gmain: Do not dead-look if calling g_main_loop_run from a GSource

This should fail and warn but not leaving the context acquired and
locked.

Add tests.

glib/gmain.c

@@ -4522,10 +4522,12 @@ g_main_loop_run (GMainLoop *loop)
       g_assert (got_ownership);
     }
 
-  if (loop->context->in_check_or_prepare)
+  if G_UNLIKELY (loop->context->in_check_or_prepare)
     {
       g_warning ("g_main_loop_run(): called recursively from within a source's "
 		 "check() or prepare() member, iteration not possible.");
+      g_main_context_release_unlocked (loop->context);
+      UNLOCK_CONTEXT (loop->context);
       g_main_loop_unref (loop);
       return;
     }

glib/tests/mainloop.c

@@ -1169,6 +1169,75 @@ test_unref_while_pending (void)
   g_assert_cmpint (n_finalized, ==, 1);
 }
 
+typedef struct {
+  GSource parent;
+  GMainLoop *loop;
+} LoopedSource;
+
+static gboolean
+prepare_loop_run (GSource *source, gint *time)
+{
+  LoopedSource *looped_source = (LoopedSource*) source;
+  *time = 0;
+
+  g_test_expect_message (G_LOG_DOMAIN, G_LOG_LEVEL_WARNING,
+                         "*called recursively from within a source's check() "
+                         "or prepare() member*");
+  g_main_loop_run (looped_source->loop);
+  g_test_assert_expected_messages ();
+
+  return FALSE;
+}
+
+static gboolean
+check_loop_run (GSource *source)
+{
+  LoopedSource *looped_source = (LoopedSource*) source;
+
+  g_test_expect_message (G_LOG_DOMAIN, G_LOG_LEVEL_WARNING,
+                         "*called recursively from within a source's check() "
+                         "or prepare() member*");
+  g_main_loop_run (looped_source->loop);
+  g_test_assert_expected_messages ();
+
+  return TRUE;
+}
+
+static gboolean
+dispatch_loop_run (GSource    *source,
+                   GSourceFunc callback,
+                   gpointer    user_data)
+{
+  LoopedSource *looped_source = (LoopedSource*) source;
+
+  g_main_loop_quit (looped_source->loop);
+
+  return FALSE;
+}
+
+static void
+test_recursive_loop_child_sources (void)
+{
+  GMainLoop *loop;
+  GSource *source;
+  GSourceFuncs loop_run_funcs = {
+    prepare_loop_run, check_loop_run, dispatch_loop_run, NULL, NULL, NULL,
+  };
+
+  loop = g_main_loop_new (NULL, FALSE);
+
+  source = g_source_new (&loop_run_funcs, sizeof (LoopedSource));
+  ((LoopedSource*)source)->loop = loop;
+
+  g_source_attach (source, NULL);
+
+  g_main_loop_run (loop);
+  g_source_unref (source);
+
+  g_main_loop_unref (loop);
+}
+
+
 #ifdef G_OS_UNIX
 
 #include <glib-unix.h>
@@ -2445,6 +2514,7 @@ main (int argc, char *argv[])
   g_test_add_func ("/mainloop/invoke", test_invoke);
   g_test_add_func ("/mainloop/child_sources", test_child_sources);
   g_test_add_func ("/mainloop/recursive_child_sources", test_recursive_child_sources);
+  g_test_add_func ("/mainloop/recursive_loop_child_sources", test_recursive_loop_child_sources);
   g_test_add_func ("/mainloop/swapping_child_sources", test_swapping_child_sources);
   g_test_add_func ("/mainloop/blocked_child_sources", test_blocked_child_sources);
   g_test_add_func ("/mainloop/source_time", test_source_time);