AsyncLogWriter.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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.
 */

#pragma once

#include <condition_variable>
#include <mutex>
#include <thread>

#include <folly/File.h>
#include <folly/Range.h>
#include <folly/Synchronized.h>
#include <folly/logging/LogWriter.h>

namespace folly {

/**
 * An abstract LogWriter implementation that provides functionality for
 * asynchronous IO operations. Users can subclass this class and provide their
 * own IO operation implementation by overriding `performIO` method. This class
 * will automatically manage incoming log messages and call the method in
 * appropriate time.
 *
 * This class performs the log I/O in a separarate thread.
 *
 * The advantage of this class over ImmediateFileWriter is that logging I/O can
 * never slow down or block your normal program operation.  If log messages are
 * generated faster than they can be written, messages will be dropped (and an
 * indication of how many messages were dropped will be written to the log file
 * when we are able to catch up a bit.)
 *
 * However, one downside is that if your program crashes, not all log messages
 * may have been written, so you may lose messages generated immediately before
 * the crash.
 */
class AsyncLogWriter : public LogWriter {
 public:
  /**
   * The default maximum buffer size.
   *
   * The comments for setMaxBufferSize() explain how this parameter is used.
   */
  static constexpr size_t kDefaultMaxBufferSize = 1024 * 1024;

  explicit AsyncLogWriter();

  virtual ~AsyncLogWriter() override;

  void writeMessage(folly::StringPiece buffer, uint32_t flags = 0) override;

  void writeMessage(std::string&& buffer, uint32_t flags = 0) override;

  /**
   * Block until the I/O thread has finished writing all messages that
   * were already enqueued when flush() was called.
   */
  void flush() override;

  /**
   * Set the maximum buffer size for this AsyncLogWriter, in bytes.
   *
   * This controls the upper bound on how much unwritten data will be buffered
   * in memory.  If messages are being logged faster than they can be written
   * to output file, new messages will be discarded if they would cause the
   * amount of buffered data to exceed this limit.
   */
  void setMaxBufferSize(size_t size);

  /**
   * Get the maximum buffer size for this AsyncLogWriter, in bytes.
   */
  size_t getMaxBufferSize() const;

  using DiscardCallback = void (*)(size_t);

  /**
   * Set a callback to be invoked when log messages have been discarded.
   *
   * Each time a batch of log messages is discarded, the callback will be
   * invoked with the number of discarded messages.
   *
   * Note: setDiscardCallback() does not wait for the I/O thread to finish
   * using the discard callback, and so the old callback may continue to be
   * invoked in the I/O thread for a brief period of time even after
   * setDiscardCallback() returns.
   */
  static void setDiscardCallback(DiscardCallback callback);

 protected:
  /**
   * Drain up the log message queue. Subclasses must call this method in their
   * destructors to avoid losing log messages at shutdown.
   */
  void cleanup();

 private:
  enum Flags : uint32_t {
    // FLAG_IO_THREAD_STARTED indicates that the constructor has started the
    // I/O thread.
    FLAG_IO_THREAD_STARTED = 0x01,
    // FLAG_DESTROYING indicates that the destructor is running and destroying
    // the I/O thread.
    FLAG_DESTROYING = 0x02,
    // FLAG_STOP indicates that the I/O thread has been asked to stop.
    // This is set either by the destructor or by preFork()
    FLAG_STOP = 0x04,
    // FLAG_IO_THREAD_STOPPED indicates that the I/O thread is about to return
    // and can now be joined.  ioCV_ will be signalled when this flag is set.
    FLAG_IO_THREAD_STOPPED = 0x08,
    // FLAG_IO_THREAD_JOINED indicates that the I/O thread has been joined.
    FLAG_IO_THREAD_JOINED = 0x10,
  };

  /*
   * A simple implementation using two queues.
   * All writer threads enqueue into one queue while the I/O thread is
   * processing the other.
   *
   * We could potentially also provide an implementation using folly::MPMCQueue
   * in the future, which may improve contention under very high write loads.
   */
  struct Data {
    std::array<std::vector<std::string>, 2> queues;
    uint32_t flags{0};
    uint64_t ioThreadCounter{0};
    size_t maxBufferBytes{kDefaultMaxBufferSize};
    size_t currentBufferSize{0};
    size_t numDiscarded{0};
    std::thread ioThread;

    std::vector<std::string>* getCurrentQueue() {
      return &queues[ioThreadCounter & 0x1];
    }
  };

  /**
   * Subclasses should override this method to provide IO operations on log
   * messages. This method will be called in a separate IO thread.
   */
  virtual void performIO(
      const std::vector<std::string>& logs, size_t numDiscarded) = 0;

  void invokeDiscardCallback(size_t numDiscarded);

  void ioThread();

  bool preFork();
  void postForkParent();
  void postForkChild();
  void stopIoThread(
      folly::Synchronized<Data, std::mutex>::LockedPtr& data,
      uint32_t extraFlags);
  void restartThread();

  folly::Synchronized<Data, std::mutex> data_;
  /**
   * messageReady_ is signaled by writer threads whenever they add a new
   * message to the current queue.
   */
  std::condition_variable messageReady_;
  /**
   * ioCV_ is signaled by the I/O thread each time it increments
   * the ioThreadCounter (once each time around its loop).
   */
  std::condition_variable ioCV_;

  /**
   * lockedData_ exists only to help pass the lock between preFork() and
   * postForkParent()/postForkChild().  We potentially could add some new
   * low-level methods to Synchronized to allow manually locking and unlocking
   * to avoid having to store this object as a member variable.
   */
  folly::Synchronized<Data, std::mutex>::LockedPtr lockedData_;

  static FOLLY_CONSTINIT std::atomic<DiscardCallback> discardCallback_;
};

} // namespace folly