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gh-160368: Detect sem_clockwait at runtime on Linux #160371

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Merged
itamaro merged 1 commit into from
Oct 23, 2026
+34 −16
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gh-160368: Detect sem_clockwait at runtime on Linux #160371

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3 changes: 3 additions & 0 deletions ....d/next/Core_and_Builtins/2026-10-21-18-00-00.gh-issue-160368.sem-clockwait.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
On Linux, allow timed semaphore waits to use the monotonic clock when
``sem_clockwait`` is available at runtime, even if Python was built with
older libc headers.
47 changes: 31 additions & 16 deletions Python/parking_lot.c
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,18 @@

#include <stdbool.h>

#if defined(_Py_USE_SEMAPHORES) && defined(CLOCK_MONOTONIC) && !defined(_Py_THREAD_SANITIZER)
# ifdef HAVE_SEM_CLOCKWAIT
# define _Py_SEM_CLOCKWAIT_AVAILABLE 1
# elif defined(__linux__) && (defined(__GNUC__) || defined(__clang__))
/* Older glibc headers do not declare sem_clockwait. A weak reference allows
such builds to use monotonic waits on newer runtimes. */
extern int sem_clockwait(sem_t *, clockid_t, const struct timespec *)
__attribute__((weak));
# define _Py_SEM_CLOCKWAIT_AVAILABLE (sem_clockwait != NULL)
# endif
#endif


typedef struct {
// The mutex protects the waiter queue and the num_waiters counter.
Expand Down Expand Up @@ -151,24 +163,27 @@ _PySemaphore_Wait(_PySemaphore *sema, PyTime_t timeout)
if (timeout >= 0) {
struct timespec ts;

#if defined(CLOCK_MONOTONIC) && defined(HAVE_SEM_CLOCKWAIT) && !defined(_Py_THREAD_SANITIZER)
PyTime_t now;
// silently ignore error: cannot report error to the caller
(void)PyTime_MonotonicRaw(&now);
PyTime_t deadline = _PyTime_Add(now, timeout);
_PyTime_AsTimespec_clamp(deadline, &ts);

err = sem_clockwait(&sema->platform_sem, CLOCK_MONOTONIC, &ts);
#else
PyTime_t now;
// silently ignore error: cannot report error to the caller
(void)PyTime_TimeRaw(&now);
PyTime_t deadline = _PyTime_Add(now, timeout);

_PyTime_AsTimespec_clamp(deadline, &ts);
#ifdef _Py_SEM_CLOCKWAIT_AVAILABLE
if (_Py_SEM_CLOCKWAIT_AVAILABLE) {
PyTime_t now;
// Silently ignore error: cannot report error to the caller.
(void)PyTime_MonotonicRaw(&now);
PyTime_t deadline = _PyTime_Add(now, timeout);
_PyTime_AsTimespec_clamp(deadline, &ts);

err = sem_timedwait(&sema->platform_sem, &ts);
err = sem_clockwait(&sema->platform_sem, CLOCK_MONOTONIC, &ts);
}
else
#endif
{
PyTime_t now;
// Silently ignore error: cannot report error to the caller.
(void)PyTime_TimeRaw(&now);
PyTime_t deadline = _PyTime_Add(now, timeout);
_PyTime_AsTimespec_clamp(deadline, &ts);

err = sem_timedwait(&sema->platform_sem, &ts);
}
}
else {
err = sem_wait(&sema->platform_sem);
Expand Down
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