linux: fix main loop issues with windowless plugins

base/message_pump_glib.cc

 // Copyright (c) 2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/message_pump_glib.h"
 
 #include <fcntl.h>
 #include <math.h>
 
 #include "base/eintr_wrapper.h"
@@ skipping 32 matching lines @@
 // system.  File descriptors can be attached to the sources.  The poll may block
 // if none of the Prepare calls returned TRUE.  It will block indefinitely, or
 // by the minimum time returned by a source in Prepare.
 //     After the poll, GLib calls Check for each source that returned FALSE
 // from Prepare.  The return value of Check has the same meaning as for Prepare,
 // making Check a second chance to tell GLib we are ready for Dispatch.
 //     Finally, GLib calls Dispatch for each source that is ready.  If Dispatch
 // returns FALSE, GLib will destroy the source.  Dispatch calls may be recursive
 // (i.e., you can call Run from them), but Prepare and Check cannot.
 //     Finalize is called when the source is destroyed.
+// NOTE: It is common for subsytems to want to process pending events while
+// doing intensive work, for example the flash plugin. They usually use the
+// following pattern (recommended by the GTK docs):
+// while (gtk_events_pending()) {
+//   gtk_main_iteration();
+// }
+//
+// gtk_events_pending just calls g_main_context_pending, which does the
+// following:
+// - Call prepare on all the sources.
+// - Do the poll with a timeout of 0 (not blocking).
+// - Call check on all the sources.
+// - *Does not* call dispatch on the sources.
+// - Return true if any of prepare() or check() returned true.
+//
+// gtk_main_iteration just calls g_main_context_iteration, which does the whole
+// thing, respecting the timeout for the poll (and block, although it is
+// expected not to if gtk_events_pending returned true), and call dispatch.
+//
+// Thus it is important to only return true from prepare or check if we
+// actually have events or work to do. We also need to make sure we keep
+// internal state consistent so that if prepare/check return true when called
+// from gtk_events_pending, they will still return true when called right
+// after, from gtk_main_iteration.
+//
+// For the GLib pump we try to follow the Windows UI pump model:
+// - Whenever we receive a wakeup event or the timer for delayed work expires,
+// we run DoWork and/or DoDelayedWork. That part will also run in the other
+// event pumps.
+// - We also run DoWork, DoDelayedWork, and possibly DoIdleWork in the main
+// loop, around event handling.
 
 struct WorkSource : public GSource {
   base::MessagePumpForUI* pump;
 };
 
 gboolean WorkSourcePrepare(GSource* source,
                            gint* timeout_ms) {
   *timeout_ms = static_cast<WorkSource*>(source)->pump->HandlePrepare();
   // We always return FALSE, so that our timeout is honored.  If we were
   // to return TRUE, the timeout would be considered to be 0 and the poll
   // would never block.  Once the poll is finished, Check will be called.
   return FALSE;
 }
 
 gboolean WorkSourceCheck(GSource* source) {
-  // Always return TRUE, and Dispatch will be called.
-  return TRUE;
+  // Only return TRUE if Dispatch should be called.
+  return static_cast<WorkSource*>(source)->pump->HandleCheck();
 }
 
 gboolean WorkSourceDispatch(GSource* source,
                             GSourceFunc unused_func,
                             gpointer unused_data) {
 
   static_cast<WorkSource*>(source)->pump->HandleDispatch();
   // Always return TRUE so our source stays registered.
   return TRUE;
 }
@@ skipping 49 matching lines @@
   static PlatformThreadId thread_id = PlatformThread::CurrentId();
   DCHECK(thread_id == PlatformThread::CurrentId()) <<
       "Running MessagePumpForUI on two different threads; "
       "this is unsupported by GLib!";
 #endif
 
   RunState state;
   state.delegate = delegate;
   state.should_quit = false;
   state.run_depth = state_ ? state_->run_depth + 1 : 1;
+  state.has_work = false;
+
+  RunState* previous_state = state_;
+  state_ = &state;
+
   // We really only do a single task for each iteration of the loop.  If we
   // have done something, assume there is likely something more to do.  This
   // will mean that we don't block on the message pump until there was nothing
   // more to do.  We also set this to true to make sure not to block on the
   // first iteration of the loop, so RunAllPending() works correctly.
-  state.more_work_is_plausible = true;
-
-  RunState* previous_state = state_;
-  state_ = &state;
+  bool more_work_is_plausible = true;
 
   // We run our own loop instead of using g_main_loop_quit in one of the
   // callbacks.  This is so we only quit our own loops, and we don't quit
   // nested loops run by others.  TODO(deanm): Is this what we want?
-  while (!state_->should_quit)
-    g_main_context_iteration(context_, true);
+  for (;;) {
+    // Don't block if we think we have more work to do.
+    bool block = !more_work_is_plausible;
+
+    // g_main_context_iteration returns true if events have been dispatched.
+    more_work_is_plausible = g_main_context_iteration(context_, block);
+    if (state_->should_quit)
+      break;
+
+    more_work_is_plausible |= state_->delegate->DoWork();
+    if (state_->should_quit)
+      break;
+
+    more_work_is_plausible |=
+        state_->delegate->DoDelayedWork(&delayed_work_time_);
+    if (state_->should_quit)
+      break;
+
+    if (more_work_is_plausible)
+      continue;
+
+    more_work_is_plausible = state_->delegate->DoIdleWork();
+    if (state_->should_quit)
+      break;
+  }
 
   state_ = previous_state;
 }
 
 // Return the timeout we want passed to poll.
 int MessagePumpForUI::HandlePrepare() {
-  // If it's likely that we have more work, don't let the pump
-  // block so that we can do some processing.
-  if (state_->more_work_is_plausible)
+  // We know we have work, but we haven't called HandleDispatch yet. Don't let
+  // the pump block so that we can do some processing.
+  if (state_->has_work)
     return 0;
 
-  // Work wasn't plausible, so we'll block.  In the case where glib fires
-  // our Dispatch(), |more_work_is_plausible| will be reset to whatever it
-  // should be.  However, so we don't get starved by more important work,
-  // we set |more_work_is_plausible| to true.  This means if we come back
-  // here without having been through Dispatch(), we will get a chance to be
-  // fired and properly do our work in Dispatch().
-  state_->more_work_is_plausible = true;
-
   // We don't think we have work to do, but make sure not to block
   // longer than the next time we need to run delayed work.
   return GetTimeIntervalMilliseconds(delayed_work_time_);
 }
 
-void MessagePumpForUI::HandleDispatch() {
+bool MessagePumpForUI::HandleCheck() {
   // We should only ever have a single message on the wakeup pipe, since we
   // are only signaled when the queue went from empty to non-empty.  The glib
   // poll will tell us whether there was data, so this read shouldn't block.
   if (wakeup_gpollfd_.revents & G_IO_IN) {
     char msg;
     if (HANDLE_EINTR(read(wakeup_pipe_read_, &msg, 1)) != 1 || msg != '!') {
       NOTREACHED() << "Error reading from the wakeup pipe.";
     }
+    // Since we ate the message, we need to record that we have more work,
+    // because HandleCheck() may be called without HandleDispatch being called
+    // afterwards.
+    state_->has_work = true;
+  }
+
+  if (state_->has_work)
+    return true;
+
+  if (GetTimeIntervalMilliseconds(delayed_work_time_) == 0) {
+    // The timer has expired. That condition will stay true until we process
+    // that delayed work, so we don't need to record this differently.
+    return true;
+  }
+
+  return false;
+}
+
+void MessagePumpForUI::HandleDispatch() {
+  state_->has_work = false;
+  if (state_->delegate->DoWork()) {
+    // NOTE: on Windows at this point we would call ScheduleWork (see
+    // MessagePumpForUI::HandleWorkMessage in message_pump_win.cc). But here,
+    // instead of posting a message on the wakeup pipe, we can avoid the
+    // syscalls and just signal that we have more work.
+    state_->has_work = true;
   }
 
   if (state_->should_quit)
     return;
 
-  state_->more_work_is_plausible = false;
-
-  if (state_->delegate->DoWork())
-    state_->more_work_is_plausible = true;
-
-  if (state_->should_quit)
-    return;
-
-  if (state_->delegate->DoDelayedWork(&delayed_work_time_))
-    state_->more_work_is_plausible = true;
-  if (state_->should_quit)
-    return;
-
-  // Don't do idle work if we think there are more important things
-  // that we could be doing.
-  if (state_->more_work_is_plausible)
-    return;
-
-  if (state_->delegate->DoIdleWork())
-    state_->more_work_is_plausible = true;
-  if (state_->should_quit)
-    return;
+  state_->delegate->DoDelayedWork(&delayed_work_time_);
 }
 
 void MessagePumpForUI::AddObserver(Observer* observer) {
   observers_.AddObserver(observer);
 }
 
 void MessagePumpForUI::RemoveObserver(Observer* observer) {
   observers_.RemoveObserver(observer);
 }
 
@@ skipping 31 matching lines @@
 }
 
 // static
 void MessagePumpForUI::EventDispatcher(GdkEvent* event, gpointer data) {
   reinterpret_cast<MessagePumpForUI*>(data)->WillProcessEvent(event);
   gtk_main_do_event(event);
   reinterpret_cast<MessagePumpForUI*>(data)->DidProcessEvent(event);
 }
 
 }  // namespace base