Repo created

TMessagesProj/jni/voip/webrtc/base/task/common/operations_controller.cc

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+// Copyright 2018 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/task/common/operations_controller.h"
+#include "base/logging.h"
+
+namespace base {
+namespace internal {
+
+OperationsController::OperationsController() = default;
+
+OperationsController::~OperationsController() {
+#if DCHECK_IS_ON()
+  // An OperationsController may only be deleted when it was either not
+  // accepting operations or after it was shutdown and there are no in flight
+  // attempts to perform operations.
+  auto value = state_and_count_.load();
+  DCHECK(
+      ExtractState(value) == State::kRejectingOperations ||
+      (ExtractState(value) == State::kShuttingDown && ExtractCount(value) == 0))
+      << value;
+#endif
+}
+
+bool OperationsController::StartAcceptingOperations() {
+  // Release semantics are required to ensure that all memory accesses made on
+  // this thread happen-before any others done on a thread which is later
+  // allowed to perform an operation.
+  auto prev_value = state_and_count_.fetch_or(kAcceptingOperationsBitMask,
+                                              std::memory_order_release);
+
+  DCHECK_EQ(ExtractState(prev_value), State::kRejectingOperations);
+  // The count is the number of rejected operations, unwind them now.
+  auto num_rejected = ExtractCount(prev_value);
+  DecrementBy(num_rejected);
+  return num_rejected != 0;
+}
+
+OperationsController::OperationToken OperationsController::TryBeginOperation() {
+  // Acquire semantics are required to ensure that a thread which is allowed to
+  // perform an operation sees all the memory side-effects that happened-before
+  // StartAcceptingOperations(). They're also required so that no operations on
+  // this thread (e.g. the operation itself) can be reordered before this one.
+  auto prev_value = state_and_count_.fetch_add(1, std::memory_order_acquire);
+
+  switch (ExtractState(prev_value)) {
+    case State::kRejectingOperations:
+      return OperationToken(nullptr);
+    case State::kAcceptingOperations:
+      return OperationToken(this);
+    case State::kShuttingDown:
+      DecrementBy(1);
+      return OperationToken(nullptr);
+  }
+}
+
+void OperationsController::ShutdownAndWaitForZeroOperations() {
+  // Acquire semantics are required to guarantee that all memory side-effects
+  // made by other threads that were allowed to perform operations are
+  // synchronized with this thread before it returns from this method.
+  auto prev_value = state_and_count_.fetch_or(kShuttingDownBitMask,
+                                              std::memory_order_acquire);
+
+  switch (ExtractState(prev_value)) {
+    case State::kRejectingOperations:
+      // The count is the number of rejected operations, unwind them now.
+      DecrementBy(ExtractCount(prev_value));
+      break;
+    case State::kAcceptingOperations:
+      if (ExtractCount(prev_value) != 0) {
+        shutdown_complete_.Wait();
+      }
+      break;
+    case State::kShuttingDown:
+      DCHECK(false) << "Multiple calls to ShutdownAndWaitForZeroOperations()";
+      break;
+  }
+}
+
+OperationsController::State OperationsController::ExtractState(uint32_t value) {
+  if (value & kShuttingDownBitMask) {
+    return State::kShuttingDown;
+  } else if (value & kAcceptingOperationsBitMask) {
+    return State::kAcceptingOperations;
+  } else {
+    return State::kRejectingOperations;
+  }
+}
+
+void OperationsController::DecrementBy(uint32_t n) {
+  // Release semantics are required to ensure that no operation on the current
+  // thread (e.g. the operation itself) can be reordered after this one.
+  auto prev_value = state_and_count_.fetch_sub(n, std::memory_order_release);
+  DCHECK_LE(n, ExtractCount(prev_value)) << "Decrement underflow";
+
+  if (ExtractState(prev_value) == State::kShuttingDown &&
+      ExtractCount(prev_value) == n) {
+    shutdown_complete_.Signal();
+  }
+}
+
+}  // namespace internal
+}  // namespace base