libc++ passes invalid values to pthread_equal

[mclow]

Bugzilla Link	42918
Resolution	FIXED
Resolved on	Aug 14, 2019 14:33
Version	unspecified
OS	FreeBSD
CC	@emaste,@mclow

Extended Description

[ received from Ed Maste at FreeBSD ]

We just had a FreeBSD bug submitted for a failure arising from
libc++'s use of pthread_equal.

FreeBSD's pthread_equal stub implementation (used in non-threaded
programs) returns true unconditionally, since a non-threaded program
cannot have two different threads. However, std::thread::id defines a
special value that should not equal any real thread, and this is may
be passed to pthread_equal. See FreeBSD PR 239550 for more details,
and PR 239038 for the user-facing issue that prompted this.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239550
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239038

Now the most expedient fix is probably for us to make pthread_equal
somewhat less of a stub, but for correctness it seems
__libcpp_thread_id_equal would need to check for libc++'s special
value before calling pthread_equal.

[mclow]

assigned to @mclow

[mclow]

I've looked at this, and it seems like the best place to fix this is in __thread_id, which is where the "no thread == 0" is imposed.

However, the recursive_timed_mutex implementation doesn't use __thread_id, but instead goes right to the underlying type __libcpp_thread_id.

Also, we have no tests :-(

[mclow]

Fixed in revision 368916.