Implement MessageLoopForUI using GLib. This gets some exercise from...

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/lazy_instance.h"
+#include "base/logging.h"
+#include "base/platform_thread.h"
+
+namespace base {
+
+static const char kWorkScheduled = '\0';
+static const char kDelayedWorkScheduled = '\1';
+
+// I wish these could be const, but g_source_new wants a non-const GSourceFunc
+// pointer.
+
+// static
+GSourceFuncs MessagePumpForUI::WorkSourceFuncs = {
+  WorkSourcePrepare,
+  WorkSourceCheck,
+  WorkSourceDispatch,
+  NULL
+};
+
+// static
+GSourceFuncs MessagePumpForUI::IdleSourceFuncs = {
+  IdleSourcePrepare,
+  IdleSourceCheck,
+  IdleSourceDispatch,
+  NULL
+};
+
+static int GetTimeIntervalMilliseconds(Time from) {
+  if (from.is_null())
+    return -1;
+
+  // Be careful here.  TimeDelta has a precision of microseconds, but we want a
+  // value in milliseconds.  If there are 5.5ms left, should the delay be 5 or
+  // 6?  It should be 6 to avoid executing delayed work too early.
+  double timeout = ceil((from - Time::Now()).InMillisecondsF());
+
+  // If this value is negative, then we need to run delayed work soon.
+  int delay = static_cast<int>(timeout);
+  if (delay < 0)
+    delay = 0;
+
+  return delay;
+}
+
+MessagePumpForUI::MessagePumpForUI()
+    : state_(NULL),
+      context_(g_main_context_default()) {
+  // Create a pipe with a non-blocking read end for use by ScheduleWork to
+  // break us out of a poll.  Create the work source and attach the file
+  // descriptor to it.
+  int pipe_fd[2];
+  CHECK(0 == pipe(pipe_fd)) << "Could not create pipe!";
+  write_fd_work_scheduled_ = pipe_fd[1];
+  read_fd_work_scheduled_ = pipe_fd[0];
+  int flags = fcntl(read_fd_work_scheduled_, F_GETFL, 0);
+  if (-1 == flags)
+    flags = 0;
+  CHECK(0 == fcntl(read_fd_work_scheduled_, F_SETFL, flags | O_NONBLOCK)) <<
+      "Could not set file descriptor to non-blocking!";
+  GPollFD poll_fd;
+  poll_fd.fd = read_fd_work_scheduled_;
+  poll_fd.events = G_IO_IN | G_IO_HUP | G_IO_ERR;
+  work_source_ = AddSource(&WorkSourceFuncs, G_PRIORITY_DEFAULT, &poll_fd);
+}
+
+MessagePumpForUI::~MessagePumpForUI() {
+  close(read_fd_work_scheduled_);
+  close(write_fd_work_scheduled_);
+  g_source_destroy(work_source_);
+  g_source_unref(work_source_);
+}
+
+struct ThreadIdTraits {
+  static void New(void* instance) {
+    int* thread_id = static_cast<int*>(instance);
+    *thread_id = PlatformThread::CurrentId();
+  }
+  static void Delete(void* instance) {
+  }
+};
+
+void MessagePumpForUI::Run(Delegate* delegate) {
+  // Make sure we only run this on one thread.  GTK only has one message pump
+  // so we can only have one UI loop per process.
+  static LazyInstance<int, ThreadIdTraits> thread_id(base::LINKER_INITIALIZED);
+  DCHECK(thread_id.Get() == PlatformThread::CurrentId()) <<
+      "Running MessagePumpForUI on two different threads; "
+      "this is unsupported by GLib!";
+
+  RunState state;
+  state.delegate = delegate;
+  state.keep_running = true;
+  // We emulate the behavior of MessagePumpDefault and try to do work at once.
+  state.should_do_work = true;
+  state.should_do_idle_work = false;
+  state.idle_source = NULL;
+
+  RunState* previous_state = state_;
+  state_ = &state;
+
+  while (state.keep_running)
+    g_main_context_iteration(context_, true);
+
+  if (state.idle_source) {
+    // This removes the source from the context and releases GLib's hold on it.
+    g_source_destroy(state.idle_source);
+    // This releases our hold and destroys the source.
+    g_source_unref(state.idle_source);
+  }
+
+  state_ = previous_state;
+}
+
+void MessagePumpForUI::Quit() {
+  DCHECK(state_) << "Quit called outside Run!";
+  state_->keep_running = false;
+}
+
+void MessagePumpForUI::ScheduleWork() {
+  // This can be called on any thread, so we don't want to touch any state
+  // variables as we would then need locks all over.  This ensures that if
+  // we are sleeping in a poll that we will wake up, and we check the pipe
+  // so we know when work was scheduled.
+  CHECK(1 == write(write_fd_work_scheduled_, &kWorkScheduled, 1)) <<
+      "Could not write to pipe!";
+}
+
+void MessagePumpForUI::ScheduleDelayedWork(const Time& delayed_work_time) {
+  delayed_work_time_ = delayed_work_time;
+  // This is an optimization.  Delayed work may not be imminent, we may just
+  // need to update our timeout to poll.  Hence we don't want to go overkill
+  // with kWorkScheduled.
+  CHECK(1 == write(write_fd_work_scheduled_, &kDelayedWorkScheduled, 1)) <<
+      "Could not write to pipe!";
+}
+
+// A brief refresher on GLib:
+//     GLib sources have four callbacks: Prepare, Check, Dispatch and Finalize.
+// On each iteration of the GLib pump, it calls each source's Prepare function.
+// This function should return TRUE if it wants GLib to call its Dispatch, and
+// FALSE otherwise.  It can also set a timeout in this case for the next time
+// Prepare should be called again (it may be called sooner).
+//     After the Prepare calls, GLib does a poll to check for events from the
+// 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.
+
+// static
+gboolean MessagePumpForUI::WorkSourcePrepare(GSource* source,
+                                             gint* timeout_ms) {
+  MessagePumpForUI* self = static_cast<WorkSource*>(source)->self;
+  RunState* state = self->state_;
+
+  if (state->should_do_work) {
+    state->should_do_idle_work = false;
+    *timeout_ms = 0;
+    return TRUE;
+  }
+
+  *timeout_ms = GetTimeIntervalMilliseconds(self->delayed_work_time_);
+
+  state->should_do_idle_work = true;
+  // We want to do idle work right before poll goes to sleep.  Obviously
+  // we are not currently asleep, but we may be about to since we have
+  // no work to do.  If we don't have an idle source ready to go it's
+  // probably because it fired already (or we just started and it hasn't
+  // been added yet) and we should add one for when we are ready.  Note
+  // that this new source will get Prepare called on this current pump
+  // iteration since it gets added at the end of the source list.
+  if (!state->idle_source) {
+    state->idle_source =
+        self->AddSource(&IdleSourceFuncs, G_PRIORITY_DEFAULT_IDLE, NULL);
+  }
+
+  return FALSE;
+}
+
+// static
+gboolean MessagePumpForUI::WorkSourceCheck(GSource* source) {
+  MessagePumpForUI* self = static_cast<WorkSource*>(source)->self;
+  RunState* state = self->state_;
+
+  // Make sure we don't attempt idle work until we are really sure we don't
+  // have other work to do.  We'll know this in the call to Prepare.
+  state->should_do_idle_work = false;
+
+  // Check if ScheduleWork or ScheduleDelayedWork was called.  This is a
+  // non-blocking read.
+  char byte;
+  while (0 < read(self->read_fd_work_scheduled_, &byte, 1)) {
+    // Don't assume we actually have work yet unless the stronger ScheduleWork
+    // was called.
+    if (byte == kWorkScheduled)
+      state->should_do_work = true;
+  }
+
+  if (state->should_do_work)
+    return TRUE;
+
+  if (!self->delayed_work_time_.is_null())
+    return self->delayed_work_time_ <= Time::Now();
+
+  return FALSE;
+}
+
+// static
+gboolean MessagePumpForUI::WorkSourceDispatch(GSource* source,
+                                              GSourceFunc unused_func,
+                                              gpointer unused_data) {
+  MessagePumpForUI* self = static_cast<WorkSource*>(source)->self;
+  RunState* state = self->state_;
+  DCHECK(!state->should_do_idle_work) <<
+      "Idle work should not be flagged while regular work exists.";
+
+  // Note that in this function we never return FALSE.  This source is owned
+  // by GLib and shared by multiple calls to Run.  It will only finally get
+  // destroyed when the loop is destroyed.
+
+  state->should_do_work = state->delegate->DoWork();
+  if (!state->keep_running)
+    return TRUE;
+
+  state->should_do_work |=
+      state->delegate->DoDelayedWork(&self->delayed_work_time_);
+
+  return TRUE;
+}
+
+// static
+gboolean MessagePumpForUI::IdleSourcePrepare(GSource* source,
+                                             gint* timeout_ms) {
+  RunState* state = static_cast<WorkSource*>(source)->self->state_;
+  *timeout_ms = 0;
+  return state->should_do_idle_work;
+}
+
+// static
+gboolean MessagePumpForUI::IdleSourceCheck(GSource* source) {
+  RunState* state = static_cast<WorkSource*>(source)->self->state_;
+  return state->should_do_idle_work;
+}
+
+// static
+gboolean MessagePumpForUI::IdleSourceDispatch(GSource* source,
+                                              GSourceFunc unused_func,
+                                              gpointer unused_data) {
+  RunState* state = static_cast<WorkSource*>(source)->self->state_;
+  // We should not do idle work unless we didn't have other work to do.
+  DCHECK(!state->should_do_work) << "Doing idle work in non-idle time!";
+  state->should_do_idle_work = false;
+  state->should_do_work = state->delegate->DoIdleWork();
+
+  // This is an optimization.  We could always remove ourselves right now,
+  // but we will just get re-added when WorkSourceCheck eventually returns
+  // FALSE.
+  if (!state->should_do_work) {
+    // This is so that when we return FALSE, GLib will not only remove us
+    // from the context, but since it holds the last reference, it will
+    // destroy us as well.
+    g_source_unref(source);
+    state->idle_source = NULL;
+  }
+
+  return state->should_do_work;
+}
+
+GSource* MessagePumpForUI::AddSource(GSourceFuncs* funcs, gint priority,
+                                     GPollFD *optional_poll_fd) {
+  GSource* source = g_source_new(funcs, sizeof(WorkSource));
+
+  // Setting the priority is actually a bit expensive since it causes GLib
+  // to resort an internal list.
+  if (priority != G_PRIORITY_DEFAULT)
+    g_source_set_priority(source, priority);
+
+  // This is needed to allow Run calls inside Dispatch.
+  g_source_set_can_recurse(source, TRUE);
+  static_cast<WorkSource*>(source)->self = this;
+
+  if (optional_poll_fd)
+    g_source_add_poll(source, optional_poll_fd);
+
+  g_source_attach(source, context_);
+  return source;
+}
+
+}  // namespace base