Code review for early drop of orphaned temp relations in autovacuum

Enterprise / PostgreSQL - Tom Lane [sss.pgh.pa.us] - 27 November 2016 20:23 UTC

Commit a734fd5d1 exposed some race conditions that existed previously in the autovac code, but were basically harmless because autovac would not try to delete orphaned relations immediately. Specifically, the test for orphaned-ness was made on a pg_class tuple that might be dead by now, allowing autovac to try to remove a table that the owning backend had just finished deleting. This resulted in a hard crash due to inadequate caution about accessing the table's catalog entries without any lock. We must take a relation lock and then recheck whether the table is still present and still looks deletable before we do anything.

Also, it seemed to me that deleting multiple tables per transaction, and trying to continue after errors, represented unjustifiable complexity. We do not expect this code path to be taken often in the field, nor even during testing, which means that prioritizing performance over correctness is a bad tradeoff. Rip all that out in favor of just starting a new transaction after each successful temp table deletion. If we're unlucky enough to get an error, which shouldn't happen anyway now that we're being more cautious, let the autovacuum worker fail as it normally would.

In passing, improve the order of operations in the initial scan loop. Now that we don't care about whether a temp table is a wraparound hazard, there's no need to perform extract_autovac_opts, get_pgstat_tabentry_relid, or relation_needs_vacanalyze for temp tables.

Also, if GetTempNamespaceBackendId returns InvalidBackendId (indicating it doesn't recognize the schema as temp), treat that as meaning it's NOT an orphaned temp table, not that it IS one, which is what happened before because BackendIdGetProc necessarily failed. The case really shouldn't come up for a table that has RELPERSISTENCE_TEMP, but the consequences if it did seem undesirable. (This might represent a back-patchable bug fix; not sure if it's worth the trouble.)

Discussion: https://postgr.es/m/21299.1480272347@sss.pgh.pa.us

dafa084 Code review for early drop of orphaned temp relations in autovacuum.
src/backend/postmaster/autovacuum.c | 255 +++++++++++++++--------------------
1 file changed, 112 insertions(+), 143 deletions(-)

Upstream: git.postgresql.org

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