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Try to handle torn reads of pg_control in frontend.

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Some of our src/bin tools read the control file without any kind of
interlocking against concurrent writes from the server.  At least ext4
and ntfs can expose partially modified contents when you do that.

For now, we'll try to tolerate this by retrying up to 10 times if the
checksum doesn't match, until we get two reads in a row with the same
bad checksum.  This is not guaranteed to reach the right conclusion, but
it seems very likely to.  Thanks to Tom Lane for this suggestion.

Various ideas for interlocking or atomicity were considered too
complicated, unportable or expensive given the lack of field reports,
but remain open for future reconsideration.

Back-patch as far as 12.  It doesn't seem like a good idea to put a
heuristic change for a very rare problem into the final release of 11.

Reviewed-by: Anton A. Melnikov <aamelnikov@inbox.ru>
Reviewed-by: David Steele <david@pgmasters.net>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20221123014224.xisi44byq3cf5psi%40awork3.anarazel.de
This commit is contained in:
Thomas Munro 2023-10-16 17:10:13 +13:00
parent 4817da51f6
commit 63a582222c
1 changed files with 30 additions and 0 deletions
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30
src/common/controldata_utils.c
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@ -56,12 +56,22 @@ get_controlfile(const char *DataDir, bool *crc_ok_p)
char ControlFilePath[MAXPGPATH];
pg_crc32c crc;
int r;
#ifdef FRONTEND
pg_crc32c last_crc;
int retries = 0;
#endif
Assert(crc_ok_p);
ControlFile = palloc_object(ControlFileData);
snprintf(ControlFilePath, MAXPGPATH, "%s/global/pg_control", DataDir);
#ifdef FRONTEND
INIT_CRC32C(last_crc);
retry:
#endif
#ifndef FRONTEND
if ((fd = OpenTransientFile(ControlFilePath, O_RDONLY | PG_BINARY)) == -1)
ereport(ERROR,
@ -117,6 +127,26 @@ get_controlfile(const char *DataDir, bool *crc_ok_p)
*crc_ok_p = EQ_CRC32C(crc, ControlFile->crc);
#ifdef FRONTEND
/*
* If the server was writing at the same time, it is possible that we read
* partially updated contents on some systems. If the CRC doesn't match,
* retry a limited number of times until we compute the same bad CRC twice
* in a row with a short sleep in between. Then the failure is unlikely
* to be due to a concurrent write.
*/
if (!*crc_ok_p &&
(retries == 0 || !EQ_CRC32C(crc, last_crc)) &&
retries < 10)
{
retries++;
last_crc = crc;
pg_usleep(10000);
goto retry;
}
#endif
/* Make sure the control file is valid byte order. */
if (ControlFile->pg_control_version % 65536 == 0 &&
ControlFile->pg_control_version / 65536 != 0)