Horton

Re: TopoSort() fix

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Michael Paquier <michael@paquier.xyz>, Andres Freund <andres@anarazel.de>, Alvaro Herrera <alvherre@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2019-07-30T00:57:15Z
Lists: pgsql-hackers 
On Mon, Jul 29, 2019 at 5:55 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> FYI, I just got done inventing a way to reach that code, and I have
> to suspect that it's impossible to do so in production, because under
> ordinary circumstances no parallel worker will take any exclusive lock
> that isn't already held by its leader.  (If you happen to know an
> easy counterexample, let's see it.)

I think the way you could make that happen would be to run a parallel
query that calls a user-defined function which does LOCK TABLE.

> Anyway, armed with this, I was able to prove that HEAD just hangs up
> on this test case; apparently the deadlock checker never detects that
> the additional holders of the advisory lock need to be rearranged.
> And removing that "break" fixes it.

Nice!

> So I'll go commit the break-ectomy, but what do people think about
> testing this better?

I think it's a great idea.  I was never very happy with the amount of
exercise I was able to give this code.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Add an isolation test to exercise parallel-worker deadlock resolution.

  2. Mark advisory-lock functions as parallel restricted, not parallel unsafe.

  3. Fix busted logic for parallel lock grouping in TopoSort().