work_serializer.h

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/*
 *
 * Copyright 2019 gRPC authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */
 
#ifndef GRPC_CORE_LIB_IOMGR_WORK_SERIALIZER_H
#define GRPC_CORE_LIB_IOMGR_WORK_SERIALIZER_H
#include <grpc/support/port_platform.h>
 
#include <atomic>
#include <functional>
 
#include "absl/synchronization/mutex.h"
 
#include "src/core/lib/debug/trace.h"
#include "src/core/lib/gprpp/debug_location.h"
#include "src/core/lib/gprpp/mpscq.h"
#include "src/core/lib/gprpp/orphanable.h"
#include "src/core/lib/gprpp/ref_counted.h"
#include "src/core/lib/iomgr/exec_ctx.h"
 
namespace grpc_core {
 
// WorkSerializer is a mechanism to schedule callbacks in a synchronized manner.
// All callbacks scheduled on a WorkSerializer instance will be executed
// serially in a borrowed thread. The API provides a FIFO guarantee to the
// execution of callbacks scheduled on the thread.
// When a thread calls Run() with a callback, the thread is considered borrowed.
// The callback might run inline, or it might run asynchronously in a different
// thread that is already inside of Run(). If the callback runs directly inline,
// other callbacks from other threads might also be executed before Run()
// returns. Since an arbitrary set of callbacks might be executed when Run() is
// called, generally no locks should be held while calling Run().
// If a thread wants to preclude the possibility of the callback being invoked
// inline in Run() (for example, if a mutex lock is held and executing callbacks
// inline would cause a deadlock), it should use Schedule() instead and then
// invoke DrainQueue() when it is safe to invoke the callback.
class ABSL_LOCKABLE WorkSerializer {
 public:
  WorkSerializer();
 
  ~WorkSerializer();
 
  // Runs a given callback on the work serializer. If there is no other thread
  // currently executing the WorkSerializer, the callback is run immediately. In
  // this case, the current thread is also borrowed for draining the queue for
  // any callbacks that get added in the meantime.
  //
  // If you want to use clang thread annotation to make sure that callback is
  // called by WorkSerializer only, you need to add the annotation to both the
  // lambda function given to Run and the actual callback function like;
  //
  //   void run_callback() {
  //     work_serializer.Run(
  //         []() ABSL_EXCLUSIVE_LOCKS_REQUIRED(work_serializer) {
  //            callback();
  //         }, DEBUG_LOCATION);
  //   }
  //   void callback() ABSL_EXCLUSIVE_LOCKS_REQUIRED(work_serializer) { ... }
  //
  // TODO(yashkt): Replace DebugLocation with absl::SourceLocation
  // once we can start using it directly.
  void Run(std::function<void()> callback, const DebugLocation& location);
 
  // Schedule \a callback to be run later when the queue of callbacks is
  // drained.
  void Schedule(std::function<void()> callback, const DebugLocation& location);
  // Drains the queue of callbacks.
  void DrainQueue();
 
 private:
  class WorkSerializerImpl;
 
  OrphanablePtr<WorkSerializerImpl> impl_;
};
 
} /* namespace grpc_core */
 
#endif /* GRPC_CORE_LIB_IOMGR_WORK_SERIALIZER_H */