Merge branch 'source-attach-trigger-wakeup' into 'main'

Add g_main_context_new_with_flags() and ownerless polling option

See merge request GNOME/glib!1960

docs/reference/glib/glib-sections.txt

@@ -841,7 +841,9 @@ G_SOURCE_REMOVE
 
 <SUBSECTION>
 GMainContext
+GMainContextFlags
 g_main_context_new
+g_main_context_new_with_flags
 g_main_context_ref
 g_main_context_unref
 g_main_context_default

glib/gmain.c

@@ -177,6 +177,15 @@
  * g_main_context_prepare(), g_main_context_query(),
  * g_main_context_check() and g_main_context_dispatch().
  *
+ * If the event loop thread releases #GMainContext ownership until the results
+ * required by g_main_context_check() are ready you must create a context with
+ * the flag %G_MAIN_CONTEXT_FLAGS_OWNERLESS_POLLING or else you'll lose
+ * g_source_attach() notifications. This happens for instance when you integrate
+ * the GLib event loop into implementations that follow the proactor pattern
+ * (i.e. in these contexts the `poll()` implementation will reclaim the thread for
+ * other tasks until the results are ready). One example of the proactor pattern
+ * is the Boost.Asio library.
+ *
  * ## State of a Main Context # {#mainloop-states}
  *
  * The operation of these functions can best be seen in terms
@@ -270,6 +279,7 @@ struct _GMainContext
   GCond cond;
   GThread *owner;
   guint owner_count;
+  GMainContextFlags flags;
   GSList *waiters;
 
   gint ref_count;  /* (atomic) */
@@ -658,6 +668,23 @@ g_main_context_new_with_next_id (guint next_id)
  **/
 GMainContext *
 g_main_context_new (void)
+{
+  return g_main_context_new_with_flags (G_MAIN_CONTEXT_FLAGS_NONE);
+}
+
+/**
+ * g_main_context_new_with_flags:
+ * @flags: a bitwise-OR combination of #GMainContextFlags flags that can only be
+ *         set at creation time.
+ *
+ * Creates a new #GMainContext structure.
+ *
+ * Returns: (transfer full): the new #GMainContext
+ *
+ * Since: 2.72
+ */
+GMainContext *
+g_main_context_new_with_flags (GMainContextFlags flags)
 {
   static gsize initialised;
   GMainContext *context;
@@ -681,6 +708,7 @@ g_main_context_new (void)
 
   context->sources = g_hash_table_new (NULL, NULL);
   context->owner = NULL;
+  context->flags = flags;
   context->waiters = NULL;
 
   context->ref_count = 1;
@@ -1248,8 +1276,12 @@ g_source_attach_unlocked (GSource      *source,
   /* If another thread has acquired the context, wake it up since it
    * might be in poll() right now.
    */
-  if (do_wakeup && context->owner && context->owner != G_THREAD_SELF)
-    g_wakeup_signal (context->wakeup);
+  if (do_wakeup &&
+      (context->flags & G_MAIN_CONTEXT_FLAGS_OWNERLESS_POLLING ||
+       (context->owner && context->owner != G_THREAD_SELF)))
+    {
+      g_wakeup_signal (context->wakeup);
+    }
 
   g_trace_mark (G_TRACE_CURRENT_TIME, 0,
                 "GLib", "g_source_attach",

glib/gmain.h

@@ -38,6 +38,26 @@ typedef enum /*< flags >*/
   G_IO_NVAL	GLIB_SYSDEF_POLLNVAL
 } GIOCondition;
 
+/**
+ * GMainContextFlags:
+ * @G_MAIN_CONTEXT_FLAGS_NONE: Default behaviour.
+ * @G_MAIN_CONTEXT_FLAGS_OWNERLESS_POLLING: Assume that polling for events will
+ * free the thread to process other jobs. That's useful if you're using
+ * `g_main_context_{prepare,query,check,dispatch}` to integrate GMainContext in
+ * other event loops.
+ *
+ * Flags to pass to g_main_context_new_with_flags() which affect the behaviour
+ * of a #GMainContext.
+ *
+ * Since: 2.72
+ */
+GLIB_AVAILABLE_TYPE_IN_2_72
+typedef enum /*< flags >*/
+{
+  G_MAIN_CONTEXT_FLAGS_NONE = 0,
+  G_MAIN_CONTEXT_FLAGS_OWNERLESS_POLLING = 1
+} GMainContextFlags;
+
 
 /**
  * GMainContext:
@@ -358,6 +378,10 @@ struct _GSourceFuncs
 
 GLIB_AVAILABLE_IN_ALL
 GMainContext *g_main_context_new       (void);
+G_GNUC_BEGIN_IGNORE_DEPRECATIONS
+GLIB_AVAILABLE_IN_2_72
+GMainContext *g_main_context_new_with_flags (GMainContextFlags flags);
+G_GNUC_END_IGNORE_DEPRECATIONS
 GLIB_AVAILABLE_IN_ALL
 GMainContext *g_main_context_ref       (GMainContext *context);
 GLIB_AVAILABLE_IN_ALL

glib/tests/mainloop.c

@@ -158,6 +158,56 @@ test_mainloop_basic (void)
   g_main_loop_unref (loop);
 }
 
+static void
+test_ownerless_polling (gconstpointer test_data)
+{
+  gboolean attach_first = GPOINTER_TO_INT (test_data);
+  GMainContext *ctx = g_main_context_new_with_flags (
+    G_MAIN_CONTEXT_FLAGS_OWNERLESS_POLLING);
+
+  GPollFD fds[20];
+  gint fds_size;
+  gint max_priority;
+  GSource *source = NULL;
+
+  g_assert_true (ctx != g_main_context_default ());
+
+  g_main_context_push_thread_default (ctx);
+
+  /* Drain events */
+  for (;;)
+    {
+      gboolean ready_to_dispatch = g_main_context_prepare (ctx, &max_priority);
+      gint timeout, nready;
+      fds_size = g_main_context_query (ctx, max_priority, &timeout, fds, G_N_ELEMENTS (fds));
+      nready = g_poll (fds, fds_size, /*timeout=*/0);
+      if (!ready_to_dispatch && nready == 0)
+        {
+          if (timeout == -1)
+            break;
+          else
+            g_usleep (timeout * 1000);
+        }
+      ready_to_dispatch = g_main_context_check (ctx, max_priority, fds, fds_size);
+      if (ready_to_dispatch)
+        g_main_context_dispatch (ctx);
+    }
+
+  if (!attach_first)
+    g_main_context_pop_thread_default (ctx);
+
+  source = g_idle_source_new ();
+  g_source_attach (source, ctx);
+  g_source_unref (source);
+
+  if (attach_first)
+    g_main_context_pop_thread_default (ctx);
+
+  g_assert_cmpint (g_poll (fds, fds_size, 0), >, 0);
+
+  g_main_context_unref (ctx);
+}
+
 static gint a;
 static gint b;
 static gint c;
@@ -2145,6 +2195,8 @@ main (int argc, char *argv[])
 #endif
   g_test_add_func ("/mainloop/nfds", test_nfds);
   g_test_add_func ("/mainloop/steal-fd", test_steal_fd);
+  g_test_add_data_func ("/mainloop/ownerless-polling/attach-first", GINT_TO_POINTER (TRUE), test_ownerless_polling);
+  g_test_add_data_func ("/mainloop/ownerless-polling/pop-first", GINT_TO_POINTER (FALSE), test_ownerless_polling);
 
   return g_test_run ();
 }