Horton

Re: LISTEN/NOTIFY testing woes

Mark Dilger <hornschnorter@gmail.com>

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From: Mark Dilger <hornschnorter@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@lists.postgresql.org
Cc: Martijn van Oosterhout <kleptog@gmail.com>
Date: 2019-11-24T18:25:57Z
Lists: pgsql-hackers 

On 11/23/19 8:50 PM, Mark Dilger wrote:
> 
> 
> On 11/23/19 5:01 PM, Tom Lane wrote:
>> I ran into a couple of issues while trying to devise a regression test
>> illustrating the LISTEN-in-serializable-transaction issue Mark Dilger
>> reported.  The first one is that an isolation test in which we expect
>> to see a cross-process NOTIFY immediately after a COMMIT turns out to
>> be not very stable: on my machine, it works as long as you're just
>> running the isolation tests by themselves, but it usually falls over
>> if I'm running check-world with any amount of parallelism.  The reason
>> for this seems to be that incoming notifies are only checked for when
>> we're about to wait for client input.  At that point we've already
>> sent the ReadyForQuery ('Z') protocol message, which will cause libpq
>> to stand down from looking for more input and return a null from
>> PQgetResult().  Depending on timing, the following Notify protocol
>> messages might arrive quickly enough that isolationtester.c sees them
>> before it goes off to do something else, but that's not very reliable.
> 
> Thanks for working on this, Tom.
> 
> I have finished reading and applying your three patches and have moved 
> on to testing them.  I hope to finish the review soon.

After applying all three patches, the stress test that originally
uncovered the assert in predicate.c no longer triggers any asserts.
`check-world` passes.  The code and comments look good.

Your patches are ready for commit.

-- 
Mark Dilger

Commits

  1. Avoid assertion failure with LISTEN in a serializable transaction.

  2. Stabilize NOTIFY behavior by transmitting notifies before ReadyForQuery.

  3. Stabilize the results of pg_notification_queue_usage().