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

I 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.

The cfbot noticed that this required a minor rebase over 7de19fbc0,
so here is one.

> 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.
> 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
> https://postgr.es/m/16908.1557521200@sss.pgh.pa.us

That's now been committed (7de19fbc0), so it's not holding back
this patch anymore.

> Also, this will change the order in which entries get made into
> postmaster.pid. I think that's OK, but we'll need to take a
> close look at pg_ctl to be sure it isn't making any invalid
> assumptions.

I took a look around and couldn't find any such problems.
pg_ctl, in particular, doesn't care about the order in which
these lines get added. I did add a comment to pidfile.h
warning people against making new assumptions in this area.

> 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.

I did that here, too.

I think this probably is committable now, though of course it'd
be good for somebody to review it (and maybe test on Windows
before it hits the buildfarm?)

regards, tom lane