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From: Philipp Stephani
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Subject: bug#54667: 29.0.50; posix_spawn breaks emacs-gdb
Date: Sun, 17 Apr 2022 20:53:26 +0200
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To: Robert Pluim
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Archived-At:
> Am 01.04.2022 um 16:44 schrieb Robert Pluim :
>=20
>>>>>> On Fri, 1 Apr 2022 15:29:50 +0200, "Herman, G=C3=A9za" =
said:
>=20
> Herman> I can confirm that it fixes the problem.
>=20
> Thanks, I=CA=BCll see if I can get around to committing it this =
weekend.
>=20
> Herman> For reference, here's the patch:
> Herman> =
https://lists.gnu.org/archive/html/emacs-devel/2022-03/msg00067.html
>=20
> Herman> (Btw., according to this email thread, vfork is faster than
> Herman> posix_spawn on Linux, so maybe it'd make sense to use =
posix_spawn only
> Herman> on non-Linux platforms?)
>=20
> In emacs-28 we only use posix_spawn on macOS because its vork is
> sub-optimal. I don=CA=BCt remember the rationale for switching to =
using it
> everywhere, it=CA=BCs undoubtedly in the archives somewhere.
My reasoning back then was:
1. Using fork/vfork has a few pitfalls (can't call async-signal-unsafe =
functions in the child process) that posix_spawn avoids (by not allowing =
arbitrary code between fork and exec). Therefore, using posix_spawn is =
simpler and more obviously correct.
2. Because posix_spawn offers less functionality than fork+exec, it =
could be faster than the latter, e.g. by having it implemented in the =
kernel and avoiding page table copies.
3. Since we only run CI on GNU/Linux, it's useful to have only one =
codepath on all Unix-like systems so that issues that only appear on =
macOS are less likely to go unnoticed.
These arguments are still mostly correct, but there are some =
counterpoints:
1. We need to keep the fork/vfork code path anyway since posix_spawn =
doesn't allow us to correctly set up a pseudoterminal, so we still need =
to deal with the pitfalls.
2. posix_spawn is indeed much faster on macOS, but not so on GNU/Linux. =
(I think it's unnecessarily slowed down on GNU/Linux by what I consider =
a bug in the POSIX standard.)
3. This is still true, but I'm not sure how much it matters given that =
the implementations of posix_spawn are completely different on GNU/Linux =
and macOS.=