(stat(...) == -1 || faccessat(...) == -1) && errno == EINTR ?!??

[Tobias Bading <tbading@web.de>]

Hi.

A few days ago I encountered strange problems at home while using GNU
Emacs on GNU/Linux with Windows SMB shares in the office automounted
through the company's VPN.

Details and my current findings on the Emacs devel mailing list:
https://lists.gnu.org/archive/html/emacs-devel/2021-02/msg00835.html.

TL;DR:
Emacs' stat() and faccessat() calls sometimes return -1 and set errno to
EINTR (according to strace). Most of the time the interruption seems to
come from a SIGIO, but at least once a SIGCHLD was in the mix as well.

The behavior of returning with errno == EINTR seems to be new and/or
some kind of bug, because otherwise Emacs would handle that case with
TEMP_FAILURE_RETRY() or something to that effect, right? I've used
pretty much the same Emacs in the office (on an older GNU/Linux release)
for ages with the same Windows SMB shares and never had this problem.

The GNU/Linux man pages of stat() and faccessat() don't mention EINTR at
all, and neither does POSIX
(https://pubs.opengroup.org/onlinepubs/9699919799/).

So I guess the one million dollar question is this:
Are stat() and faccessat() permitted to return -1 with errno == EINTR,
or does that violate the POSIX (or some other) spec?

I'll probably write a small test program next to check if I can
reproduce this problem outside of Emacs, maybe with a simple SIGALRM and
an endless loop of stat()/faccessat() calls. If that reproduces the
problem it would be nice to know whether using SA_RESTART in sigaction()
has any effect.

Any ideas are very welcome... :)

Tobias

PS: I'm running Ubuntu focal (20.04.2 LTS, amd64), which currently uses
libc-bin 2.31-0ubuntu9.2, kernel linux-image-5.4.0-64-generic and
openconnect 8.05-1 to access the VPN.