bpo-41710: PyThread_acquire_lock_timed() uses sem_clockwait() (GH-28662)

On Unix, if the sem_clockwait() function is available in the C
library (glibc 2.30 and newer), the threading.Lock.acquire() method
now uses the monotonic clock (time.CLOCK_MONOTONIC) for the timeout,
rather than using the system clock (time.CLOCK_REALTIME), to not be
affected by system clock changes.

configure now checks if the sem_clockwait() function is available.

Author: vstinner

Doc/whatsnew/3.11.rst

@@ -239,6 +239,16 @@ sqlite3
   (Contributed by Aviv Palivoda, Daniel Shahaf, and Erlend E. Aasland in
   :issue:`16379`.)
 
+threading
+---------
+
+* On Unix, if the ``sem_clockwait()`` function is available in the C library
+  (glibc 2.30 and newer), the :meth:`threading.Lock.acquire` method now uses
+  the monotonic clock (:data:`time.CLOCK_MONOTONIC`) for the timeout, rather
+  than using the system clock (:data:`time.CLOCK_REALTIME`), to not be affected
+  by system clock changes.
+  (Contributed by Livius and Victor Stinner in :issue:`41710`.)
+
 time
 ----
 

Misc/NEWS.d/next/Library/2021-09-30-23-00-18.bpo-41710.svuloZ.rst

@@ -0,0 +1,5 @@
+On Unix, if the ``sem_clockwait()`` function is available in the C library
+(glibc 2.30 and newer), the :meth:`threading.Lock.acquire` method now uses the
+monotonic clock (:data:`time.CLOCK_MONOTONIC`) for the timeout, rather than
+using the system clock (:data:`time.CLOCK_REALTIME`), to not be affected by
+system clock changes. Patch by Victor Stinner.

Python/thread_pthread.h

@@ -92,7 +92,7 @@
  * mutexes and condition variables:
  */
 #if (defined(_POSIX_SEMAPHORES) && !defined(HAVE_BROKEN_POSIX_SEMAPHORES) && \
-     defined(HAVE_SEM_TIMEDWAIT))
+     (defined(HAVE_SEM_TIMEDWAIT) || defined(HAVE_SEM_CLOCKWAIT)))
 #  define USE_SEMAPHORES
 #else
 #  undef USE_SEMAPHORES
@@ -461,17 +461,34 @@ PyThread_acquire_lock_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds,
         timeout = _PyTime_FromNanoseconds(-1);
     }
 
+#ifdef HAVE_SEM_CLOCKWAIT
+    struct timespec abs_timeout;
+    // Local scope for deadline
+    {
+        _PyTime_t deadline = _PyTime_GetMonotonicClock() + timeout;
+        _PyTime_AsTimespec_clamp(deadline, &abs_timeout);
+    }
+#else
     _PyTime_t deadline = 0;
-    if (timeout > 0 && !intr_flag) {
+    if (timeout > 0
+        && !intr_flag
+        )
+    {
         deadline = _PyTime_GetMonotonicClock() + timeout;
     }
+#endif
 
     while (1) {
         if (timeout > 0) {
-            _PyTime_t t = _PyTime_GetSystemClock() + timeout;
+#ifdef HAVE_SEM_CLOCKWAIT
+            status = fix_status(sem_clockwait(thelock, CLOCK_MONOTONIC,
+                                              &abs_timeout));
+#else
+            _PyTime_t abs_timeout = _PyTime_GetSystemClock() + timeout;
             struct timespec ts;
-            _PyTime_AsTimespec_clamp(t, &ts);
+            _PyTime_AsTimespec_clamp(abs_timeout, &ts);
             status = fix_status(sem_timedwait(thelock, &ts));
+#endif
         }
         else if (timeout == 0) {
             status = fix_status(sem_trywait(thelock));
@@ -486,6 +503,9 @@ PyThread_acquire_lock_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds,
             break;
         }
 
+        // sem_clockwait() uses an absolute timeout, there is no need
+        // to recompute the relative timeout.
+#ifndef HAVE_SEM_CLOCKWAIT
         if (timeout > 0) {
             /* wait interrupted by a signal (EINTR): recompute the timeout */
             _PyTime_t timeout = deadline - _PyTime_GetMonotonicClock();
@@ -494,17 +514,24 @@ PyThread_acquire_lock_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds,
                 break;
             }
         }
+#endif
     }
 
     /* Don't check the status if we're stopping because of an interrupt.  */
     if (!(intr_flag && status == EINTR)) {
         if (timeout > 0) {
-            if (status != ETIMEDOUT)
+            if (status != ETIMEDOUT) {
+#ifdef HAVE_SEM_CLOCKWAIT
+                CHECK_STATUS("sem_clockwait");
+#else
                 CHECK_STATUS("sem_timedwait");
+#endif
+            }
         }
         else if (timeout == 0) {
-            if (status != EAGAIN)
+            if (status != EAGAIN) {
                 CHECK_STATUS("sem_trywait");
+            }
         }
         else {
             CHECK_STATUS("sem_wait");