From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Paquier <michael@paquier.xyz>
cc: Andres Freund <andres@anarazel.de>, Julien Rouhaud <rjuju123@gmail.com>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Add an optional timeout clause to isolationtester step.
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Comments: In-reply-to Michael Paquier <michael@paquier.xyz>
	message dated "Tue, 10 Mar 2020 11:55:36 +0900"
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Date: Tue, 10 Mar 2020 00:09:12 -0400
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Michael Paquier <michael@paquier.xyz> writes:
> On Mon, Mar 09, 2020 at 10:32:27PM -0400, Tom Lane wrote:
>> It strikes me to wonder whether we could improve matters by teaching
>> isolationtester to watch for particular values in a connected backend's
>> pg_stat_activity.wait_event_type/wait_event columns.  Those columns
>> didn't exist when isolationtester was designed, IIRC, so it's not
>> surprising that they're not used in the current design.  But we could
>> use them perhaps to detect that a backend has arrived at some state
>> that's not a heavyweight-lock-wait state.

> Interesting idea.  So that would be basically an equivalent of
> PostgresNode::poll_query_until but for the isolation tester?

No, more like the existing isolationtester wait query, which watches
for something being blocked on a heavyweight lock.  Right now, that
one depends on a bespoke function pg_isolation_test_session_is_blocked(),
but it used to be a query on pg_stat_activity/pg_locks.

> In short
> we gain a meta-command that runs a SELECT query that waits until the
> query defined in the command returns true.  The polling interval may
> be tricky to set though.

I think it'd be just the same as the polling interval for the existing
wait query.  We'd have to have some way to mark a script step to say
what to check to decide that it's blocked ...

			regards, tom lane