firestore: re-write BackgroundQueue to NOT use the deprecated AsyncTask thread pool

This change results in a significant performance improvement of queries with a large
number of documents in their result set.

In my testing, the performance improved by 91% from 3022 ms down to 265 ms on a real
device! For some reason, there were _no_ performance gains on the Android emulator.
This must have something to do with how different Android OS versions manage the
threads in the (deprecated) AsyncTask thread pool.

The test environment was as follows:
- Pixel 7 Pro real device running Android 16.
- Test called collectionRef.whereGreaterThan("foo", 50).get(Source.CACHE) 200 times.
- Calculated the average amount of time it took for the query results to be received.
- collectionRef is a Firestore collection containing 10,000 documents, of which 50%
  match the "whereGreaterThan" filter.
- Local cache had no other documents in it.
- Test app was compiled in "release" mode with "proguard-android-optimize.txt" r8 configuration.

Author: dconeybe

firebase-firestore/CHANGELOG.md

@@ -3,6 +3,9 @@
 - [changed] Bumped internal dependencies.
 - [changed] Improve the performance of queries in collections that contain many deleted documents.
   [#7295](//github.com/firebase/firebase-android-sdk/issues/7295)
+- [changed] Improve query performance in large result sets by replacing the deprecated AsyncTask
+  thread pool with a self-managed thread pool.
+  [#NNNN](//github.com/firebase/firebase-android-sdk/issues/NNNN)
 
 # 26.0.0
 

firebase-firestore/src/main/java/com/google/firebase/firestore/local/SQLiteDocumentOverlayCache.java

@@ -26,7 +26,6 @@
 import com.google.firebase.firestore.model.mutation.Mutation;
 import com.google.firebase.firestore.model.mutation.Overlay;
 import com.google.firebase.firestore.util.BackgroundQueue;
-import com.google.firebase.firestore.util.Executors;
 import com.google.firestore.v1.Write;
 import com.google.protobuf.InvalidProtocolBufferException;
 import java.util.ArrayList;
@@ -35,7 +34,6 @@
 import java.util.List;
 import java.util.Map;
 import java.util.SortedSet;
-import java.util.concurrent.Executor;
 
 public class SQLiteDocumentOverlayCache implements DocumentOverlayCache {
   private final SQLitePersistence db;
@@ -204,10 +202,7 @@ private void processOverlaysInBackground(
     byte[] rawMutation = row.getBlob(0);
     int largestBatchId = row.getInt(1);
 
-    // Since scheduling background tasks incurs overhead, we only dispatch to a
-    // background thread if there are still some documents remaining.
-    Executor executor = row.isLast() ? Executors.DIRECT_EXECUTOR : backgroundQueue;
-    executor.execute(
+    backgroundQueue.submit(
         () -> {
           Overlay overlay = decodeOverlay(rawMutation, largestBatchId);
           synchronized (results) {

firebase-firestore/src/main/java/com/google/firebase/firestore/local/SQLiteRemoteDocumentCache.java

@@ -33,7 +33,6 @@
 import com.google.firebase.firestore.model.ResourcePath;
 import com.google.firebase.firestore.model.SnapshotVersion;
 import com.google.firebase.firestore.util.BackgroundQueue;
-import com.google.firebase.firestore.util.Executors;
 import com.google.firebase.firestore.util.Function;
 import com.google.protobuf.InvalidProtocolBufferException;
 import com.google.protobuf.MessageLite;
@@ -47,7 +46,6 @@
 import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
-import java.util.concurrent.Executor;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
@@ -308,10 +306,7 @@ private void processRowInBackground(
     boolean documentTypeIsNull = row.isNull(3);
     String path = row.getString(4);
 
-    // Since scheduling background tasks incurs overhead, we only dispatch to a
-    // background thread if there are still some documents remaining.
-    Executor executor = row.isLast() ? Executors.DIRECT_EXECUTOR : backgroundQueue;
-    executor.execute(
+    backgroundQueue.submit(
         () -> {
           MutableDocument document =
               decodeMaybeDocument(rawDocument, readTimeSeconds, readTimeNanos);

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

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * 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 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.
+ *
+ * Instances of this class are _not_ thread-safe. All methods of an instance of this class must be
+ * called from the same thread. The behavior of an instance is undefined if any of the methods are
+ * called from multiple threads.
+ */
+internal class BackgroundQueue {
+
+  private var state: State = State.Submitting()
+
+  /**
+   * Submit a task for asynchronous execution on the executor of the owning [BackgroundQueue].
+   *
+   * @throws IllegalStateException if [drain] has been called.
+   */
+  fun submit(runnable: Runnable) {
+    val submittingState = this.state
+    check(submittingState is State.Submitting) { "submit() may not be called after drain()" }
+
+    submittingState.run {
+      taskCount++
+      executor.execute {
+        try {
+          runnable.run()
+        } finally {
+          completedTasks.release()
+        }
+      }
+    }
+  }
+
+  /**
+   * Blocks until all tasks submitted via calls to [submit] have completed.
+   *
+   * @throws IllegalStateException if called more than once.
+   */
+  fun drain() {
+    val submittingState = this.state
+    check(submittingState is State.Submitting) { "drain() may not be called more than once" }
+    this.state = State.Draining
+
+    submittingState.run { completedTasks.acquire(taskCount) }
+  }
+
+  private sealed interface State {
+    class Submitting : State {
+      val completedTasks = Semaphore(0)
+      var taskCount: Int = 0
+    }
+    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/BackgroundQueue.kt

@@ -40,14 +40,12 @@
 import com.google.firebase.firestore.model.ResourcePath;
 import com.google.firebase.firestore.testutil.ComparatorTester;
 import com.google.firebase.firestore.util.BackgroundQueue;
-import com.google.firebase.firestore.util.Executors;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
-import java.util.concurrent.Executor;
 import org.junit.Assert;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -991,10 +989,7 @@ public void testSynchronousMatchesOrderBy() {
 
     while (iterator.hasNext()) {
       MutableDocument doc = iterator.next();
-      // Only put the processing in the backgroundQueue if there are more documents
-      // in the list. This behavior matches SQLiteRemoteDocumentCache.getAll(...)
-      Executor executor = iterator.hasNext() ? backgroundQueue : Executors.DIRECT_EXECUTOR;
-      executor.execute(
+      backgroundQueue.submit(
           () -> {
             // We call query.matches() to indirectly test query.matchesOrderBy()
             boolean result = query.matches(doc);