callback_scheduler.h

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/*
 * Copyright 2026 The A11 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 A11_CONCURRENCY_CALLBACK_SCHEDULER_H_
#define A11_CONCURRENCY_CALLBACK_SCHEDULER_H_
 
#include <cstddef>
#include <deque>
 
#include <absl/functional/any_invocable.h>
 
#include "thread/boost_primitives.h"
 
namespace a11::internal {
 
// A fair, stackless callback pump. Instances must have process lifetime because
// posted callbacks retain a raw pointer to the scheduler, while queued work
// owns the state it operates on.
class CallbackScheduler {
 public:
  static constexpr size_t kDefaultMaxCallbacksPerTurn = 64;
  static constexpr size_t kDefaultMaxConcurrentTurns = 2;
 
  explicit CallbackScheduler(
      size_t max_callbacks_per_turn = kDefaultMaxCallbacksPerTurn,
      size_t max_concurrent_turns = kDefaultMaxConcurrentTurns)
      : max_callbacks_per_turn_(max_callbacks_per_turn),
        max_concurrent_turns_(max_concurrent_turns < 2 ? 2
                                                       : max_concurrent_turns) {
  }
 
  CallbackScheduler(const CallbackScheduler&) = delete;
  CallbackScheduler& operator=(const CallbackScheduler&) = delete;
 
  void Schedule(absl::AnyInvocable<void() &&> callback);
 
 private:
  void Run();
 
  const size_t max_callbacks_per_turn_;
  const size_t max_concurrent_turns_;
  thread::Mutex mu_;
  std::deque<absl::AnyInvocable<void() &&>> callbacks_ ABSL_GUARDED_BY(mu_);
  size_t active_turns_ ABSL_GUARDED_BY(mu_) = 0;
};
 
}  // namespace a11::internal
 
#endif  // A11_CONCURRENCY_CALLBACK_SCHEDULER_H_