When Python is built on Alpine Linux or in any other configuration that uses musl libc, calls to Lock.acquire() can't be interrupted by signals. This bug is caught by test_lock_acquire_interruption and test_rlock_acquire_interruption in Lib/test/test_threadsignals.py, both of which fail when Python is built on musl.

POSIX explicitly says that sem_timedwait ever failing with EINTR is optional:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/sem_timedwait.html#tag_16_508_05

And musl deliberately chooses not to support it:
http://www.openwall.com/lists/musl/2018/02/24/3
http://git.musl-libc.org/cgit/musl/commit/?id=c0ed5a201b2bdb6d1896064bec0020c9973db0a1

However, we incorrectly rely on it for correct operation. Our documentation https://docs.python.org/3.6/library/threading.html#threading.Lock.acquire just says that "Lock acquires can now be interrupted by signals on POSIX", not that any optional POSIX features are required for it.

A similar bug was #11223, which was the same problem but with pthread_cond_timedwait, which is used when semaphores aren't available.