[issue23428] Use the monotonic clock for thread conditions on POSIX platforms

[STINNER Victor]

STINNER Victor added the comment:

> Python/condvar.h and Python/thread_pthread.h should use the monotonic clock CLOCK_MONOTONIC

Oh, I forgot that Python/thread_pthread.h only uses pthread_cond_timedwait() if semaphores are emulated with mutexes+conditional variables.

On most platforms, PyThread_acquire_lock_timed() is implemented with sem_timedwait(). Problem: sem_timedwait() requires an absolute time using the CLOCK_REALTIME clock and the clock is not yet configurable on Linux :-(

See the feature request in the glibc: "Bug 14717 - Allow choice of clock source for calls to sem_timedwait() and pthread_mutex_timedwait()" opened in 2012:
https://sourceware.org/bugzilla/show_bug.cgi?id=14717

Note: QNX provides sem_timedwait_monotonic().

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