Re: BUG #15804: Assertion failure when using logging_collector with EXEC_BACKEND

From: Michael Paquier <michael(at)paquier(dot)xyz>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Yuli Khodorkovskiy <yuli(dot)khodorkovskiy(at)crunchydata(dot)com>, pgsql-bugs(at)lists(dot)postgresql(dot)org
Subject: Re: BUG #15804: Assertion failure when using logging_collector with EXEC_BACKEND
Date: 2019-05-21 04:45:12
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On Mon, May 20, 2019 at 11:01:05AM -0400, Tom Lane wrote:
> In other words, the right way to think about this is less "move
> syslogger launch to earlier" and more "move port opening to later".

> I did some cursory testing of that idea with the attached patch,
> which simply relocates the port opening logic to below where
> syslogger start is (though "git diff" insists on presenting it
> differently :-(). I also moved and recommented the emission
> of the "starting ..." log entry. It works under EXEC_BACKEND,
> but I'm not fool enough to believe that that proves it works
> under Windows :-(.

I can see what you have done, no worries.

> One issue with this is that we can't be sure we have sole control
> of the postmaster port number at the time we create shmem.

Couldn't that also be a problem for some users? This means that
there is a window where the port won't be opened but the external PID
file can be found. Some tools could be broken as an effect of that.

> Hence, to avoid undesirable conflicts of shmem, we should change
> things to base the shmem key on the datadir's ID not the port
> number, as was already speculated about in

This would be actually nice to change as you suggest.

> Another point is that we want to be sure this doesn't change
> the order in which lockfiles are released at shutdown. That
> seems OK (I confirmed by strace'ing that the postmaster's
> final syscalls are still done in the same order) but it could
> use some additional eyeballs on it.
> There may be some other reorderings that would be a good idea.
> In particular I'm thinking that the CreateOptsFile call should
> be pushed down, so that it doesn't get written until we know
> that the port number is OK.

Is this thread a good place to discuss all those changes by the way?
After all the recent activity in this area, a new thread and a
discussion for v13 would be I think appropriate. I would rather not
play with the devil for v12.

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