Thread: pgsql: Prevent concurrent SimpleLruTruncate() for any given SLRU.
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 Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/566372b3d6435639e4cc4476d79b8505a0297c87 Modified Files -------------- doc/src/sgml/catalogs.sgml | 4 +++- doc/src/sgml/monitoring.sgml | 16 ++++++++++++++ src/backend/access/transam/slru.c | 8 +++++++ src/backend/access/transam/subtrans.c | 4 ++-- src/backend/commands/async.c | 37 +++++++++++++++++++++++--------- src/backend/commands/vacuum.c | 13 +++++++++++ src/backend/storage/lmgr/lmgr.c | 20 +++++++++++++++++ src/backend/storage/lmgr/lwlocknames.txt | 3 +++ src/backend/utils/adt/lockfuncs.c | 12 +++++++++++ src/include/storage/lmgr.h | 3 +++ src/include/storage/lock.h | 10 +++++++++ 11 files changed, 117 insertions(+), 13 deletions(-)
Noah Misch <noah@leadboat.com> writes:
> Prevent concurrent SimpleLruTruncate() for any given SLRU.
I find it fairly scary that you've changed enum LockTagType in the
back branches. Are we quite certain that no extensions have compiled-in
values of that enum?
Safer from an ABI standpoint would be to add the new value at the end,
at least in the back branches.
regards, tom lane
On Sat, Aug 15, 2020 at 04:39:05PM -0400, Tom Lane wrote: > Noah Misch <noah@leadboat.com> writes: > > Prevent concurrent SimpleLruTruncate() for any given SLRU. > > I find it fairly scary that you've changed enum LockTagType in the > back branches. Are we quite certain that no extensions have compiled-in > values of that enum? > > Safer from an ABI standpoint would be to add the new value at the end, > at least in the back branches. Yeah, that was negligent. Several PGXN modules do refer to locktags. I'll move it.