Feedback-ID: id4a34324:Fastmail
Date: Thu, 30 Nov 2023 17:42:44 -0800
From: Andres Freund <andres@anarazel.de>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Tristan Partin <tristan@neon.tech>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	Thomas Munro <thomas.munro@gmail.com>
Subject: Re: Refactoring backend fork+exec code
Message-ID: <20231201014244.zdngaok5wb74blms@awork3.anarazel.de>
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Hi,

On 2023-12-01 01:36:13 +0200, Heikki Linnakangas wrote:
> On 30/11/2023 22:26, Andres Freund wrote:
> > On 2023-11-30 01:36:25 +0200, Heikki Linnakangas wrote:
> > >  From a96b6e92fdeaa947bf32774c425419b8f987b8e2 Mon Sep 17 00:00:00 2001
> > > From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
> > > Date: Thu, 30 Nov 2023 00:01:25 +0200
> > > Subject: [PATCH v3 1/7] Refactor CreateSharedMemoryAndSemaphores
> > >
> > > For clarity, have separate functions for *creating* the shared memory
> > > and semaphores at postmaster or single-user backend startup, and
> > > for *attaching* to existing shared memory structures in EXEC_BACKEND
> > > case. CreateSharedMemoryAndSemaphores() is now called only at
> > > postmaster startup, and a new AttachSharedMemoryStructs() function is
> > > called at backend startup in EXEC_BACKEND mode.
> >
> > I assume CreateSharedMemoryAndSemaphores() is still called during crash
> > restart?
>
> Yes.
>
> >  I wonder if it shouldn't split three ways:
> > 1) create
> > 2) initialize
> > 3) attach
>
> Why? What would be the difference between create and initialize phases?

Mainly because I somehow mis-remembered how we deal with the shared memory
allocation when crashing. I somehow had remembered that we reused the same
allocation across restarts, but reinitialized it from scratch. There's a
kernel of truth to that, because we can end up re-attaching to an existing
sysv shared memory segment. But not more. Perhaps I was confusing things with
the listen sockets?

Andres