core/src/main/java/io/grpc/internal/SerializeReentrantCallsDirectExecutor.java

/*
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 * modification, are permitted provided that the following conditions are
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 *
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 *    * Redistributions in binary form must reproduce the above
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 * in the documentation and/or other materials provided with the
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 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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package io.grpc.internal;

import com.google.common.base.Preconditions;
import java.util.ArrayDeque;
import java.util.concurrent.Executor;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * Executes a task directly in the calling thread, unless it's a reentrant call in which case the
 * task is enqueued and executed once the calling task completes.
 *
 * <p>The {@code Executor} assumes that reentrant calls are rare and its fast path is thus
 * optimized for that - avoiding queuing and additional object creation altogether.
 *
 * <p>This class is not thread-safe.
 */
class SerializeReentrantCallsDirectExecutor implements Executor {

  private static final Logger log =
      Logger.getLogger(SerializeReentrantCallsDirectExecutor.class.getName());

  private boolean executing;
  // Lazily initialized if a reentrant call is detected.
  private ArrayDeque<Runnable> taskQueue;

  @Override
  public void execute(Runnable task) {
    Preconditions.checkNotNull(task, "'task' must not be null.");
    if (!executing) {
      executing = true;
      try {
        task.run();
      } catch (Throwable t) {
        log.log(Level.SEVERE, "Exception while executing runnable " + task, t);
      } finally {
        if (taskQueue != null) {
          completeQueuedTasks();
        }
        executing = false;
      }
    } else {
      enqueue(task);
    }
  }

  private void completeQueuedTasks() {
    Runnable task = null;
    while ((task = taskQueue.poll()) != null) {
      try {
        task.run();
      } catch (Throwable t) {
        // Log it and keep going
        log.log(Level.SEVERE, "Exception while executing runnable " + task, t);
      }
    }
  }

  private void enqueue(Runnable r) {
    if (taskQueue == null) {
      taskQueue = new ArrayDeque<Runnable>(4);
    }
    taskQueue.add(r);
  }
}