Bug fix. autovacuum.c do_worker_start() associates memory allocations with TopMemoryContext rather than 'Autovacuum start worker (tmp)'

Hi Hackers,

This patch ensures get_database_list() switches back to the memory
context in use upon entry rather than returning with TopMemoryContext
as the context.

This will address memory allocations in autovacuum.c being associated
with TopMemoryContext when they should not be.

autovacuum.c do_start_worker() with current context
'Autovacuum start worker (tmp)' invokes get_database_list(). Upon
return, the current context has been changed to TopMemoryContext by
AtCommit_Memory() as part of an internal transaction. Further down
in the do_start_worker(), pgstat_fetch_stat_dbentry() is invoked.
Previously this didn't pose a issue, however recent changes altered
how pgstat_fetch_stat_dbentry() is implemented. The new
implementation has a branch utilizing palloc. The patch ensures these
allocations are associated with the 'Autovacuum start worker (tmp)'
context rather than the TopMemoryContext. Prior to the change,
leaving an idle laptop PG instance running just shy of 3 days saw the
autovacuum launcher process grow to 42MB with most of that growth in
TopMemoryContext due to the palloc allocations issued with autovacuum
worker startup.

--
Reid Thompson
Senior Software Engineer
Crunchy Data, Inc.

reid(dot)thompson(at)crunchydata(dot)com
www.crunchydata.com

--
Reid Thompson
Senior Software Engineer
Crunchy Data, Inc.

reid(dot)thompson(at)crunchydata(dot)com
www.crunchydata.com