postgres.git / summary / log / commit / refs

commit    00a0a428ef503d50b74b8a08d6365a4a0ce04c14
Author:   Andres Freund <andres@anarazel.de>
Date:     Sat Mar 28 18:52:11 2020 +0000

    Ensure snapshot is registered within ScanPgRelation().
    
    In 9.4 I added support to use a historical snapshot in
    ScanPgRelation(), while adding logical decoding. Unfortunately a
    conflict with the concurrent removal of SnapshotNow was incorrectly
    resolved, leading to an unregistered snapshot being used.
    
    It is not correct to use an unregistered (or non-active) snapshot for
    anything non-trivial, because catalog invalidations can cause the
    snapshot to be invalidated.
    
    Luckily it seems unlikely to actively cause problems in practice, as
    ScanPgRelation() requires that we already have a lock on the relation,
    we only look for a single row, and we don't appear to rely on the
    result's tid to be correct. It however is clearly wrong and potential
    negative consequences would likely be hard to find. So it seems worth
    backpatching the fix, even without a concrete hazard.
    
    Discussion: https://postgr.es/m/20200229052459.wzhqnbhrriezg4v2@alap3.anarazel.de
    Backpatch: 9.5-


 src/backend/utils/cache/relcache.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 462b1a9636a..e29efa10239 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -313,7 +313,7 @@ ScanPgRelation(Oid targetRelId, bool indexOK, bool force_non_historic)
 	Relation	pg_class_desc;
 	SysScanDesc pg_class_scan;
 	ScanKeyData key[1];
-	Snapshot	snapshot;
+	Snapshot	snapshot = NULL;
 
 	/*
 	 * If something goes wrong during backend startup, we might find ourselves
@@ -343,12 +343,12 @@ ScanPgRelation(Oid targetRelId, bool indexOK, bool force_non_historic)
 	/*
 	 * The caller might need a tuple that's newer than the one the historic
 	 * snapshot; currently the only case requiring to do so is looking up the
-	 * relfilenode of non mapped system relations during decoding.
+	 * relfilenode of non mapped system relations during decoding. That
+	 * snapshot cant't change in the midst of a relcache build, so there's no
+	 * need to register the snapshot.
 	 */
 	if (force_non_historic)
 		snapshot = GetNonHistoricCatalogSnapshot(RelationRelationId);
-	else
-		snapshot = GetCatalogSnapshot(RelationRelationId);
 
 	pg_class_scan = systable_beginscan(pg_class_desc, ClassOidIndexId,
 									   indexOK && criticalRelcachesBuilt,


[parent: deeda011b316]