systemclock: Add gst_system_clock_set_default

Used for setting the default system clock that is obtained through
gst_system_clock_obtain(), which is sometimes needed for unit
testing.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=711269

docs/gst/gstreamer-sections.txt

@@ -2552,6 +2552,7 @@ gst_structure_get_type
 GstClockType
 GstSystemClock
 gst_system_clock_obtain
+gst_system_clock_set_default
 <SUBSECTION Standard>
 GstSystemClockClass
 GstSystemClockPrivate

gst/gstsystemclock.c

@@ -112,6 +112,7 @@ enum
 
 /* the one instance of the systemclock */
 static GstClock *_the_system_clock = NULL;
+static gboolean _external_default_clock = FALSE;
 
 static void gst_system_clock_dispose (GObject * object);
 static void gst_system_clock_set_property (GObject * object, guint prop_id,
@@ -280,6 +281,43 @@ gst_system_clock_get_property (GObject * object, guint prop_id, GValue * value,
   }
 }
 
+/**
+ * gst_system_clock_set_default:
+ * @new_clock: a #GstClock
+ *
+ * Sets the default system clock that can be obtained with
+ * gst_system_clock_obtain.
+ *
+ * This is mostly used for testing and debugging purposes when you
+ * want to have control over the time reported by the default system
+ * clock.
+ *
+ * MT safe.
+ */
+void
+gst_system_clock_set_default (GstClock * new_clock)
+{
+  GstClock *clock;
+
+  g_mutex_lock (&_gst_sysclock_mutex);
+  clock = _the_system_clock;
+
+  if (clock != NULL)
+    g_object_unref (clock);
+
+  if (new_clock == NULL) {
+    GST_CAT_DEBUG (GST_CAT_CLOCK, "resetting default system clock");
+    _external_default_clock = FALSE;
+  } else {
+    GST_CAT_DEBUG (GST_CAT_CLOCK, "setting new default system clock to %p",
+        new_clock);
+    _external_default_clock = TRUE;
+    g_object_ref (new_clock);
+  }
+  _the_system_clock = new_clock;
+  g_mutex_unlock (&_gst_sysclock_mutex);
+}
+
 /**
  * gst_system_clock_obtain:
  *
@@ -301,6 +339,7 @@ gst_system_clock_obtain (void)
 
   if (clock == NULL) {
     GST_CAT_DEBUG (GST_CAT_CLOCK, "creating new static system clock");
+    g_assert (_external_default_clock == FALSE);
     clock = g_object_new (GST_TYPE_SYSTEM_CLOCK,
         "name", "GstSystemClock", NULL);
 

gst/gstsystemclock.h

@@ -81,6 +81,7 @@ struct _GstSystemClockClass {
 GType                   gst_system_clock_get_type       (void);
 
 GstClock*               gst_system_clock_obtain         (void);
+void                    gst_system_clock_set_default    (GstClock *new_clock);
 
 G_END_DECLS
 

tests/check/gst/gstsystemclock.c

@@ -117,6 +117,38 @@ notify_callback (GstClock * clock, GstClockTime time,
   return FALSE;
 }
 
+GST_START_TEST (test_set_default)
+{
+  GstClock *clock, *static_clock;
+
+  /* obtain the default system clock, which keeps a static ref and bumps the
+   * refcount before returning */
+  static_clock = gst_system_clock_obtain ();
+  fail_unless (static_clock != NULL, "Could not create default system clock");
+  g_assert_cmpint (GST_OBJECT_REFCOUNT (static_clock), ==, 2);
+
+  /* set a new default clock to a different instance which should replace the
+   * static clock with this one, and unref the static clock */
+  clock = g_object_new (GST_TYPE_SYSTEM_CLOCK, "name", "TestClock", NULL);
+  gst_system_clock_set_default (clock);
+  g_assert_cmpint (GST_OBJECT_REFCOUNT (static_clock), ==, 1);
+  static_clock = gst_system_clock_obtain ();
+  fail_unless (static_clock == clock);
+  g_assert_cmpint (GST_OBJECT_REFCOUNT (clock), ==, 3);
+  g_object_unref (static_clock);
+
+  /* Reset the default clock to the static one */
+  gst_system_clock_set_default (NULL);
+  static_clock = gst_system_clock_obtain ();
+  fail_unless (static_clock != clock);
+  g_assert_cmpint (GST_OBJECT_REFCOUNT (clock), ==, 1);
+  g_assert_cmpint (GST_OBJECT_REFCOUNT (static_clock), ==, 2);
+  g_object_unref (clock);
+  g_object_unref (static_clock);
+}
+
+GST_END_TEST;
+
 GST_START_TEST (test_single_shot)
 {
   GstClock *clock;
@@ -654,6 +686,7 @@ gst_systemclock_suite (void)
   tcase_add_test (tc_chain, test_diff);
   tcase_add_test (tc_chain, test_mixed);
   tcase_add_test (tc_chain, test_async_full);
+  tcase_add_test (tc_chain, test_set_default);
 
   return s;
 }

win32/common/libgstreamer.def

@@ -1127,6 +1127,7 @@ EXPORTS
 	gst_structure_to_string
 	gst_system_clock_get_type
 	gst_system_clock_obtain
+	gst_system_clock_set_default
 	gst_tag_exists
 	gst_tag_flag_get_type
 	gst_tag_get_description