content: /*
   * Copyright (C) 2022 The Android Open Source Project
   *
   * 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.
   */
  package androidx.test.espresso.util.concurrent;
  
  import static androidx.test.internal.util.Checks.checkNotNull;
  
  import android.util.Log;
  import androidx.annotation.GuardedBy;
  import androidx.annotation.Nullable;
  import androidx.annotation.RestrictTo;
  import androidx.annotation.RestrictTo.Scope;
  import com.google.common.util.concurrent.ListenableFuture;
  import java.util.concurrent.Executor;
  
  /**
   * Minimal fork of Guava's ExecutionList to avoid the full dependency.
   *
   * @hide
   */
  @RestrictTo(Scope.LIBRARY)
  final class ExecutionList {
  
    /**
     * The runnable, executor pairs to execute. This acts as a stack threaded through the {@link
     * RunnableExecutorPair#next} field.
     */
    private RunnableExecutorPair runnables;
  
    @GuardedBy("this")
    private boolean executed;
  
    /** Creates a new, empty {@link ExecutionList}. */
    ExecutionList() {}
  
    /**
     * Adds the {@code Runnable} and accompanying {@code Executor} to the list of listeners to
     * execute. If execution has already begun, the listener is executed immediately.
     *
     * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
     * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
     * documentation.
     */
    void add(Runnable runnable, Executor executor) {
      // Fail fast on a null. We throw NPE here because the contract of Executor states that it throws
      // NPE on null listener, so we propagate that contract up into the add method as well.
      checkNotNull(runnable, "Runnable was null.");
      checkNotNull(executor, "Executor was null.");
  
      // Lock while we check state. We must maintain the lock while adding the new pair so that
      // another thread can't run the list out from under us. We only add to the list if we have not
      // yet started execution.
      synchronized (this) {
        if (!executed) {
          runnables = new RunnableExecutorPair(runnable, executor, runnables);
          return;
        }
      }
      // Execute the runnable immediately. Because of scheduling this may end up getting called before
      // some of the previously added runnables, but we're OK with that. If we want to change the
      // contract to guarantee ordering among runnables we'd have to modify the logic here to allow
      // it.
      executeListener(runnable, executor);
    }
  
    /**
     * Runs this execution list, executing all existing pairs in the order they were added. However,
     * note that listeners added after this point may be executed before those previously added, and
     * note that the execution order of all listeners is ultimately chosen by the implementations of
     * the supplied executors.
     *
     * <p>This method is idempotent. Calling it several times in parallel is semantically equivalent
     * to calling it exactly once.
     *
     * @since 10.0 (present in 1.0 as {@code run})
     */
    void execute() {
      // Lock while we update our state so the add method above will finish adding any listeners
      // before we start to run them.
      RunnableExecutorPair list;
      synchronized (this) {
        if (executed) {
          return;
        }
        executed = true;
        list = runnables;
        runnables = null; // allow GC to free listeners even if this stays around for a while.
      }
      // If we succeeded then list holds all the runnables we to execute. The pairs in the stack are
      // in the opposite order from how they were added so we need to reverse the list to fulfill our
      // contract.
      // This is somewhat annoying, but turns out to be very fast in practice. Alternatively, we could
      // drop the contract on the method that enforces this queue like behavior since depending on it
      // is likely to be a bug anyway.
  
      // N.B. All writes to the list and the next pointers must have happened before the above
      // synchronized block, so we can iterate the list without the lock held here.
      RunnableExecutorPair reversedList = null;
      while (list != null) {
        RunnableExecutorPair tmp = list;
        list = list.next;
        tmp.next = reversedList;
        reversedList = tmp;
      }
      while (reversedList != null) {
        executeListener(reversedList.runnable, reversedList.executor);
        reversedList = reversedList.next;
      }
    }
  
    /**
     * Submits the given runnable to the given {@link Executor} catching and logging all {@linkplain
     * RuntimeException runtime exceptions} thrown by the executor.
     */
    private static void executeListener(Runnable runnable, Executor executor) {
      try {
        executor.execute(runnable);
      } catch (RuntimeException e) {
        // Log it and keep going -- bad runnable and/or executor. Don't punish the other runnables if
        // we're given a bad one. We only catch RuntimeException because we want Errors to propagate
        // up.
        Log.e(
            "ExecutionList",
            "RuntimeException while executing runnable " + runnable + " with executor " + executor,
            e);
      }
    }
  
    private static final class RunnableExecutorPair {
      final Runnable runnable;
      final Executor executor;
      @Nullable RunnableExecutorPair next;
  
      RunnableExecutorPair(
          Runnable runnable, Executor executor, @Nullable RunnableExecutorPair next) {
        this.runnable = runnable;
        this.executor = executor;
        this.next = next;
      }
    }
  }
  