| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Michael Paquier <michael(at)paquier(dot)xyz>, Andres Freund <andres(at)anarazel(dot)de>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: TopoSort() fix |
| Date: | 2019-07-30 00:57:15 |
| Message-ID: | CA+TgmoaUQ+XvS+3U3GBsprSqUq1PBXk2RU_cbSopj2q+GFO=Lw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Jul 29, 2019 at 5:55 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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
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