From 3fbbf64ceecbd0fe675850070ea1e5d77c401c4f Mon Sep 17 00:00:00 2001 From: Noah Misch Date: Sat, 15 Aug 2020 10:15:53 -0700 Subject: Prevent concurrent SimpleLruTruncate() for any given SLRU. 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 --- src/backend/access/transam/subtrans.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'src/backend/access/transam/subtrans.c') diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c index dde72b7d2e1..6702350e124 100644 --- a/src/backend/access/transam/subtrans.c +++ b/src/backend/access/transam/subtrans.c @@ -334,8 +334,8 @@ ExtendSUBTRANS(TransactionId newestXact) /* * Remove all SUBTRANS segments before the one holding the passed transaction ID * - * This is normally called during checkpoint, with oldestXact being the - * oldest TransactionXmin of any running transaction. + * oldestXact is the oldest TransactionXmin of any running transaction. This + * is called only during checkpoint. */ void TruncateSUBTRANS(TransactionId oldestXact) -- cgit v1.2.3