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Fix low-probability bug in relcache startup: write_irels wrote the
pg_internal.init file in-place, which meant that if another backend started at about the same time, it might read the incomplete file. init_irels tries to guard against that, but I have now seen a crash due to reading bad data from a partly-written file. (This may indicate a kernel bug on my platform? Not sure.) Anyway, clearly the safest course is to write the new pg_internal.init file under a unique temporary filename, and rename it into place only after it's all written.
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src/backend/utils/cache/relcache.c
vendored
27
src/backend/utils/cache/relcache.c
vendored
@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $Header: /cvsroot/pgsql/src/backend/utils/cache/relcache.c,v 1.93 2000/03/17 02:36:27 tgl Exp $
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* $Header: /cvsroot/pgsql/src/backend/utils/cache/relcache.c,v 1.94 2000/03/31 19:39:22 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -33,6 +33,7 @@
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#include <errno.h>
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#include <sys/file.h>
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#include <fcntl.h>
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#include <unistd.h>
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#include "postgres.h"
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@ -2266,14 +2267,26 @@ write_irels(void)
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int i;
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int relno;
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RelationBuildDescInfo bi;
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char tempfilename[MAXPGPATH];
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char finalfilename[MAXPGPATH];
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/*
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* We must write a temporary file and rename it into place. Otherwise,
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* another backend starting at about the same time might crash trying to
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* read the partially-complete file.
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*/
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snprintf(tempfilename, sizeof(tempfilename), "%s%c%s.%d",
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DatabasePath, SEP_CHAR, RELCACHE_INIT_FILENAME, MyProcPid);
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snprintf(finalfilename, sizeof(finalfilename), "%s%c%s",
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DatabasePath, SEP_CHAR, RELCACHE_INIT_FILENAME);
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#ifndef __CYGWIN32__
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fd = FileNameOpenFile(RELCACHE_INIT_FILENAME, O_WRONLY | O_CREAT | O_TRUNC, 0600);
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fd = PathNameOpenFile(tempfilename, O_WRONLY | O_CREAT | O_TRUNC, 0600);
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#else
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fd = FileNameOpenFile(RELCACHE_INIT_FILENAME, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, 0600);
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fd = PathNameOpenFile(tempfilename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, 0600);
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#endif
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if (fd < 0)
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elog(FATAL, "cannot create init file %s", RELCACHE_INIT_FILENAME);
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elog(FATAL, "cannot create init file %s", tempfilename);
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FileSeek(fd, 0L, SEEK_SET);
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@ -2397,4 +2410,10 @@ write_irels(void)
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}
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FileClose(fd);
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/*
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* And rename the temp file to its final name, deleting any previously-
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* existing init file.
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*/
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rename(tempfilename, finalfilename);
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}
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