pgsql: Make pgstat tabstat lookup hash table less fragile.

Make pgstat tabstat lookup hash table less fragile.

Code review for commit 090010f2e.

Fix cases where an elog(ERROR) partway through a function would leave the
persistent data structures in a corrupt state. pgstat_report_stat got this
wrong by invalidating PgStat_TableEntry structs before removing hashtable
entries pointing to them, and get_tabstat_entry got it wrong by ignoring
the possibility of palloc failure after it had already created a hashtable
entry.

Also, avoid leaking a memory context per transaction, which the previous
code did through misunderstanding hash_create's API. We do not need to
create a context to hold the hash table; hash_create will do that.
(The leak wasn't that large, amounting to only a memory context header
per iteration, but it's still surprising that nobody noticed it yet.)

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5d00b764cd682f6071b35033f508a431d9d3f920

Modified Files
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src/backend/postmaster/pgstat.c | 124 +++++++++++++++++++++-------------------
1 file changed, 64 insertions(+), 60 deletions(-)