Fix g_cond_wait_until() vs. monotonic time

We've had a relatively rocky path with g_cond_wait_until() on systems
that either don't support pthread_condattr_setclock() or where
g_get_monotonic_time() is not based on CLOCK_MONOTONIC (ie: Android and
Mac OS).

Fortunately, both of these platforms seem to share
pthread_cond_timedwait_relative_np() which allows us to implement
g_cond_wait_until() without races.

With this patch, we now require that one of pthread_condattr_setclock()
or pthread_cond_timedwait_relative_np() exists.  A quick look around
suggests that this is true for all platforms that we care about.

This patch removes our use of pthread_cond_timedwait_monotonic() and
pthread_cond_timedwait_monotonic_np() which were Android-only APIs.

https://bugzilla.gnome.org/show_bug.cgi?id=673607

1 files changed, 34 insertions, 11 deletions

diff --git a/glib/gthread-posix.c b/glib/gthread-posix.c
index 374cdede7..4a01f4dad 100644
--- a/glib/gthread-posix.c
+++ b/glib/gthread-posix.c
@@ -45,6 +45,7 @@
 #include "gslice.h"
 #include "gmessages.h"
 #include "gstrfuncs.h"
+#include "gmain.h"
 
 #include <stdlib.h>
 #include <stdio.h>
@@ -853,19 +854,41 @@ g_cond_wait_until (GCond  *cond,
   struct timespec ts;
   gint status;
 
-  ts.tv_sec = end_time / 1000000;
-  ts.tv_nsec = (end_time % 1000000) * 1000;
+#ifdef HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE_NP
+  /* end_time is given relative to the monotonic clock as returned by
+   * g_get_monotonic_time().
+   *
+   * Since this pthreads wants the relative time, convert it back again.
+   */
+  {
+    gint64 now = g_get_monotonic_time ();
+    gint64 relative;
 
-#if defined(HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC)
-  if ((status = pthread_cond_timedwait_monotonic (g_cond_get_impl (cond), g_mutex_get_impl (mutex), &ts)) == 0)
-    return TRUE;
-#elif defined(HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC_NP)
-  if ((status = pthread_cond_timedwait_monotonic_np (g_cond_get_impl (cond), g_mutex_get_impl (mutex), &ts)) == 0)
-    return TRUE;
+    if (end_time <= now)
+      return FALSE;
+
+    relative = end_time - now;
+
+    ts.tv_sec = relative / 1000000;
+    ts.tv_nsec = (relative % 1000000) * 1000;
+
+    if ((status = pthread_cond_timedwait_relative_np (g_cond_get_impl (cond), g_mutex_get_impl (mutex), &ts)) == 0)
+      return TRUE;
+  }
+#elif defined (HAVE_PTHREAD_CONDATTR_SETCLOCK) && defined (CLOCK_MONOTONIC)
+  /* This is the exact check we used during init to set the clock to
+   * monotonic, so if we're in this branch, timedwait() will already be
+   * expecting a monotonic clock.
+   */
+  {
+    ts.tv_sec = end_time / 1000000;
+    ts.tv_nsec = (end_time % 1000000) * 1000;
+
+    if ((status = pthread_cond_timedwait (g_cond_get_impl (cond), g_mutex_get_impl (mutex), &ts)) == 0)
+      return TRUE;
+  }
 #else
-  /* Pray that the cond is actually using the monotonic clock */
-  if ((status = pthread_cond_timedwait (g_cond_get_impl (cond), g_mutex_get_impl (mutex), &ts)) == 0)
-    return TRUE;
+#error Cannot support GCond on your platform.
 #endif
 
   if G_UNLIKELY (status != ETIMEDOUT)