removal of dangling temp tables

We recently ran into a funny situation, where autovacuum would not
remove very old temp tables. The relfrozenxid of those tables was about
to reach the max freeze age, so monitoring started to complain. It
turned out that autovacuum saw that the backendId was used by a live
backend ... but that session was in reality not using those temp tables,
and had not used any temp table at all. They were left-overs from a
crash months ago, and since the session was not using temp tables, they
had not been removed. DISCARD ALL may have run, but had no effect.

I think the best way to fix this is to call RemoveTempRelations()
unconditionally at session start (without doing the rest of the temp
table setup, just the removal.)

In versions earlier than pg11, related issues occur if you have a crash
with autovacuum off and/or workers disabled, and temp tables are leaked
in backendID 1 or 2; then start with normal values. In that case, those
backendIDs are used by the logical replication launcher and the
autovacuum launcher, so autovacuum does not remove them either. This
was fixed in PG11 inadvertently by this commit:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=943576bddcb52971041d9f5f806789921fa107ee
The reason the commit fixes it is that now the databaseID of the PGPROC
entry is compared to the temp table's database; and for those worker
processes, the DatabaseId is InvalidOid so it all works out.

This isn't a terribly interesting bug, as restarting with changed
worker/autovacuum options after a crash that happens to leak temp tables
should be quite rare. But anyway we can fix this second issue in prior
branches by adding a comparison to databaseId to the existing 'if' test
in autovacuum; easy enough, no compatibility concerns. This should also
cover the case that one session crashes leaking temp tables, then the
same session connects to a different database for a long time. (I
didn't verify this last point to be a real problem.)

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services