Refactor: move Task Queue to its own class (flutter/engine#9301)

* Refactor to move Task Queue to its own class

- This is to help with sharing task queue among
  multiple message loops going forward.

- currently there is 1:1 mapping between task queue
  and message loop, we are still maintaining the semantics
  for this change.

* Add mutex include

* Add unit tests for task queue

* fix formatting

* license

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -134,6 +134,9 @@ FILE: ../../../flutter/fml/message_loop.cc
 FILE: ../../../flutter/fml/message_loop.h
 FILE: ../../../flutter/fml/message_loop_impl.cc
 FILE: ../../../flutter/fml/message_loop_impl.h
+FILE: ../../../flutter/fml/message_loop_task_queue.cc
+FILE: ../../../flutter/fml/message_loop_task_queue.h
+FILE: ../../../flutter/fml/message_loop_task_queue_unittests.cc
 FILE: ../../../flutter/fml/message_loop_unittests.cc
 FILE: ../../../flutter/fml/message_unittests.cc
 FILE: ../../../flutter/fml/native_library.h

engine/src/flutter/fml/BUILD.gn

@@ -46,6 +46,8 @@ source_set("fml") {
     "message_loop.h",
     "message_loop_impl.cc",
     "message_loop_impl.h",
+    "message_loop_task_queue.cc",
+    "message_loop_task_queue.h",
     "native_library.h",
     "paths.cc",
     "paths.h",
@@ -198,6 +200,7 @@ executable("fml_unittests") {
     "file_unittest.cc",
     "memory/ref_counted_unittest.cc",
     "memory/weak_ptr_unittest.cc",
+    "message_loop_task_queue_unittests.cc",
     "message_loop_unittests.cc",
     "message_unittests.cc",
     "paths_unittests.cc",

engine/src/flutter/fml/message_loop_impl.cc

@@ -39,13 +39,22 @@ fml::RefPtr<MessageLoopImpl> MessageLoopImpl::Create() {
 #endif
 }
 
-MessageLoopImpl::MessageLoopImpl() : order_(0), terminated_(false) {}
+MessageLoopImpl::MessageLoopImpl() : terminated_(false) {
+  task_queue_ = std::make_unique<MessageLoopTaskQueue>();
+}
 
 MessageLoopImpl::~MessageLoopImpl() = default;
 
 void MessageLoopImpl::PostTask(fml::closure task, fml::TimePoint target_time) {
   FML_DCHECK(task != nullptr);
-  RegisterTask(task, target_time);
+  FML_DCHECK(task != nullptr);
+  if (terminated_) {
+    // If the message loop has already been terminated, PostTask should destruct
+    // |task| synchronously within this function.
+    return;
+  }
+  const auto wake_up = task_queue_->RegisterTask(task, target_time);
+  WakeUp(wake_up);
 }
 
 void MessageLoopImpl::AddTaskObserver(intptr_t key, fml::closure callback) {
@@ -53,16 +62,14 @@ void MessageLoopImpl::AddTaskObserver(intptr_t key, fml::closure callback) {
   FML_DCHECK(MessageLoop::GetCurrent().GetLoopImpl().get() == this)
       << "Message loop task observer must be added on the same thread as the "
          "loop.";
-  std::lock_guard<std::mutex> observers_lock(observers_mutex_);
-  task_observers_[key] = std::move(callback);
+  task_queue_->AddTaskObserver(key, callback);
 }
 
 void MessageLoopImpl::RemoveTaskObserver(intptr_t key) {
   FML_DCHECK(MessageLoop::GetCurrent().GetLoopImpl().get() == this)
       << "Message loop task observer must be removed from the same thread as "
          "the loop.";
-  std::lock_guard<std::mutex> observers_lock(observers_mutex_);
-  task_observers_.erase(key);
+  task_queue_->RemoveTaskObserver(key);
 }
 
 void MessageLoopImpl::DoRun() {
@@ -88,8 +95,7 @@ void MessageLoopImpl::DoRun() {
   // should be destructed on the message loop's thread. We have just returned
   // from the implementations |Run| method which we know is on the correct
   // thread. Drop all pending tasks on the floor.
-  std::lock_guard<std::mutex> lock(delayed_tasks_mutex_);
-  delayed_tasks_ = {};
+  task_queue_->Dispose();
 }
 
 void MessageLoopImpl::DoTerminate() {
@@ -109,31 +115,8 @@ void MessageLoopImpl::SwapTaskQueues(const fml::RefPtr<MessageLoopImpl>& other)
   std::unique_lock<std::mutex> t2(other->tasks_flushing_mutex_,
                                   std::defer_lock);
 
-  // task_observers locks
-  std::unique_lock<std::mutex> o1(observers_mutex_, std::defer_lock);
-  std::unique_lock<std::mutex> o2(other->observers_mutex_, std::defer_lock);
-
-  // delayed_tasks locks
-  std::unique_lock<std::mutex> d1(delayed_tasks_mutex_, std::defer_lock);
-  std::unique_lock<std::mutex> d2(other->delayed_tasks_mutex_, std::defer_lock);
-
-  std::lock(t1, t2, o1, o2, d1, d2);
-
-  std::swap(task_observers_, other->task_observers_);
-  std::swap(delayed_tasks_, other->delayed_tasks_);
-}
-
-void MessageLoopImpl::RegisterTask(fml::closure task,
-                                   fml::TimePoint target_time) {
-  FML_DCHECK(task != nullptr);
-  if (terminated_) {
-    // If the message loop has already been terminated, PostTask should destruct
-    // |task| synchronously within this function.
-    return;
-  }
-  std::lock_guard<std::mutex> lock(delayed_tasks_mutex_);
-  delayed_tasks_.push({++order_, std::move(task), target_time});
-  WakeUp(delayed_tasks_.top().GetTargetTime());
+  std::lock(t1, t2);
+  task_queue_->Swap(*other->task_queue_);
 }
 
 void MessageLoopImpl::FlushTasks(FlushType type) {
@@ -148,36 +131,12 @@ void MessageLoopImpl::FlushTasks(FlushType type) {
   // will lead us to run invocations on the wrong thread.
   std::lock_guard<std::mutex> task_flush_lock(tasks_flushing_mutex_);
 
-  {
-    std::lock_guard<std::mutex> lock(delayed_tasks_mutex_);
-
-    if (delayed_tasks_.empty()) {
-      return;
-    }
-
-    auto now = fml::TimePoint::Now();
-    while (!delayed_tasks_.empty()) {
-      const auto& top = delayed_tasks_.top();
-      if (top.GetTargetTime() > now) {
-        break;
-      }
-      invocations.emplace_back(std::move(top.GetTask()));
-      delayed_tasks_.pop();
-      if (type == FlushType::kSingle) {
-        break;
-      }
-    }
-
-    WakeUp(delayed_tasks_.empty() ? fml::TimePoint::Max()
-                                  : delayed_tasks_.top().GetTargetTime());
-  }
+  const auto wake_up = task_queue_->GetTasksToRunNow(type, invocations);
+  WakeUp(wake_up);
 
   for (const auto& invocation : invocations) {
     invocation();
-    std::lock_guard<std::mutex> observers_lock(observers_mutex_);
-    for (const auto& observer : task_observers_) {
-      observer.second();
-    }
+    task_queue_->NotifyObservers();
   }
 }
 

engine/src/flutter/fml/message_loop_impl.h

@@ -17,6 +17,7 @@
 #include "flutter/fml/macros.h"
 #include "flutter/fml/memory/ref_counted.h"
 #include "flutter/fml/message_loop.h"
+#include "flutter/fml/message_loop_task_queue.h"
 #include "flutter/fml/synchronization/thread_annotations.h"
 #include "flutter/fml/time/time_point.h"
 
@@ -61,21 +62,9 @@ class MessageLoopImpl : public fml::RefCountedThreadSafe<MessageLoopImpl> {
  private:
   std::mutex tasks_flushing_mutex_;
 
-  std::mutex observers_mutex_;
-  std::map<intptr_t, fml::closure> task_observers_
-      FML_GUARDED_BY(observers_mutex_);
-
-  std::mutex delayed_tasks_mutex_;
-  DelayedTaskQueue delayed_tasks_ FML_GUARDED_BY(delayed_tasks_mutex_);
-  size_t order_ FML_GUARDED_BY(delayed_tasks_mutex_);
+  std::unique_ptr<MessageLoopTaskQueue> task_queue_;
   std::atomic_bool terminated_;
 
-  void RegisterTask(fml::closure task, fml::TimePoint target_time);
-
-  enum class FlushType {
-    kSingle,
-    kAll,
-  };
   void FlushTasks(FlushType type);
 
   FML_DISALLOW_COPY_AND_ASSIGN(MessageLoopImpl);

engine/src/flutter/fml/message_loop_task_queue.cc

@@ -0,0 +1,97 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#define FML_USED_ON_EMBEDDER
+
+#include "flutter/fml/message_loop_task_queue.h"
+
+namespace fml {
+
+MessageLoopTaskQueue::MessageLoopTaskQueue() : order_(0) {}
+
+MessageLoopTaskQueue::~MessageLoopTaskQueue() = default;
+
+void MessageLoopTaskQueue::Dispose() {
+  std::lock_guard<std::mutex> lock(delayed_tasks_mutex_);
+  delayed_tasks_ = {};
+}
+
+fml::TimePoint MessageLoopTaskQueue::RegisterTask(fml::closure task,
+                                                  fml::TimePoint target_time) {
+  std::lock_guard<std::mutex> lock(delayed_tasks_mutex_);
+  delayed_tasks_.push({++order_, std::move(task), target_time});
+  return delayed_tasks_.top().GetTargetTime();
+}
+
+bool MessageLoopTaskQueue::HasPendingTasks() {
+  std::lock_guard<std::mutex> lock(delayed_tasks_mutex_);
+  return !delayed_tasks_.empty();
+}
+
+fml::TimePoint MessageLoopTaskQueue::GetTasksToRunNow(
+    FlushType type,
+    std::vector<fml::closure>& invocations) {
+  std::lock_guard<std::mutex> lock(delayed_tasks_mutex_);
+
+  const auto now = fml::TimePoint::Now();
+  while (!delayed_tasks_.empty()) {
+    const auto& top = delayed_tasks_.top();
+    if (top.GetTargetTime() > now) {
+      break;
+    }
+    invocations.emplace_back(std::move(top.GetTask()));
+    delayed_tasks_.pop();
+    if (type == FlushType::kSingle) {
+      break;
+    }
+  }
+
+  if (delayed_tasks_.empty()) {
+    return fml::TimePoint::Max();
+  } else {
+    return delayed_tasks_.top().GetTargetTime();
+  }
+}
+
+size_t MessageLoopTaskQueue::GetNumPendingTasks() {
+  std::lock_guard<std::mutex> lock(delayed_tasks_mutex_);
+  return delayed_tasks_.size();
+}
+
+void MessageLoopTaskQueue::AddTaskObserver(intptr_t key,
+                                           fml::closure callback) {
+  std::lock_guard<std::mutex> observers_lock(observers_mutex_);
+  task_observers_[key] = std::move(callback);
+}
+
+void MessageLoopTaskQueue::RemoveTaskObserver(intptr_t key) {
+  std::lock_guard<std::mutex> observers_lock(observers_mutex_);
+  task_observers_.erase(key);
+}
+
+void MessageLoopTaskQueue::NotifyObservers() {
+  std::lock_guard<std::mutex> observers_lock(observers_mutex_);
+  for (const auto& observer : task_observers_) {
+    observer.second();
+  }
+}
+
+// Thread safety analysis disabled as it does not account for defered locks.
+void MessageLoopTaskQueue::Swap(MessageLoopTaskQueue& other)
+    FML_NO_THREAD_SAFETY_ANALYSIS {
+  // task_observers locks
+  std::unique_lock<std::mutex> o1(observers_mutex_, std::defer_lock);
+  std::unique_lock<std::mutex> o2(other.observers_mutex_, std::defer_lock);
+
+  // delayed_tasks locks
+  std::unique_lock<std::mutex> d1(delayed_tasks_mutex_, std::defer_lock);
+  std::unique_lock<std::mutex> d2(other.delayed_tasks_mutex_, std::defer_lock);
+
+  std::lock(o1, o2, d1, d2);
+
+  std::swap(task_observers_, other.task_observers_);
+  std::swap(delayed_tasks_, other.delayed_tasks_);
+}
+
+}  // namespace fml

engine/src/flutter/fml/message_loop_task_queue.h

@@ -0,0 +1,75 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_FML_MESSAGE_LOOP_TASK_QUEUE_H_
+#define FLUTTER_FML_MESSAGE_LOOP_TASK_QUEUE_H_
+
+#include <map>
+#include <mutex>
+#include <vector>
+
+#include "flutter/fml/closure.h"
+#include "flutter/fml/delayed_task.h"
+#include "flutter/fml/macros.h"
+#include "flutter/fml/memory/ref_counted.h"
+#include "flutter/fml/synchronization/thread_annotations.h"
+
+namespace fml {
+
+enum class FlushType {
+  kSingle,
+  kAll,
+};
+
+// This class keeps track of all the tasks and observers that
+// need to be run on it's MessageLoopImpl.
+class MessageLoopTaskQueue {
+ public:
+  // Lifecycle.
+
+  MessageLoopTaskQueue();
+
+  ~MessageLoopTaskQueue();
+
+  void Dispose();
+
+  // Tasks methods.
+
+  fml::TimePoint RegisterTask(fml::closure task, fml::TimePoint target_time);
+
+  bool HasPendingTasks();
+
+  // Returns the wake up time.
+  fml::TimePoint GetTasksToRunNow(FlushType type,
+                                  std::vector<fml::closure>& invocations);
+
+  size_t GetNumPendingTasks();
+
+  // Observers methods.
+
+  void AddTaskObserver(intptr_t key, fml::closure callback);
+
+  void RemoveTaskObserver(intptr_t key);
+
+  void NotifyObservers();
+
+  // Misc.
+
+  void Swap(MessageLoopTaskQueue& other);
+
+ private:
+  std::mutex observers_mutex_;
+  std::map<intptr_t, fml::closure> task_observers_
+      FML_GUARDED_BY(observers_mutex_);
+
+  std::mutex delayed_tasks_mutex_;
+  DelayedTaskQueue delayed_tasks_ FML_GUARDED_BY(delayed_tasks_mutex_);
+  size_t order_ FML_GUARDED_BY(delayed_tasks_mutex_);
+
+  FML_DISALLOW_COPY_ASSIGN_AND_MOVE(MessageLoopTaskQueue);
+};
+
+}  // namespace fml
+
+#endif  // FLUTTER_FML_MESSAGE_LOOP_TASK_QUEUE_H_

engine/src/flutter/fml/message_loop_task_queue_unittests.cc

@@ -0,0 +1,70 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#define FML_USED_ON_EMBEDDER
+
+#include "flutter/fml/message_loop_task_queue.h"
+#include "gtest/gtest.h"
+
+TEST(MessageLoopTaskQueue, StartsWithNoPendingTasks) {
+  auto task_queue = std::make_unique<fml::MessageLoopTaskQueue>();
+  ASSERT_FALSE(task_queue->HasPendingTasks());
+}
+
+TEST(MessageLoopTaskQueue, RegisterOneTask) {
+  auto task_queue = std::make_unique<fml::MessageLoopTaskQueue>();
+  const auto time = fml::TimePoint::Max();
+  const auto wake_time = task_queue->RegisterTask([] {}, time);
+  ASSERT_TRUE(task_queue->HasPendingTasks());
+  ASSERT_TRUE(task_queue->GetNumPendingTasks() == 1);
+  ASSERT_TRUE(wake_time == time);
+}
+
+TEST(MessageLoopTaskQueue, RegisterTwoTasksAndCount) {
+  auto task_queue = std::make_unique<fml::MessageLoopTaskQueue>();
+  task_queue->RegisterTask([] {}, fml::TimePoint::Now());
+  task_queue->RegisterTask([] {}, fml::TimePoint::Max());
+  ASSERT_TRUE(task_queue->HasPendingTasks());
+  ASSERT_TRUE(task_queue->GetNumPendingTasks() == 2);
+}
+
+TEST(MessageLoopTaskQueue, PreserveTaskOrdering) {
+  auto task_queue = std::make_unique<fml::MessageLoopTaskQueue>();
+  int test_val = 0;
+
+  // order: 0
+  task_queue->RegisterTask([&test_val]() { test_val = 1; },
+                           fml::TimePoint::Now());
+
+  // order: 1
+  task_queue->RegisterTask([&test_val]() { test_val = 2; },
+                           fml::TimePoint::Now());
+
+  std::vector<fml::closure> invocations;
+  task_queue->GetTasksToRunNow(fml::FlushType::kAll, invocations);
+
+  int expected_value = 1;
+
+  for (auto& invocation : invocations) {
+    invocation();
+    ASSERT_TRUE(test_val == expected_value);
+    expected_value++;
+  }
+}
+
+TEST(MessageLoopTaskQueue, AddRemoveNotifyObservers) {
+  auto task_queue = std::make_unique<fml::MessageLoopTaskQueue>();
+
+  int test_val = 0;
+  intptr_t key = 123;
+
+  task_queue->AddTaskObserver(key, [&test_val]() { test_val = 1; });
+  task_queue->NotifyObservers();
+  ASSERT_TRUE(test_val == 1);
+
+  test_val = 0;
+  task_queue->RemoveTaskObserver(key);
+  task_queue->NotifyObservers();
+  ASSERT_TRUE(test_val == 0);
+}