL_NO_LOG)
477  return InvalidBuffer;
478  /* OK to extend the file */
479  /* we do this in recovery only - no rel-extension lock needed */
481  buffer = InvalidBuffer;
482  do
483  {
484  if (buffer != InvalidBuffer)
485  {
486  if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
488  ReleaseBuffer(buffer);
489  }
490  buffer = ReadBufferWithoutRelcache(rnode, forknum,
491  P_NEW, mode, NULL);
492  }
493  while (BufferGetBlockNumber(buffer) < blkno);
494  /* Handle the corner case that P_NEW returns non-consecutive pages */
495  if (BufferGetBlockNumber(buffer) != blkno)
496  {
497  if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
499  ReleaseBuffer(buffer);
500  buffer = ReadBufferWithoutRelcache(rnode, forknum, blkno,
501  mode, NULL);
502  }
503  }
504 
505  if (mode == RBM_NORMAL)
506  {
507  /* check that page has been initialized */
508  Page page = (Page) BufferGetPage(buffer);
509 
510  /*
511  * We assume that PageIsNew is safe without a lock. During recovery,
512  * there should be no other backends that could modify the buffer at
513  * the same time.
514  */
515  if (PageIsNew(page))
516  {
517  ReleaseBuffer(buffer);
518  log_invalid_page(rnode, forknum, blkno, true);
519  return InvalidBuffer;
520  }
521  }
522 
523  return buffer;
524 }
525 
526 /*
527  * Struct actually returned by XLogFakeRelcacheEntry, though the declared
528  * return type is Relation.
529  */
530 typedef struct
531 {
532  RelationData reldata; /* Note: this must be first */
535 
537 
538 /*
539  * Create a fake relation cache entry for a physical relation
540  *
541  * It's often convenient to use the same functions in XLOG replay as in the
542  * main codepath, but those functions typically work with a relcache entry.
543  * We don't have a working relation cache during XLOG replay, but this
544  * function can be used to create a fake relcache entry instead. Only the
545  * fields related to physical storage, like rd_rel, are initialized, so the
546  * fake entry is only usable in low-level operations like ReadBuffer().
547  *
548  * Caller must free the returned entry with FreeFakeRelcacheEntry().
549  */
550 Relation
552 {
553  FakeRelCacheEntry fakeentry;
554  Relation rel;
555 
557 
558  /* Allocate the Relation struct and all related space in one block. */
559  fakeentry = palloc0(sizeof(FakeRelCacheEntryData));
560  rel = (Relation) fakeentry;
561 
562  rel->rd_rel = &fakeentry->pgc;
563  rel->rd_node = rnode;
564  /* We will never be working with temp rels during recovery */
566 
567  /* It must be a permanent table if we're in recovery. */
568  rel->rd_rel->relpersistence = RELPERSISTENCE_PERMANENT;
569 
570  /* We don't know the name of the relation; use relfilenode instead */
571  sprintf(RelationGetRelationName(rel), "%u", rnode.relNode);
572 
573  /*
574  * We set up the lockRelId in case anything tries to lock the dummy
575  * relation. Note that this is fairly bogus since relNode may be
576  * different from the relation's OID. It shouldn't really matter though,
577  * since we are presumably running by ourselves and can't have any lock
578  * conflicts ...
579  */
580  rel->rd_lockInfo.lockRelId.dbId = rnode.dbNode;
581  rel->rd_lockInfo.lockRelId.relId = rnode.relNode;
582 
583  rel->rd_smgr = NULL;
584 
585  return rel;
586 }
587 
588 /*
589  * Free a fake relation cache entry.
590  */
591 void
593 {
594  /* make sure the fakerel is not referenced by the SmgrRelation anymore */
595  if (fakerel->rd_smgr != NULL)
596  smgrclearowner(&fakerel->rd_smgr, fakerel->rd_smgr);
597  pfree(fakerel);
598 }
599 
600 /*
601  * Drop a relation during XLOG replay
602  *
603  * This is called when the relation is about to be deleted; we need to remove
604  * any open "invalid-page" records for the relation.
605  */
606 void
608 {
609  forget_invalid_pages(rnode, forknum, 0);
610 }
611 
612 /*
613  * Drop a whole database during XLOG replay
614  *
615  * As above, but for DROP DATABASE instead of dropping a single rel
616  */
617 void
619 {
620  /*
621  * This is unnecessarily heavy-handed, as it will close SMgrRelation
622  * objects for other databases as well. DROP DATABASE occurs seldom enough
623  * that it's not worth introducing a variant of smgrclose for just this
624  * purpose. XXX: Or should we rather leave the smgr entries dangling?
625  */
626  smgrcloseall();
627 
629 }
630 
631 /*
632  * Truncate a relation during XLOG replay
633  *
634  * We need to clean up any open "invalid-page" records for the dropped pages.
635  */
636 void
638  BlockNumber nblocks)
639 {
640  forget_invalid_pages(rnode, forkNum, nblocks);
641 }
642 
643 /*
644  * Read 'count' bytes from WAL into 'buf', starting at location 'startptr'
645  * in timeline 'tli'.
646  *
647  * Will open, and keep open, one WAL segment stored in the static file
648  * descriptor 'sendFile'. This means if XLogRead is used once, there will
649  * always be one descriptor left open until the process ends, but never
650  * more than one.
651  *
652  * XXX This is very similar to pg_waldump's XLogDumpXLogRead and to XLogRead
653  * in walsender.c but for small differences (such as lack of elog() in
654  * frontend). Probably these should be merged at some point.
655  */
656 static void
657 XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
658 {
659  char *p;
660  XLogRecPtr recptr;
661  Size nbytes;
662 
663  /* state maintained across calls */
664  static int sendFile = -1;
665  static XLogSegNo sendSegNo = 0;
666  static TimeLineID sendTLI = 0;
667  static uint32 sendOff = 0;
668 
669  p = buf;
670  recptr = startptr;
671  nbytes = count;
672 
673  while (nbytes > 0)
674  {
675  uint32 startoff;
676  int segbytes;
677  int readbytes;
678 
679  startoff = recptr % XLogSegSize;
680 
681  /* Do we need to switch to a different xlog segment? */
682  if (sendFile < 0 || !XLByteInSeg(recptr, sendSegNo) ||
683  sendTLI != tli)
684  {
685  char path[MAXPGPATH];
686 
687  if (sendFile >= 0)
688  close(sendFile);
689 
690  XLByteToSeg(recptr, sendSegNo);
691 
692  XLogFilePath(path, tli, sendSegNo);
693 
694  sendFile = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
695 
696  if (sendFile < 0)
697  {
698  if (errno == ENOENT)
699  ereport(ERROR,
701  errmsg("requested WAL segment %s has already been removed",
702  path)));
703  else
704  ereport(ERROR,
706  errmsg("could not open file \"%s\": %m",
707  path)));
708  }
709  sendOff = 0;
710  sendTLI = tli;
711  }
712 
713  /* Need to seek in the file? */
714  if (sendOff != startoff)
715  {
716  if (lseek(sendFile, (off_t) startoff, SEEK_SET) < 0)
717  {
718  char path[MAXPGPATH];
719 
720  XLogFilePath(path, tli, sendSegNo);
721 
722  ereport(ERROR,
724  errmsg("could not seek in log segment %s to offset %u: %m",
725  path, startoff)));
726  }
727  sendOff = startoff;
728  }
729 
730  /* How many bytes are within this segment? */
731  if (nbytes > (XLogSegSize - startoff))
732  segbytes = XLogSegSize - startoff;
733  else
734  segbytes = nbytes;
735 
737  readbytes = read(sendFile, p, segbytes);
739  if (readbytes <= 0)
740  {
741  char path[MAXPGPATH];
742 
743  XLogFilePath(path, tli, sendSegNo);
744 
745  ereport(ERROR,
747  errmsg("could not read from log segment %s, offset %u, length %lu: %m",
748  path, sendOff, (unsigned long) segbytes)));
749  }
750 
751  /* Update state for read */
752  recptr += readbytes;
753 
754  sendOff += readbytes;
755  nbytes -= readbytes;
756  p += readbytes;
757  }
758 }
759 
760 /*
761  * Determine which timeline to read an xlog page from and set the
762  * XLogReaderState's currTLI to that timeline ID.
763  *
764  * We care about timelines in xlogreader when we might be reading xlog
765  * generated prior to a promotion, either if we're currently a standby in
766  * recovery or if we're a promoted master reading xlogs generated by the old
767  * master before our promotion.
768  *
769  * wantPage must be set to the start address of the page to read and
770  * wantLength to the amount of the page that will be read, up to
771  * XLOG_BLCKSZ. If the amount to be read isn't known, pass XLOG_BLCKSZ.
772  *
773  * We switch to an xlog segment from the new timeline eagerly when on a
774  * historical timeline, as soon as we reach the start of the xlog segment
775  * containing the timeline switch. The server copied the segment to the new
776  * timeline so all the data up to the switch point is the same, but there's no
777  * guarantee the old segment will still exist. It may have been deleted or
778  * renamed with a .partial suffix so we can't necessarily keep reading from
779  * the old TLI even though tliSwitchPoint says it's OK.
780  *
781  * We can't just check the timeline when we read a page on a different segment
782  * to the last page. We could've received a timeline switch from a cascading
783  * upstream, so the current segment ends apruptly (possibly getting renamed to
784  * .partial) and we have to switch to a new one. Even in the middle of reading
785  * a page we could have to dump the cached page and switch to a new TLI.
786  *
787  * Because of this, callers MAY NOT assume that currTLI is the timeline that
788  * will be in a page's xlp_tli; the page may begin on an older timeline or we
789  * might be reading from historical timeline data on a segment that's been
790  * copied to a new timeline.
791  *
792  * The caller must also make sure it doesn't read past the current replay
793  * position (using GetWalRcvWriteRecPtr) if executing in recovery, so it
794  * doesn't fail to notice that the current timeline became historical. The
795  * caller must also update ThisTimeLineID with the result of
796  * GetWalRcvWriteRecPtr and must check RecoveryInProgress().
797  */
798 void
800 {
801  const XLogRecPtr lastReadPage = state->readSegNo * XLogSegSize + state->readOff;
802 
803  Assert(wantPage != InvalidXLogRecPtr && wantPage % XLOG_BLCKSZ == 0);
804  Assert(wantLength <= XLOG_BLCKSZ);
805  Assert(state->readLen == 0 || state->readLen <= XLOG_BLCKSZ);
806 
807  /*
808  * If the desired page is currently read in and valid, we have nothing to do.
809  *
810  * The caller should've ensured that it didn't previously advance readOff
811  * past the valid limit of this timeline, so it doesn't matter if the current
812  * TLI has since become historical.
813  */
814  if (lastReadPage == wantPage &&
815  state->readLen != 0 &&
816  lastReadPage + state->readLen >= wantPage + Min(wantLength,XLOG_BLCKSZ-1))
817  return;
818 
819  /*
820  * If we're reading from the current timeline, it hasn't become historical
821  * and the page we're reading is after the last page read, we can again
822  * just carry on. (Seeking backwards requires a check to make sure the older
823  * page isn't on a prior timeline).
824  *
825  * ThisTimeLineID might've become historical since we last looked, but the
826  * caller is required not to read past the flush limit it saw at the time
827  * it looked up the timeline. There's nothing we can do about it if
828  * StartupXLOG() renames it to .partial concurrently.
829  */
830  if (state->currTLI == ThisTimeLineID && wantPage >= lastReadPage)
831  {
833  return;
834  }
835 
836  /*
837  * If we're just reading pages from a previously validated historical
838  * timeline and the timeline we're reading from is valid until the
839  * end of the current segment we can just keep reading.
840  */
841  if (state->currTLIValidUntil != InvalidXLogRecPtr &&
842  state->currTLI != ThisTimeLineID &&
843  state->currTLI != 0 &&
844  (wantPage + wantLength) / XLogSegSize < state->currTLIValidUntil / XLogSegSize)
845  return;
846 
847  /*
848  * If we reach this point we're either looking up a page for random access,
849  * the current timeline just became historical, or we're reading from a new
850  * segment containing a timeline switch. In all cases we need to determine
851  * the newest timeline on the segment.
852  *
853  * If it's the current timeline we can just keep reading from here unless
854  * we detect a timeline switch that makes the current timeline historical.
855  * If it's a historical timeline we can read all the segment on the newest
856  * timeline because it contains all the old timelines' data too. So only
857  * one switch check is required.
858  */
859  {
860  /*
861  * We need to re-read the timeline history in case it's been changed
862  * by a promotion or replay from a cascaded replica.
863  */
864  List *timelineHistory = readTimeLineHistory(ThisTimeLineID);
865 
866  XLogRecPtr endOfSegment = (((wantPage / XLogSegSize) + 1) * XLogSegSize) - 1;
867 
868  Assert(wantPage / XLogSegSize == endOfSegment / XLogSegSize);
869 
870  /* Find the timeline of the last LSN on the segment containing wantPage. */
871  state->currTLI = tliOfPointInHistory(endOfSegment, timelineHistory);
872  state->currTLIValidUntil = tliSwitchPoint(state->currTLI, timelineHistory,
873  &state->nextTLI);
874 
876  wantPage + wantLength < state->currTLIValidUntil);
877 
878  list_free_deep(timelineHistory);
879 
880  elog(DEBUG3, "switched to timeline %u valid until %X/%X",
881  state->currTLI,
882  (uint32)(state->currTLIValidUntil >> 32),
883  (uint32)(state->currTLIValidUntil));
884  }
885 }
886 
887 /*
888  * read_page callback for reading local xlog files
889  *
890  * Public because it would likely be very helpful for someone writing another
891  * output method outside walsender, e.g. in a bgworker.
892  *
893  * TODO: The walsender has its own version of this, but it relies on the
894  * walsender's latch being set whenever WAL is flushed. No such infrastructure
895  * exists for normal backends, so we have to do a check/sleep/repeat style of
896  * loop for now.
897  */
898 int
900  int reqLen, XLogRecPtr targetRecPtr, char *cur_page,
901  TimeLineID *pageTLI)
902 {
903  XLogRecPtr read_upto,
904  loc;
905  int count;
906 
907  loc = targetPagePtr + reqLen;
908 
909  /* Loop waiting for xlog to be available if necessary */
910  while (1)
911  {
912  /*
913  * Determine the limit of xlog we can currently read to, and what the
914  * most recent timeline is.
915  *
916  * RecoveryInProgress() will update ThisTimeLineID when it first
917  * notices recovery finishes, so we only have to maintain it for the
918  * local process until recovery ends.
919  */
920  if (!RecoveryInProgress())
921  read_upto = GetFlushRecPtr();
922  else
923  read_upto = GetXLogReplayRecPtr(&ThisTimeLineID);
924 
925  *pageTLI = ThisTimeLineID;
926 
927  /*
928  * Check which timeline to get the record from.
929  *
930  * We have to do it each time through the loop because if we're in
931  * recovery as a cascading standby, the current timeline might've
932  * become historical. We can't rely on RecoveryInProgress() because
933  * in a standby configuration like
934  *
935  * A => B => C
936  *
937  * if we're a logical decoding session on C, and B gets promoted, our
938  * timeline will change while we remain in recovery.
939  *
940  * We can't just keep reading from the old timeline as the last WAL
941  * archive in the timeline will get renamed to .partial by StartupXLOG().
942  *
943  * If that happens after our caller updated ThisTimeLineID but before
944  * we actually read the xlog page, we might still try to read from the
945  * old (now renamed) segment and fail. There's not much we can do about
946  * this, but it can only happen when we're a leaf of a cascading
947  * standby whose master gets promoted while we're decoding, so a
948  * one-off ERROR isn't too bad.
949  */
950  XLogReadDetermineTimeline(state, targetPagePtr, reqLen);
951 
952  if (state->currTLI == ThisTimeLineID)
953  {
954 
955  if (loc <= read_upto)
956  break;
957 
959  pg_usleep(1000L);
960  }
961  else
962  {
963  /*
964  * We're on a historical timeline, so limit reading to the switch
965  * point where we moved to the next timeline.
966  *
967  * We don't need to GetFlushRecPtr or GetXLogReplayRecPtr. We know
968  * about the new timeline, so we must've received past the end of
969  * it.
970  */
971  read_upto = state->currTLIValidUntil;
972 
973  /*
974  * Setting pageTLI to our wanted record's TLI is slightly wrong;
975  * the page might begin on an older timeline if it contains a
976  * timeline switch, since its xlog segment will have been copied
977  * from the prior timeline. This is pretty harmless though, as
978  * nothing cares so long as the timeline doesn't go backwards. We
979  * should read the page header instead; FIXME someday.
980  */
981  *pageTLI = state->currTLI;
982 
983  /* No need to wait on a historical timeline */
984  break;
985  }
986  }
987 
988  if (targetPagePtr + XLOG_BLCKSZ <= read_upto)
989  {
990  /*
991  * more than one block available; read only that block, have caller
992  * come back if they need more.
993  */
994  count = XLOG_BLCKSZ;
995  }
996  else if (targetPagePtr + reqLen > read_upto)
997  {
998  /* not enough data there */
999  return -1;
1000  }
1001  else
1002  {
1003  /* enough bytes available to satisfy the request */
1004  count = read_upto - targetPagePtr;
1005  }
1006 
1007  /*
1008  * Even though we just determined how much of the page can be validly read
1009  * as 'count', read the whole page anyway. It's guaranteed to be
1010  * zero-padded up to the page boundary if it's incomplete.
1011  */
1012  XLogRead(cur_page, *pageTLI, targetPagePtr, XLOG_BLCKSZ);
1013 
1014  /* number of valid bytes in the buffer */
1015  return count;
1016 }
#define XLogSegSize
Definition: xlog_internal.h:92
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Definition: xlogutils.c:211
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