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

When Python is built with older Linux libc headers, timed semaphore waits use the real-time clock even if `sem_clockwait` is available at runtime. Use a weak reference on supported compilers to select `sem_clockwait` at runtime, falling back to `sem_timedwait` when the symbol is absent. ThreadSanitizer builds continue to use the real-time clock.

Addresses [astral-sh/python-build-standalone#1176](astral-sh/python-build-standalone#1176).

Author: zanieb

Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-18-00-00.gh-issue-160368.sem-clockwait.rst

@@ -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.

Python/parking_lot.c

@@ -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.
@@ -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);