Dear Maintainer,

In the example below, glibc 2.34 from experimental causes a spurious
EINTR error in the poll() call from the child thread. It seems that
thread cancellation causes the poll() to be spuriously interrupted,
even though the cancellation is explicitly disabled at that time.

As far as I understand, POSIX allows (or even requires) thread
cancellation to be essentially like a signal interruption, save for
ending the thread. But that is *only* from the moment that cancellation
is effected. Cancellation cannot be effected while it is disabled by
definition, so the behaviour from glibc seems wrong here.

This regression is known to break the test suite from the VLC package.
Rolling back to 2.33 from unstable solves the problem.
----8<----

#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <poll.h>
#include <pthread.h>

static void *thread(void *data)
{
        int canc;

        (void) data;
        pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &canc);

        if (poll(NULL, 0, 2000) < 0) {
                perror("Unexpected poll error");
                abort();
        }
        pthread_setcancelstate(canc, NULL);
        return NULL;
}

int main(void)
{
        pthread_t th;
        void *ret;
        struct timespec ts = { 0, 100*1000*1000 };

        if (pthread_create(&th, NULL, thread, NULL)) {
                perror("pthread_create");
                return 1;
        }


        clock_nanosleep(CLOCK_MONOTONIC, 0, &ts, NULL);
        pthread_cancel(th);
        pthread_join(th, &ret);
        assert(ret == NULL);
        return 0;
}

---->8----

Hi,

Thanks for the example, it's very useful to reproduce and understand the
issue.

poll() is a cancellation point. It appears to me that POSIX actually
allows this behaviour for cancellation points:

"The side effects of acting upon a cancellation request while suspended
during a call of a function are the same as the side effects that may be
seen in a single-threaded program when a call to a function is
interrupted by a signal and the given function returns [EINTR]. Any such
side effects occur before any cancellation cleanup handlers are called."

https://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_09.html
https://sourceware.org/git/?p=glibc.git;a=commit;h=26cfbb7162ad364d53d69f6d482f2d87b5950524

Regards,
Aurelien

	Hi,

Le lauantaina 26. maaliskuuta 2022, 21.34.38 EET Aurelien Jarno a écrit :

I don't think so...

AFAIU, in POSIX, "acting upon" a cancellation request means to move the
cancellation request past the pending state, in other words, actually cancel
the thread. That quote clarifies that the signal-like interruption is observed
in the cancelled thread flow of execution before the cancellation clean-up
handlers.

Otherwise the next paragraph would not make much sense, particularly the last
sentence:

And it would get even more nonsensical / contradictory in the following
section:

This paragraph clarifies that cancellation cannot occur recursively / more than
once per thread.

Assuming that "acting upon a cancellation request" would be permissible with
cancellation disabled, this sentence would imply that cancellation is disabled
permanently, and the thread will never get cancelled at all.

We believe that the bug you reported is fixed in the latest version of
glibc, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1008174@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Aurelien Jarno <aurel32@debian.org> (supplier of updated glibc package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Sat, 16 Apr 2022 13:10:56 +0200
Source: glibc
Architecture: source
Version: 2.34-0experimental4
Distribution: experimental
Urgency: medium
Maintainer: GNU Libc Maintainers <debian-glibc@lists.debian.org>
Changed-By: Aurelien Jarno <aurel32@debian.org>
Closes: 1006692 1008174
Changes:
 glibc (2.34-0experimental4) experimental; urgency=medium
 .
   [ Aurelien Jarno ]
   * debian/patches/git-updates.diff: update from upstream stable branch:
     - Fixes spurious EINTR when thread cancellation is disabled.  Closes:
       #1008174.
     - debian/patches/any/local-revert-bz13979.diff: refresh.
 .
   [ Johannes Schauer Marin Rodrigues ]
   * debian/debhelper.in/libc.postinst: do not re-exec init if DPKG_ROOT is
     set.  Closes: #1006692.
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