Re: race condition when writing pg_control

On Sat, May 18, 2024 at 05:29:12PM +1200, Thomas Munro wrote:
> On Fri, May 17, 2024 at 4:46 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> > The specific problem here is that LocalProcessControlFile() runs in
> > every launched child for EXEC_BACKEND builds.  Windows uses
> > EXEC_BACKEND, and Windows' NTFS file system is one of the two file
> > systems known to this list to have the concurrent read/write data
> > mashing problem (the other being ext4).

> First idea idea I've come up with to avoid all of that: pass a copy of
> the "proto-controlfile", to coin a term for the one read early in
> postmaster startup by LocalProcessControlFile().  As far as I know,
> the only reason we need it is to suck some settings out of it that
> don't change while a cluster is running (mostly can't change after
> initdb, and checksums can only be {en,dis}abled while down).  Right?
> Children can just "import" that sucker instead of calling
> LocalProcessControlFile() to figure out the size of WAL segments yada
> yada, I think?  Later they will attach to the real one in shared
> memory for all future purposes, once normal interlocking is allowed.

I like that strategy, particularly because it recreates what !EXEC_BACKEND
backends inherit from the postmaster.  It might prevent future bugs that would
have been specific to EXEC_BACKEND.

> I dunno.  Draft patch attached.  Better plans welcome.  This passes CI
> on Linux systems afflicted by EXEC_BACKEND, and Windows.  Thoughts?

Looks reasonable.  I didn't check over every detail, given the draft status.