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Commit43c9790

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Try to handle torn reads of pg_control in frontend.
Some of our src/bin tools read the control file without any kind ofinterlocking against concurrent writes from the server. At least ext4and 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 thechecksum doesn't match, until we get two reads in a row with the samebad checksum. This is not guaranteed to reach the right conclusion, butit seems very likely to. Thanks to Tom Lane for this suggestion.Various ideas for interlocking or atomicity were considered toocomplicated, 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 aheuristic 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
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‎src/common/controldata_utils.c

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Original file line numberDiff line numberDiff line change
@@ -55,12 +55,22 @@ get_controlfile(const char *DataDir, bool *crc_ok_p)
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charControlFilePath[MAXPGPATH];
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pg_crc32ccrc;
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intr;
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#ifdefFRONTEND
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pg_crc32clast_crc;
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intretries=0;
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#endif
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AssertArg(crc_ok_p);
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ControlFile=palloc(sizeof(ControlFileData));
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snprintf(ControlFilePath,MAXPGPATH,"%s/global/pg_control",DataDir);
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#ifdefFRONTEND
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INIT_CRC32C(last_crc);
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retry:
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#endif
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#ifndefFRONTEND
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if ((fd=OpenTransientFile(ControlFilePath,O_RDONLY |PG_BINARY))==-1)
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ereport(ERROR,
@@ -128,6 +138,26 @@ get_controlfile(const char *DataDir, bool *crc_ok_p)
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*crc_ok_p=EQ_CRC32C(crc,ControlFile->crc);
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#ifdefFRONTEND
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/*
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* If the server was writing at the same time, it is possible that we read
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* partially updated contents on some systems. If the CRC doesn't match,
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* retry a limited number of times until we compute the same bad CRC twice
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* in a row with a short sleep in between. Then the failure is unlikely
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* to be due to a concurrent write.
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*/
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if (!*crc_ok_p&&
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(retries==0|| !EQ_CRC32C(crc,last_crc))&&
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retries<10)
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{
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retries++;
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last_crc=crc;
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pg_usleep(10000);
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gotoretry;
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}
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#endif
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/* Make sure the control file is valid byte order. */
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if (ControlFile->pg_control_version %65536==0&&
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ControlFile->pg_control_version /65536!=0)

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