Adds ability to tell Event::Wait to yield.

This will be used by simulated time controller to allow processing other
tasks while waiting on an Event. This makes posting of blocking tasks
possible.

Bug: webrtc:10365
Change-Id: Ic3fb156d545eed2c036939121b89295433176e26
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128121
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27214}

rtc_base/synchronization/BUILD.gn

@@ -35,3 +35,27 @@ rtc_source_set("rw_lock_wrapper") {
     ]
   }
 }
+
+rtc_source_set("yield_policy") {
+  sources = [
+    "yield_policy.cc",
+    "yield_policy.h",
+  ]
+  deps = [
+    "//third_party/abseil-cpp/absl/base:core_headers",
+  ]
+}
+
+if (rtc_include_tests) {
+  rtc_source_set("synchronization_unittests") {
+    testonly = true
+    sources = [
+      "yield_policy_unittest.cc",
+    ]
+    deps = [
+      ":yield_policy",
+      "..:rtc_event",
+      "../../test:test_support",
+    ]
+  }
+}

rtc_base/synchronization/yield_policy.cc

@@ -0,0 +1,32 @@
+/*
+ *  Copyright 2019 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#include "rtc_base/synchronization/yield_policy.h"
+
+#include "absl/base/attributes.h"
+
+namespace rtc {
+namespace {
+ABSL_CONST_INIT thread_local YieldInterface* current_yield_policy = nullptr;
+}
+
+ScopedYieldPolicy::ScopedYieldPolicy(YieldInterface* policy)
+    : previous_(current_yield_policy) {
+  current_yield_policy = policy;
+}
+
+ScopedYieldPolicy::~ScopedYieldPolicy() {
+  current_yield_policy = previous_;
+}
+
+void ScopedYieldPolicy::YieldExecution() {
+  if (current_yield_policy)
+    current_yield_policy->YieldExecution();
+}
+}  // namespace rtc

rtc_base/synchronization/yield_policy.h

@@ -0,0 +1,36 @@
+/*
+ *  Copyright 2019 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#ifndef RTC_BASE_SYNCHRONIZATION_YIELD_POLICY_H_
+#define RTC_BASE_SYNCHRONIZATION_YIELD_POLICY_H_
+
+namespace rtc {
+class YieldInterface {
+ public:
+  virtual ~YieldInterface() = default;
+  virtual void YieldExecution() = 0;
+};
+
+// Sets the current thread-local yield policy while it's in scope and reverts
+// to the previous policy when it leaves the scope.
+class ScopedYieldPolicy final {
+ public:
+  explicit ScopedYieldPolicy(YieldInterface* policy);
+  ~ScopedYieldPolicy();
+  // Will yield as specified by the currently active thread-local yield policy
+  // (which by default is a no-op).
+  static void YieldExecution();
+
+ private:
+  YieldInterface* const previous_;
+};
+
+}  // namespace rtc
+
+#endif  // RTC_BASE_SYNCHRONIZATION_YIELD_POLICY_H_

rtc_base/synchronization/yield_policy_unittest.cc

@@ -0,0 +1,67 @@
+/*
+ *  Copyright 2019 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include <thread>  // Not allowed in production per Chromium style guide.
+
+#include "rtc_base/event.h"
+#include "rtc_base/synchronization/yield_policy.h"
+#include "test/gmock.h"
+#include "test/gtest.h"
+
+namespace rtc {
+namespace {
+class MockYieldHandler : public YieldInterface {
+ public:
+  MOCK_METHOD0(YieldExecution, void());
+};
+}  // namespace
+TEST(YieldPolicyTest, HandlerReceivesYieldSignalWhenSet) {
+  testing::StrictMock<MockYieldHandler> handler;
+  {
+    Event event;
+    EXPECT_CALL(handler, YieldExecution()).Times(1);
+    ScopedYieldPolicy policy(&handler);
+    event.Set();
+    event.Wait(Event::kForever);
+  }
+  {
+    Event event;
+    EXPECT_CALL(handler, YieldExecution()).Times(0);
+    event.Set();
+    event.Wait(Event::kForever);
+  }
+}
+
+TEST(YieldPolicyTest, IsThreadLocal) {
+  Event events[3];
+  std::thread other_thread([&]() {
+    testing::StrictMock<MockYieldHandler> local_handler;
+    // The local handler is never called as we never Wait on this thread.
+    EXPECT_CALL(local_handler, YieldExecution()).Times(0);
+    ScopedYieldPolicy policy(&local_handler);
+    events[0].Set();
+    events[1].Set();
+    events[2].Set();
+  });
+
+  // Waiting until the other thread has entered the scoped policy.
+  events[0].Wait(Event::kForever);
+  // Wait on this thread should not trigger the handler of that policy as it's
+  // thread local.
+  events[1].Wait(Event::kForever);
+
+  // We can set a policy that's active on this thread independently.
+  testing::StrictMock<MockYieldHandler> main_handler;
+  EXPECT_CALL(main_handler, YieldExecution()).Times(1);
+  ScopedYieldPolicy policy(&main_handler);
+  events[2].Wait(Event::kForever);
+  other_thread.join();
+}
+}  // namespace rtc