[Bug libstdc++/95989] Segmentation fault compiling with static libraries and using jthread::request_stop

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95989

--- Comment #16 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:08b4d325711d5c6f68ac29443aba3fd7aa173ac8

commit r11-5183-g08b4d325711d5c6f68ac29443aba3fd7aa173ac8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Nov 19 21:07:06 2020 +0000

    libstdc++: Avoid calling undefined __gthread_self weak symbol [PR 95989]

    Since glibc 2.27 the pthread_self symbol has been defined in libc rather
    than libpthread. Because we only call pthread_self through a weak alias
    it's possible for statically linked executables to end up without a
    definition of pthread_self. This crashes when trying to call an
    undefined weak symbol.

    We can use the __GLIBC_PREREQ version check to detect the version of
    glibc where pthread_self is no longer in libpthread, and call it
    directly rather than through the weak reference.

    It would be better to check for pthread_self in libc during configure
    instead of hardcoding the __GLIBC_PREREQ check. That would be
    complicated by the fact that prior to glibc 2.27 libc.a didn't have the
    pthread_self symbol, but libc.so.6 did.  The configure checks would need
    to try to link both statically and dynamically, and the result would
    depend on whether the static libc.a happens to be installed during
    configure (which could vary between different systems using the same
    version of glibc). Doing it properly is left for a future date, as that
    will be needed anyway after glibc moves all pthread symbols from
    libpthread to libc. When that happens we should revisit the whole
    approach of using weak symbols for pthread symbols.

    For the purposes of std::this_thread::get_id() we call
    pthread_self() directly when using glibc 2.27 or later. Otherwise, if
    __gthread_active_p() is true then we know the libpthread symbol is
    available so we call that. Otherwise, we are single-threaded and just
    use ((__gthread_t)1) as the thread ID.

    An undesirable consequence of this change is that code compiled prior to
    the change might inline the old definition of this_thread::get_id()
    which always returns (__gthread_t)1 in a program that isn't linked to
    libpthread. Code compiled after the change will use pthread_self() and
    so get a real TID. That could result in the main thread having different
    thread::id values in different translation units. This seems acceptable,
    as there are not expected to be many uses of thread::id in programs
    that aren't linked to libpthread.

    An earlier version of this patch also changed __gthread_self() to use
    __GLIBC_PREREQ(2, 27) and only use the weak symbol for older glibc. Tha
    might still make sense to do, but isn't needed by libstdc++ now.

    libstdc++-v3/ChangeLog:

            PR libstdc++/95989
            * config/os/gnu-linux/os_defines.h (_GLIBCXX_NATIVE_THREAD_ID):
            Define new macro to get reliable thread ID.
            * include/bits/std_thread.h: (this_thread::get_id): Use new
            macro if it's defined.
            * testsuite/30_threads/jthread/95989.cc: New test.
            * testsuite/30_threads/this_thread/95989.cc: New test.