The attached test case exhibits a bug I'm about to fix in musl, and
which also exists in glibc, whereby happening to get the same TID as a
previous mutex owner who exited with the mutex locked allows the new
thread to unlock the mutex. See also:

http://austingroupbugs.net/view.php?id=755

Sadly it's rather slow and impractical to add to libc-test. I'm not
sure if it's possible to run it in a container with a smaller pid/tid
space to get the reuse to happen sooner. It can also have false
negatives if some other process claims the tid before it gets reused
for a new thread in the test process. Ideas to make the test more
practical and reliable would be nice.

Rich