[glib] gmain: only signal GWakeup right before or during a blocking poll

[Colin Walters <walters src gnome org>]

commit 9ba95e25b74adf8d62effeaf6567074ac932811c
Author: Paolo Bonzini <pbonzini redhat com>
Date:   Tue Apr 4 09:56:47 2017 +0200

    gmain: only signal GWakeup right before or during a blocking poll
    
    Since commit e4ee307 ("Do not wake up main loop if change is from same
    thread", bug 761102), GMainContext uses context->owner to decide if the
    event loop is being run in the current thread.  However, what really
    matters is the phase in the prepare/query/poll/check/dispatch sequence.
    Wakeups are only needed between the end of prepare and the end of poll,
    and then only if prepare found that no sources were ready.
    
    There is no need to take threads into account, because prepare, check
    and all callers of conditional_wakeup all look at the new need_wakeup
    flag inside LOCK_CONTEXT/UNLOCK_CONTEXT.
    
    With this change, g_main_context_is_owner and g_main_context_wait are
    the only functions for which acquire/release matters, just like before
    commit e4ee307.
    
    Signed-off-by: Paolo Bonzini <bonzini gnu org>

 glib/gmain.c |   30 +++++++++++++++++++-----------
 1 files changed, 19 insertions(+), 11 deletions(-)
---
diff --git a/glib/gmain.c b/glib/gmain.c
index ab2908e..af0a29a 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -277,7 +277,8 @@ struct _GMainContext
 
   guint next_id;
   GList *source_lists;
-  gint in_check_or_prepare;
+  gboolean in_check_or_prepare;
+  gboolean need_wakeup;
 
   GPollRec *poll_records;
   guint n_poll_records;
@@ -651,6 +652,7 @@ g_main_context_new (void)
   
   context->pending_dispatches = g_ptr_array_new ();
   
+  context->need_wakeup = FALSE;
   context->time_is_fresh = FALSE;
   
   context->wakeup = g_wakeup_new ();
@@ -1127,17 +1129,11 @@ source_remove_from_context (GSource      *source,
 static void
 conditional_wakeup (GMainContext *context)
 {
-  /* We want to signal wakeups in two cases:
-   *  1 When the context is owned by another thread
-   *  2 When the context owner is NULL (two subcases)
-   *   2a Possible if the context has never been acquired
-   *   2b Or if the context has no current owner
-   *
-   * At least case 2a) is necessary to ensure backwards compatibility with
-   * qemu's use of GMainContext.
-   * https://bugzilla.gnome.org/show_bug.cgi?id=761102#c14
+  /* This flag is set if at the start of prepare() we have no other ready
+   * sources, and hence would wait in poll(). In that case, any other threads
+   * attaching sources will need to signal a wakeup.
    */
-  if (context->owner != G_THREAD_SELF)
+  if (context->need_wakeup)
     g_wakeup_signal (context->wakeup);
 }
 
@@ -3469,6 +3465,10 @@ g_main_context_prepare (GMainContext *context,
   
   LOCK_CONTEXT (context);
 
+  /* context->need_wakeup is protected by LOCK_CONTEXT/UNLOCK_CONTEXT,
+   * so need not set it yet.
+   */
+
   context->time_is_fresh = FALSE;
 
   if (context->in_check_or_prepare)
@@ -3594,6 +3594,8 @@ g_main_context_prepare (GMainContext *context,
        }
     }
   g_source_iter_clear (&iter);
+  /* See conditional_wakeup() where this is used */
+  context->need_wakeup = (n_ready == 0);
 
   TRACE (GLIB_MAIN_CONTEXT_AFTER_PREPARE (context, current_priority, n_ready));
 
@@ -3728,6 +3730,12 @@ g_main_context_check (GMainContext *context,
 
   TRACE (GLIB_MAIN_CONTEXT_BEFORE_CHECK (context, max_priority, fds, n_fds));
 
+  /* We don't need to wakeup during check or dispatch, because
+   * all sources will be re-evaluated during prepare/query.
+   */
+  context->need_wakeup = FALSE;
+
+  /* And if we have a wakeup pending, acknowledge it */
   for (i = 0; i < n_fds; i++)
     {
       if (fds[i].fd == context->wake_up_rec.fd)