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Prevent race condition while reading relmapper file.

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Contrary to the comment here, POSIX does not guarantee atomicity of a
read(), if another process calls write() concurrently. Or at least Linux
does not. Add locking to load_relmap_file() to avoid the race condition.

Fixes bug #17064. Thanks to Alexander Lakhin for the report and test case.

Backpatch-through: 9.6, all supported versions.
Discussion: https://www.postgresql.org/message-id/17064-bb0d7904ef72add3@postgresql.org
This commit is contained in:
Heikki Linnakangas 2021-06-24 10:45:23 +03:00
parent a73bd49c69
commit caac196507
1 changed files with 20 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

34
src/backend/utils/cache/relmapper.c vendored
View File

@ -136,7 +136,7 @@ static void apply_map_update(RelMapFile *map, Oid relationId, Oid fileNode,
bool add_okay);
static void merge_map_updates(RelMapFile *map, const RelMapFile *updates,
bool add_okay);
static void load_relmap_file(bool shared);
static void load_relmap_file(bool shared, bool lock_held);
static void write_relmap_file(bool shared, RelMapFile *newmap,
bool write_wal, bool send_sinval, bool preserve_files,
Oid dbid, Oid tsid, const char *dbpath);
@ -405,12 +405,12 @@ RelationMapInvalidate(bool shared)
if (shared)
{
if (shared_map.magic == RELMAPPER_FILEMAGIC)
load_relmap_file(true);
load_relmap_file(true, false);
}
else
{
if (local_map.magic == RELMAPPER_FILEMAGIC)
load_relmap_file(false);
load_relmap_file(false, false);
}
}
@ -425,9 +425,9 @@ void
RelationMapInvalidateAll(void)
{
if (shared_map.magic == RELMAPPER_FILEMAGIC)
load_relmap_file(true);
load_relmap_file(true, false);
if (local_map.magic == RELMAPPER_FILEMAGIC)
load_relmap_file(false);
load_relmap_file(false, false);
}
/*
@ -612,7 +612,7 @@ RelationMapInitializePhase2(void)
/*
* Load the shared map file, die on error.
*/
load_relmap_file(true);
load_relmap_file(true, false);
}
/*
@ -633,7 +633,7 @@ RelationMapInitializePhase3(void)
/*
* Load the local map file, die on error.
*/
load_relmap_file(false);
load_relmap_file(false, false);
}
/*
@ -695,7 +695,7 @@ RestoreRelationMap(char *startAddress)
* Note that the local case requires DatabasePath to be set up.
*/
static void
load_relmap_file(bool shared)
load_relmap_file(bool shared, bool lock_held)
{
RelMapFile *map;
char mapfilename[MAXPGPATH];
@ -725,12 +725,15 @@ load_relmap_file(bool shared)
mapfilename)));
/*
* Note: we could take RelationMappingLock in shared mode here, but it
* seems unnecessary since our read() should be atomic against any
* concurrent updater's write(). If the file is updated shortly after we
* look, the sinval signaling mechanism will make us re-read it before we
* are able to access any relation that's affected by the change.
* Grab the lock to prevent the file from being updated while we read it,
* unless the caller is already holding the lock. If the file is updated
* shortly after we look, the sinval signaling mechanism will make us
* re-read it before we are able to access any relation that's affected by
* the change.
*/
if (!lock_held)
LWLockAcquire(RelationMappingLock, LW_SHARED);
pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_READ);
r = read(fd, map, sizeof(RelMapFile));
if (r != sizeof(RelMapFile))
@ -747,6 +750,9 @@ load_relmap_file(bool shared)
}
pgstat_report_wait_end();
if (!lock_held)
LWLockRelease(RelationMappingLock);
if (CloseTransientFile(fd))
ereport(FATAL,
(errcode_for_file_access(),
@ -969,7 +975,7 @@ perform_relmap_update(bool shared, const RelMapFile *updates)
LWLockAcquire(RelationMappingLock, LW_EXCLUSIVE);
/* Be certain we see any other updates just made */
load_relmap_file(shared);
load_relmap_file(shared, true);
/* Prepare updated data in a local variable */
if (shared)