rts/OSThreads: Fix reference clock of timedWaitCondition

Previously `timedWaitCondition` assumed that timeouts were referenced
against `CLOCK_MONOTONIC`. This is wrong; by default
`pthread_cond_timedwait` references against `CLOCK_REALTIME`, although
this can be overridden using `pthread_condattr_setclock`.

Fix this and add support for using `CLOCK_MONOTONIC` whenever possible
as it is more robust against system time changes and is likely cheaper
to query. Unfortunately, this is complicated by the fact that older
versions of Darwin did not provide `clock_gettime`, which means we also
need to introduce a fallback path using `gettimeofday`.

Fixes #20144.

3 files changed, 46 insertions, 13 deletions

diff --git a/includes/rts/OSThreads.h b/includes/rts/OSThreads.h
index 08d90de06e..d24a1313a6 100644
--- a/includes/rts/OSThreads.h
+++ b/includes/rts/OSThreads.h
@@ -27,7 +27,17 @@
 #include <pthread.h>
 #include <errno.h>
 
-typedef pthread_cond_t  Condition;
+typedef struct {
+    pthread_cond_t cond;
+
+    // Which clock are pthread_cond_timedwait calls referenced against?
+    // N.B. Some older Darwin releases don't support clock_gettime. However, we
+    // do want to reference to CLOCK_MONOTONIC whenever possible as it is more
+    // robust against system time changes and is likely cheaper to query.
+#if defined(HAVE_CLOCK_GETTIME) && defined(HAVE_PTHREAD_CONDATTR_SETCLOCK)
+    clockid_t timeout_clk;
+#endif
+} Condition;
 typedef pthread_mutex_t Mutex;
 typedef pthread_t       OSThreadId;
 typedef pthread_key_t   ThreadLocalKey;
diff --git a/m4/fp_check_pthreads.m4 b/m4/fp_check_pthreads.m4
index 8160f76d4b..ee5b364e93 100644
--- a/m4/fp_check_pthreads.m4
+++ b/m4/fp_check_pthreads.m4
@@ -115,4 +115,6 @@ AC_DEFUN([FP_CHECK_PTHREADS],
     ],
     AC_MSG_RESULT(no)
   )
+
+  AC_CHECK_FUNCS_ONCE([pthread_condattr_setclock])
 ])
diff --git a/rts/posix/OSThreads.c b/rts/posix/OSThreads.c
index 647b4840d3..980080731f 100644
--- a/rts/posix/OSThreads.c
+++ b/rts/posix/OSThreads.c
@@ -91,6 +91,9 @@
 #include <numa.h>
 #endif
 
+// For gettimeofday()
+#include <sys/time.h>
+
 // TODO does this need configure magic?
 #include <time.h>
 
@@ -104,44 +107,62 @@
 void
 initCondition( Condition* pCond )
 {
-  CHECK(pthread_cond_init(pCond, NULL) == 0);
+  pthread_condattr_t attr;
+  CHECK(pthread_condattr_init(&attr) == 0);
+#if defined(HAVE_CLOCK_GETTIME) && defined(HAVE_PTHREAD_CONDATTR_SETCLOCK)
+  pCond->timeout_clk = CLOCK_REALTIME;
+  if (pthread_condattr_setclock(&attr, CLOCK_MONOTONIC) == 0) {
+      pCond->timeout_clk = CLOCK_MONOTONIC;
+  }
+#endif
+  CHECK(pthread_cond_init(&pCond->cond, &attr) == 0);
+  CHECK(pthread_condattr_destroy(&attr) == 0);
 }
 
 void
 closeCondition( Condition* pCond )
 {
-  CHECK(pthread_cond_destroy(pCond) == 0);
+  CHECK(pthread_cond_destroy(&pCond->cond) == 0);
 }
 
 void
 broadcastCondition ( Condition* pCond )
 {
-  CHECK(pthread_cond_broadcast(pCond) == 0);
+  CHECK(pthread_cond_broadcast(&pCond->cond) == 0);
 }
 
 void
 signalCondition ( Condition* pCond )
 {
-  CHECK(pthread_cond_signal(pCond) == 0);
+  CHECK(pthread_cond_signal(&pCond->cond) == 0);
 }
 
 void
 waitCondition ( Condition* pCond, Mutex* pMut )
 {
-  CHECK(pthread_cond_wait(pCond,pMut) == 0);
+  CHECK(pthread_cond_wait(&pCond->cond, pMut) == 0);
 }
 
 bool
 timedWaitCondition ( Condition* pCond, Mutex* pMut, Time timeout) {
-    timeout += getMonotonicNSec();
-    uint64_t secs = TimeToSeconds(timeout);
+    struct timespec ts;
+
+#if defined(HAVE_CLOCK_GETTIME) && defined(HAVE_PTHREAD_CONDATTR_SETCLOCK)
+    CHECK(clock_gettime(pCond->timeout_clk, &ts) == 0);
+#else
+    struct timeval tv;
+    CHECK(gettimeofday(&tv, NULL) == 0);
+    ts.tv_sec = tv.tv_sec;
+    ts.tv_nsec = 1000 * tv.tv_usec;
+#endif
 
-    const struct timespec t = (struct timespec) {
-        .tv_sec = secs,
-        .tv_nsec = TimeToNS(timeout - SecondsToTime(secs))
-    };
+    uint64_t sec = TimeToSeconds(timeout);
+    ts.tv_sec += sec;
+    ts.tv_nsec += TimeToNS(timeout - SecondsToTime(sec));
+    ts.tv_sec += ts.tv_nsec / 1000000000;
+    ts.tv_nsec %= 1000000000;
 
-    int ret = pthread_cond_timedwait(pCond,pMut, &t);
+    int ret = pthread_cond_timedwait(&pCond->cond, pMut, &ts);
     switch (ret) {
     case ETIMEDOUT:
         return false;