Thread: pgsql: Assert that a snapshot is active or registered before it's used
pgsql: Assert that a snapshot is active or registered before it's used
From
Heikki Linnakangas
Date:
Assert that a snapshot is active or registered before it's used The comment in GetTransactionSnapshot() said that you "should call RegisterSnapshot or PushActiveSnapshot on the returned snap if it is to be used very long". That felt too unclear to me. Make the comment more strongly worded. To enforce that rule and to catch potential bugs where a snapshot might get invalidated while it's still in use, add an assertion to HeapTupleSatisfiesMVCC() to check that the snapshot is registered or pushed to active stack. No new bugs were found by this, but it seems like good future-proofing. It's not a great place for the check; HeapTupleSatisfiesMVCC() is in fact safe to call with an unregistered snapshot, and the assertion won't catch other unsafe uses. But it goes a long way in practice. Fix a few cases that were playing fast and loose with that and just assumed that the snapshot cannot be invalidated during a scan. Those assumptions were not wrong, but they're not performance critical, so let's drop the excuses and just register the snapshot. These were false positives found by the new assertion. Discussion: https://www.postgresql.org/message-id/7c56f180-b9e1-481e-8c1d-efa63de3ecbb@iki.fi Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/8076c00592e40e8dbd1fce7a98b20d4bf075e4ba Modified Files -------------- src/backend/access/heap/heapam_visibility.c | 9 +++++++++ src/backend/access/index/genam.c | 8 ++------ src/backend/commands/dbcommands.c | 3 ++- src/backend/utils/cache/relcache.c | 15 +++++++++------ src/backend/utils/time/snapmgr.c | 8 ++++---- 5 files changed, 26 insertions(+), 17 deletions(-)