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Commitdac1ff3

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Replace durable_rename_excl() by durable_rename(), take two
durable_rename_excl() attempts to avoid overwriting any existing filesby using link() and unlink(), and it falls back to rename() on someplatforms (aka WIN32), which offers no such overwrite protection. Mostcallers use durable_rename_excl() just in case there is an existingfile, but in practice there shouldn't be one (see below for moredetails).Furthermore, failures during durable_rename_excl() can result inmultiple hard links to the same file. As per Nathan's tests, it ispossible to end up with two links to the same file in pg_wal after acrash just before unlink() during WAL recycling. Specifically, the testproduced links to the same file for the current WAL file and the nextone because the half-recycled WAL file was re-recycled upon restarting,leading to WAL corruption.This change replaces all the calls of durable_rename_excl() todurable_rename(). This removes the protection against accidentallyoverwriting an existing file, but some platforms are already livingwithout it and ordinarily there shouldn't be one. The function itselfis left around in case any extensions are using it. It will be removedon HEAD via a follow-up commit.Here is a summary of the existing callers of durable_rename_excl() (seesecond discussion link at the bottom), replaced by this commit. First,basic_archive used it to avoid overwriting an archive concurrentlycreated by another server, but as mentioned above, it will stilloverwrite files on some platforms. Second, xlog.c uses it to recyclepast WAL segments, where an overwrite should not happen (origin of thechange atf0e37a8) because there are protections about the WAL segmentto select when recycling an entry. The third and last area is relatedto the write of timeline history files. writeTimeLineHistory() willwrite a new timeline history file at the end of recovery on promotion,so there should be no such files for the same timeline.What remains is writeTimeLineHistoryFile(), that can be used in parallelby a WAL receiver and the startup process, and some digging of thebuildfarm shows that EEXIST from a WAL receiver can happen with an errorof "could not link file \"pg_wal/xlogtemp.NN\" to \"pg_wal/MM.history\",which would cause an automatic restart of the WAL receiver as it ispromoted to FATAL, hence this should improve the stability of the WALreceiver as rename() would overwrite an existing TLI history filealready fetched by the startup process at recovery.This is a bug fix, but knowing the unlikeliness of the problem involvingone or more crashes at an exceptionally bad moment, no backpatch isdone. Also, I want to be careful with such changes (aaa3aed did theopposite of this change by removing HAVE_WORKING_LINK so as Windowswould do a link() rather than a rename() but this was notconcurrent-safe). A backpatch could be revisited in the future. Thisis the second time this change is attempted,ccfbd92 being the firstone, but this time no assertions are added for the case of a TLI historyfile written concurrently by the WAL receiver or the startup processbecause we can expect one to exist (some of the TAP tests are able totrigger with a proper timing).Author: Nathan BossartReviewed-by: Robert Haas, Kyotaro Horiguchi, Michael PaquierDiscussion:https://postgr.es/m/20220407182954.GA1231544@nathanxps13Discussion:https://postgr.es/m/Ym6GZbqQdlalSKSG@paquier.xyz
1 parent2ce648f commitdac1ff3

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3 files changed

+11
-21
lines changed

3 files changed

+11
-21
lines changed

‎contrib/basic_archive/basic_archive.c

Lines changed: 3 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -282,9 +282,10 @@ basic_archive_file_internal(const char *file, const char *path)
282282

283283
/*
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* Sync the temporary file to disk and move it to its final destination.
285-
* This will fail if destination already exists.
285+
* Note that this will overwrite any existing file, but this is only
286+
* possible if someone else created the file since the stat() above.
286287
*/
287-
(void)durable_rename_excl(temp,destination,ERROR);
288+
(void)durable_rename(temp,destination,ERROR);
288289

289290
ereport(DEBUG1,
290291
(errmsg("archived \"%s\" via basic_archive",file)));

‎src/backend/access/transam/timeline.c

Lines changed: 5 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -441,12 +441,8 @@ writeTimeLineHistory(TimeLineID newTLI, TimeLineID parentTLI,
441441
* Now move the completed history file into place with its final name.
442442
*/
443443
TLHistoryFilePath(path,newTLI);
444-
445-
/*
446-
* Perform the rename using link if available, paranoidly trying to avoid
447-
* overwriting an existing file (there shouldn't be one).
448-
*/
449-
durable_rename_excl(tmppath,path,ERROR);
444+
Assert(access(path,F_OK)!=0&&errno==ENOENT);
445+
durable_rename(tmppath,path,ERROR);
450446

451447
/* The history file can be archived immediately. */
452448
if (XLogArchivingActive())
@@ -516,15 +512,11 @@ writeTimeLineHistoryFile(TimeLineID tli, char *content, int size)
516512
errmsg("could not close file \"%s\": %m",tmppath)));
517513

518514
/*
519-
* Now move the completed history file into place with its final name.
515+
* Now move the completed history file into place with its final name,
516+
* replacing any existing file with the same name.
520517
*/
521518
TLHistoryFilePath(path,tli);
522-
523-
/*
524-
* Perform the rename using link if available, paranoidly trying to avoid
525-
* overwriting an existing file (there shouldn't be one).
526-
*/
527-
durable_rename_excl(tmppath,path,ERROR);
519+
durable_rename(tmppath,path,ERROR);
528520
}
529521

530522
/*

‎src/backend/access/transam/xlog.c

Lines changed: 3 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -3323,14 +3323,11 @@ InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
33233323
}
33243324
}
33253325

3326-
/*
3327-
* Perform the rename using link if available, paranoidly trying to avoid
3328-
* overwriting an existing file (there shouldn't be one).
3329-
*/
3330-
if (durable_rename_excl(tmppath,path,LOG)!=0)
3326+
Assert(access(path,F_OK)!=0&&errno==ENOENT);
3327+
if (durable_rename(tmppath,path,LOG)!=0)
33313328
{
33323329
LWLockRelease(ControlFileLock);
3333-
/*durable_rename_excl already emitted log message */
3330+
/*durable_rename already emitted log message */
33343331
return false;
33353332
}
33363333

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