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

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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.
+
+#ifndef BASE_TASK_COMMON_OPERATIONS_CONTROLLER_H_
+#define BASE_TASK_COMMON_OPERATIONS_CONTROLLER_H_
+
+#include <atomic>
+#include <cstdint>
+
+#include "base/synchronization/waitable_event.h"
+
+namespace base {
+namespace internal {
+
+// A lock-free thread-safe controller to manage critical multi-threaded
+// operations without locks.
+//
+// The controller is used to determine if operations are allowed, and to keep
+// track of how many are currently active. Users will call TryBeginOperation()
+// before starting such operations. If the call succeeds the user can run the
+// operation and the controller will keep track of it until the user signals
+// that the operation is completed. No operations are allowed before
+// StartAcceptingOperations() is called, or after
+// ShutdownAndWaitForZeroOperations() is called.
+//
+// There is no explicit way of telling the controller when an operation is
+// completed, instead for convenience TryBeginOperation() will return a RAII
+// like object that will do so on destruction.
+//
+// For example:
+//
+// OperationsController controller_;
+//
+// void SetUp() {
+//   controller_.StartAcceptingOperations();
+// }
+//
+// void TearDown() {
+//   controller_.ShutdownAndWaitForZeroOperations();
+// }
+//
+// void MaybeRunOperation() {
+//   auto operation_token = controller_.TryBeginOperation();
+//   if (operation_token) {
+//     Process();
+//   }
+// }
+//
+// This class is thread-safe.
+// But note that StartAcceptingOperations can never be called after
+// ShutdownAndWaitForZeroOperations.
+class BASE_EXPORT OperationsController {
+ public:
+  // The owner of an OperationToken which evaluates to true can safely perform
+  // an operation while being certain it happens-after
+  // StartAcceptingOperations() and happens-before
+  // ShutdownAndWaitForZeroOperations(). Releasing this OperationToken
+  // relinquishes this right.
+  //
+  // This class is thread-safe
+  class OperationToken {
+   public:
+    ~OperationToken() {
+      if (outer_)
+        outer_->DecrementBy(1);
+    }
+    OperationToken(const OperationToken&) = delete;
+    OperationToken(OperationToken&& other) {
+      this->outer_ = other.outer_;
+      other.outer_ = nullptr;
+    }
+
+    operator bool() const { return !!outer_; }
+
+   private:
+    friend class OperationsController;
+    explicit OperationToken(OperationsController* outer) : outer_(outer) {}
+    OperationsController* outer_;
+  };
+
+  OperationsController();
+
+  // Users must call ShutdownAndWaitForZeroOperations() before destroying an
+  // instance of this class if StartAcceptingOperations() was called.
+  ~OperationsController();
+
+  OperationsController(const OperationsController&) = delete;
+  OperationsController& operator=(const OperationsController&) = delete;
+
+  // Starts to accept operations (before this point TryBeginOperation() returns
+  // an invalid token). Returns true if an attempt to perform an operation was
+  // made and denied before StartAcceptingOperations() was called. Can be called
+  // at most once, never after ShutdownAndWaitForZeroOperations().
+  bool StartAcceptingOperations();
+
+  // Returns a RAII like object that implicitly converts to true if operations
+  // are allowed i.e. if this call happens-after StartAcceptingOperations() and
+  // happens-before Shutdown(), otherwise the object will convert to false. On
+  // successful return, this OperationsController will keep track of the
+  // operation until the returned object goes out of scope.
+  OperationToken TryBeginOperation();
+
+  // Prevents further calls to TryBeginOperation() from succeeding and waits for
+  // all the ongoing operations to complete.
+  //
+  // Attention: Can only be called once.
+  void ShutdownAndWaitForZeroOperations();
+
+ private:
+  // Atomic representation of the state of this class. We use the upper 2 bits
+  // to keep track of flag like values and the remainder bits are used as a
+  // counter. The 2 flags are used to represent 3 different states:
+  //
+  // State                   | AcceptOperations Bit | ShuttingDown Bit
+  // --------------------------------------------------------------
+  // kRejectingOperations    | 0                    | 0
+  // kAcceptingOperations    | 1                    | 0
+  // kShuttingDown           | *                    | 1
+  //
+  // The counter keeps track of the rejected operations when we are in
+  // the kRejectingOperations state, the number of inflight operations
+  // otherwise. If the count reaches zero and we are in the shutting down state
+  // |shutdown_complete_| will be signaled.
+  static constexpr uint32_t kShuttingDownBitMask = uint32_t{1} << 31;
+  static constexpr uint32_t kAcceptingOperationsBitMask = uint32_t{1} << 30;
+  static constexpr uint32_t kFlagsBitMask =
+      (kShuttingDownBitMask | kAcceptingOperationsBitMask);
+  static constexpr uint32_t kCountBitMask = ~kFlagsBitMask;
+  enum class State {
+    kRejectingOperations,
+    kAcceptingOperations,
+    kShuttingDown,
+  };
+
+  // Helper methods for the bit fiddling. Pass a |state_and_count_| value to
+  // extract state or count out of it.
+  static uint32_t ExtractCount(uint32_t value) { return value & kCountBitMask; }
+  static State ExtractState(uint32_t value);
+
+  // Decrements the counter by |n| and signals |shutdown_complete_| if needed.
+  void DecrementBy(uint32_t n);
+
+  std::atomic<uint32_t> state_and_count_{0};
+  WaitableEvent shutdown_complete_;
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_TASK_COMMON_OPERATIONS_CONTROLLER_H_