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Prevent concurrent SimpleLruTruncate() for any given SLRU.

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The SimpleLruTruncate() header comment states the new coding rule.  To
achieve this, add locktype "frozenid" and two LWLocks.  This closes a
rare opportunity for data loss, which manifested as "apparent
wraparound" or "could not access status of transaction" errors.  Data
loss is more likely in pg_multixact, due to released branches' thin
margin between multiStopLimit and multiWrapLimit.  If a user's physical
replication primary logged ":  apparent wraparound" messages, the user
should rebuild standbys of that primary regardless of symptoms.  At less
risk is a cluster having emitted "not accepting commands" errors or
"must be vacuumed" warnings at some point.  One can test a cluster for
this data loss by running VACUUM FREEZE in every database.  Back-patch
to 9.5 (all supported versions).

Discussion: https://postgr.es/m/20190218073103.GA1434723@rfd.leadboat.com
This commit is contained in:
Noah Misch
2020-08-15 10:15:53 -07:00
parent 912fb290c5
commit 30e68a2abb
11 changed files with 119 additions and 15 deletions
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3
src/include/storage/lmgr.h
View File

@@ -59,6 +59,9 @@ extern bool ConditionalLockRelationForExtension(Relation relation,
LOCKMODE lockmode);
extern int RelationExtensionLockWaiterCount(Relation relation);
/* Lock to recompute pg_database.datfrozenxid in the current database */
extern void LockDatabaseFrozenIds(LOCKMODE lockmode);
/* Lock a page (currently only used within indexes) */
extern void LockPage(Relation relation, BlockNumber blkno, LOCKMODE lockmode);
extern bool ConditionalLockPage(Relation relation, BlockNumber blkno, LOCKMODE lockmode);

10
src/include/storage/lock.h
View File

@@ -139,6 +139,7 @@ typedef enum LockTagType
{
LOCKTAG_RELATION, /* whole relation */
LOCKTAG_RELATION_EXTEND, /* the right to extend a relation */
LOCKTAG_DATABASE_FROZEN_IDS, /* pg_database.datfrozenxid */
LOCKTAG_PAGE, /* one page of a relation */
LOCKTAG_TUPLE, /* one physical tuple */
LOCKTAG_TRANSACTION, /* transaction (for waiting for xact done) */
@@ -195,6 +196,15 @@ typedef struct LOCKTAG
(locktag).locktag_type = LOCKTAG_RELATION_EXTEND, \
(locktag).locktag_lockmethodid = DEFAULT_LOCKMETHOD)
/* ID info for frozen IDs is DB OID */
#define SET_LOCKTAG_DATABASE_FROZEN_IDS(locktag,dboid) \
((locktag).locktag_field1 = (dboid), \
(locktag).locktag_field2 = 0, \
(locktag).locktag_field3 = 0, \
(locktag).locktag_field4 = 0, \
(locktag).locktag_type = LOCKTAG_DATABASE_FROZEN_IDS, \
(locktag).locktag_lockmethodid = DEFAULT_LOCKMETHOD)
/* ID info for a page is RELATION info + BlockNumber */
#define SET_LOCKTAG_PAGE(locktag,dboid,reloid,blocknum) \
((locktag).locktag_field1 = (dboid), \