firestore: replace deprecated AsyncTask-based executor with modern Kotlin dispatchers

firebase-firestore/CHANGELOG.md

@@ -8,6 +8,8 @@
   [#7376](//github.com/firebase/firebase-android-sdk/issues/7376)
 - [changed] Improve query performance via internal memoization of calculated document data.
   [#7370](//github.com/firebase/firebase-android-sdk/issues/7370)
+- [changed] Replace deprecated AsyncTask-based executor with mordern Kotlin dispatchers.
+  [#NNNN](//github.com/firebase/firebase-android-sdk/issues/NNNN)
 
 # 26.0.0
 

firebase-firestore/src/main/java/com/google/firebase/firestore/Firestore.kt

@@ -21,8 +21,7 @@ import com.google.firebase.Firebase
 import com.google.firebase.FirebaseApp
 import com.google.firebase.components.Component
 import com.google.firebase.components.ComponentRegistrar
-import com.google.firebase.firestore.*
-import com.google.firebase.firestore.util.Executors.BACKGROUND_EXECUTOR
+import com.google.firebase.firestore.util.Executors.newSequentialExecutor
 import kotlinx.coroutines.cancel
 import kotlinx.coroutines.channels.awaitClose
 import kotlinx.coroutines.channels.trySendBlocking
@@ -233,7 +232,9 @@ fun DocumentReference.snapshots(
 ): Flow<DocumentSnapshot> {
   return callbackFlow {
     val registration =
-      addSnapshotListener(BACKGROUND_EXECUTOR, metadataChanges) { snapshot, exception ->
+      addSnapshotListener(newSequentialExecutor("DocRef.snapshots"), metadataChanges) {
+        snapshot,
+        exception ->
         if (exception != null) {
           cancel(message = "Error getting DocumentReference snapshot", cause = exception)
         } else if (snapshot != null) {
@@ -257,7 +258,9 @@ fun Query.snapshots(
 ): Flow<QuerySnapshot> {
   return callbackFlow {
     val registration =
-      addSnapshotListener(BACKGROUND_EXECUTOR, metadataChanges) { snapshot, exception ->
+      addSnapshotListener(newSequentialExecutor("Query.snapshots"), metadataChanges) {
+        snapshot,
+        exception ->
         if (exception != null) {
           cancel(message = "Error getting Query snapshot", cause = exception)
         } else if (snapshot != null) {

firebase-firestore/src/main/java/com/google/firebase/firestore/remote/GrpcCallProvider.java

@@ -237,7 +237,7 @@ private void initChannelTask() {
     // the AsyncQueue.
     this.channelTask =
         Tasks.call(
-            Executors.BACKGROUND_EXECUTOR,
+            Executors.SHORT_WORKLOAD_EXECUTOR,
             () -> {
               ManagedChannel channel = initChannel(context, databaseInfo);
               asyncQueue.enqueueAndForget(() -> onConnectivityStateChange(channel));

firebase-firestore/src/main/java/com/google/firebase/firestore/util/BackgroundQueue.kt

@@ -15,10 +15,7 @@
  */
 package com.google.firebase.firestore.util
 
-import java.util.concurrent.Executor
 import java.util.concurrent.Semaphore
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.asExecutor
 
 /**
  * Manages CPU-bound work on background threads to enable parallel processing.
@@ -41,7 +38,7 @@ internal class BackgroundQueue {
     check(submittingState is State.Submitting) { "submit() may not be called after drain()" }
 
     submittingState.taskCount++
-    executor.execute {
+    Executors.CPU_WORKLOAD_EXECUTOR.execute {
       try {
         runnable.run()
       } finally {
@@ -70,17 +67,4 @@ internal class BackgroundQueue {
     }
     object Draining : State
   }
-
-  companion object {
-
-    /**
-     * The maximum amount of parallelism shared by all instances of this class.
-     *
-     * This is equal to the number of processor cores available, or 2, whichever is larger.
-     */
-    val maxParallelism = Runtime.getRuntime().availableProcessors().coerceAtLeast(2)
-
-    private val executor: Executor =
-      Dispatchers.IO.limitedParallelism(maxParallelism, "firestore.BackgroundQueue").asExecutor()
-  }
 }

firebase-firestore/src/main/java/com/google/firebase/firestore/util/Executors.java

@@ -14,20 +14,23 @@
 
 package com.google.firebase.firestore.util;
 
-import android.os.AsyncTask;
+import static kotlinx.coroutines.ExecutorsKt.asExecutor;
+
 import com.google.android.gms.tasks.TaskExecutors;
 import java.util.concurrent.Executor;
+import kotlinx.coroutines.Dispatchers;
 
 /** Helper class for executors. */
 public final class Executors {
   /**
-   * The maximum number of tasks we submit to AsyncTask.THREAD_POOL_EXECUTOR.
-   *
-   * <p>The limit is based on the number of core threads spun by THREAD_POOL_EXECUTOR and is well
-   * below the queue size limit of 120 pending tasks. Limiting our usage of the THREAD_POOL_EXECUTOR
-   * allows other users to schedule their own operations on the shared THREAD_POOL_EXECUTOR.
+   * The number of physical CPU cores available for multithreaded execution, or 2, whichever is
+   * larger.
+   * <p>
+   * CPU-bound tasks should never use more than this number of concurrent threads as doing so will
+   * almost certainly reduce throughput due to the overhead of context switching.
    */
-  private static final int ASYNC_THREAD_POOL_MAXIMUM_CONCURRENCY = 4;
+  public static final int HARDWARE_CONCURRENCY =
+      Math.max(2, Runtime.getRuntime().availableProcessors());
 
   /**
    * The default executor for user visible callbacks. It is an executor scheduling callbacks on
@@ -38,10 +41,46 @@ public final class Executors {
   /** An executor that executes the provided runnable immediately on the current thread. */
   public static final Executor DIRECT_EXECUTOR = Runnable::run;
 
-  /** An executor that runs tasks in parallel on Android's AsyncTask.THREAD_POOL_EXECUTOR. */
-  public static final Executor BACKGROUND_EXECUTOR =
-      new ThrottledForwardingExecutor(
-          ASYNC_THREAD_POOL_MAXIMUM_CONCURRENCY, AsyncTask.THREAD_POOL_EXECUTOR);
+  /**
+   * An executor suitable for short tasks that perform little or no blocking.
+   */
+  public static final Executor SHORT_WORKLOAD_EXECUTOR =
+      asExecutor(
+          Dispatchers.getIO()
+              .limitedParallelism(HARDWARE_CONCURRENCY, "firestore.SHORT_WORKLOAD_EXECUTOR"));
+
+  /**
+   * An executor suitable for IO-bound workloads. New threads are usually created to satisfy demand,
+   * and, therefore, tasks do not usually wait in a queue for execution.
+   */
+  public static final Executor IO_WORKLOAD_EXECUTOR = asExecutor(Dispatchers.getIO());
+
+  /**
+   * An executor suitable for CPU-bound workloads. No more tasks than available CPU cores will
+   * execute concurrently, while other tasks line up and wait for a thread to become available, and
+   * are scheduled in an arbitrary order.
+   */
+  public static final Executor CPU_WORKLOAD_EXECUTOR =
+      asExecutor(
+          Dispatchers.getIO()
+              .limitedParallelism(HARDWARE_CONCURRENCY, "firestore.CPU_WORKLOAD_EXECUTOR"));
+
+  /**
+   * Creates and returns a new {@link Executor} that executes tasks sequentially.
+   * <p>
+   * The implementation guarantees that tasks are executed sequentially and that a happens-before
+   * relation is established between them. This means that tasks run by this executor do _not_ need
+   * to synchronize access to shared resources, such as using "synchronized" blocks or "volatile"
+   * variables. See `kotlinx.coroutines.limitedParallelism` for full details.
+   * <p>
+   * Note that there is no guarantee that tasks will all run on the _same thread_.
+   *
+   * @param name a brief name to assign to the executor, for debugging purposes.
+   * @return the newly-created executor.
+   */
+  public static Executor newSequentialExecutor(String name) {
+    return asExecutor(Dispatchers.getIO().limitedParallelism(1, "firestore.seq." + name));
+  }
 
   private Executors() {
     // Private constructor to prevent initialization

firebase-firestore/src/test/java/com/google/firebase/firestore/integration/FirebaseFirestoreTest.java

@@ -16,7 +16,7 @@
 
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.firebase.firestore.testutil.TestUtil.map;
-import static com.google.firebase.firestore.util.Executors.BACKGROUND_EXECUTOR;
+import static com.google.firebase.firestore.util.Executors.SHORT_WORKLOAD_EXECUTOR;
 import static org.mockito.Mockito.verify;
 
 import androidx.annotation.NonNull;
@@ -78,9 +78,9 @@ public static String getResourcePrefixValue(DatabaseId databaseId) {
 
   private static <T> Task<T> waitFor(Task<T> task) throws InterruptedException {
     CountDownLatch countDownLatch = new CountDownLatch(1);
-    task.addOnSuccessListener(BACKGROUND_EXECUTOR, t -> countDownLatch.countDown());
-    task.addOnFailureListener(BACKGROUND_EXECUTOR, e -> countDownLatch.countDown());
-    task.addOnCanceledListener(BACKGROUND_EXECUTOR, () -> countDownLatch.countDown());
+    task.addOnSuccessListener(SHORT_WORKLOAD_EXECUTOR, t -> countDownLatch.countDown());
+    task.addOnFailureListener(SHORT_WORKLOAD_EXECUTOR, e -> countDownLatch.countDown());
+    task.addOnCanceledListener(SHORT_WORKLOAD_EXECUTOR, () -> countDownLatch.countDown());
     countDownLatch.await(15, TimeUnit.SECONDS);
     return task;
   }