Horton

Re: stopgap fix for signal handling during restore_command

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Thomas Munro <thomas.munro@gmail.com>, Michael Paquier <michael@paquier.xyz>, Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Fujii Masao <fujii@postgresql.org>, Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-03-01T23:13:04Z
Lists: pgsql-hackers 
Hi,

On 2023-03-01 14:47:51 -0800, Nathan Bossart wrote:
> On Tue, Feb 28, 2023 at 08:36:03PM -0800, Nathan Bossart wrote:
> > On Sun, Feb 26, 2023 at 12:12:27PM -0800, Andres Freund wrote:
> >> Partially I just want something that can easily be searched for, that can have
> >> comments attached to it documenting why what it is doing is safe.
> >> 
> >> It'd not be a huge amount of work to have a slow and restricted string
> >> interpolation support, to make it easier to write messages. Converting floats
> >> is probably too hard to do safely, and I'm not sure %m can safely be
> >> supported. But basic things like %d would be pretty simple.
> >> 
> >> Basically a loop around the format string that directly writes to stderr using
> >> write(), and only supports a signal safe subset of normal format strings.
> > 
> > Got it, thanks.  I will try to put something together along these lines,
> > although I don't know if I'll pick up the interpolation support in this
> > thread.
> 
> Here is an attempt at adding a signal safe function for writing to STDERR.

Cool.

> I didn't add support for format strings, but looking ahead, I think one
> challenge will be avoiding va_start() and friends.  In any case, IMO format
> string support probably deserves its own thread.

Makes sense to split that off.

FWIW, I think we could rely on va_start() et al to be signal safe. The
standardese isn't super clear about this, because they aren't functions, and
posix only talks about functions being async signal safe...

Greetings,

Andres Freund

Commits

  1. windows: msvc: Define STDIN/OUT/ERR_FILENO.

  2. Avoid calling proc_exit() in processes forked by system().

  3. Move extra code out of the Pre/PostRestoreCommand() section.