Fix race condition introduced by background platform channels

shell/platform/android/io/flutter/embedding/engine/dart/DartMessenger.java

@@ -7,6 +7,7 @@
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.annotation.UiThread;
+import com.google.common.util.concurrent.SettableFuture;
 import io.flutter.Log;
 import io.flutter.embedding.engine.FlutterJNI;
 import io.flutter.plugin.common.BinaryMessenger;
@@ -32,8 +33,15 @@ class DartMessenger implements BinaryMessenger, PlatformMessageHandler {
 
   @NonNull private final FlutterJNI flutterJNI;
 
+  /**
+   * Maps a channel name to an object that contains the task queue and the handler associated with
+   * the channel.
+   */
   @NonNull private final ConcurrentHashMap<String, HandlerInfo> messageHandlers;
 
+  /** Maps a channel name to a future that resolves when the channel is defined. */
+  @NonNull private final ConcurrentHashMap<String, SettableFuture<Void>> definedChannels;
+
   @NonNull private final Map<Integer, BinaryMessenger.BinaryReply> pendingReplies;
   private int nextReplyId = 1;
 
@@ -46,6 +54,7 @@ class DartMessenger implements BinaryMessenger, PlatformMessageHandler {
   DartMessenger(@NonNull FlutterJNI flutterJNI, @NonNull TaskQueueFactory taskQueueFactory) {
     this.flutterJNI = flutterJNI;
     this.messageHandlers = new ConcurrentHashMap<>();
+    this.definedChannels = new ConcurrentHashMap<>();
     this.pendingReplies = new HashMap<>();
     this.createdTaskQueues = new WeakHashMap<TaskQueue, DartMessengerTaskQueue>();
     this.taskQueueFactory = taskQueueFactory;
@@ -124,18 +133,22 @@ public void setMessageHandler(
     if (handler == null) {
       Log.v(TAG, "Removing handler for channel '" + channel + "'");
       messageHandlers.remove(channel);
-    } else {
-      DartMessengerTaskQueue dartMessengerTaskQueue = null;
-      if (taskQueue != null) {
-        dartMessengerTaskQueue = createdTaskQueues.get(taskQueue);
-        if (dartMessengerTaskQueue == null) {
-          throw new IllegalArgumentException(
-              "Unrecognized TaskQueue, use BinaryMessenger to create your TaskQueue (ex makeBackgroundTaskQueue).");
-        }
+      return;
+    }
+    DartMessengerTaskQueue dartMessengerTaskQueue = null;
+    if (taskQueue != null) {
+      dartMessengerTaskQueue = createdTaskQueues.get(taskQueue);
+      if (dartMessengerTaskQueue == null) {
+        throw new IllegalArgumentException(
+            "Unrecognized TaskQueue, use BinaryMessenger to create your TaskQueue (ex makeBackgroundTaskQueue).");
       }
-      Log.v(TAG, "Setting handler for channel '" + channel + "'");
-      messageHandlers.put(channel, new HandlerInfo(handler, dartMessengerTaskQueue));
     }
+    Log.v(TAG, "Setting handler for channel '" + channel + "'");
+    if (!definedChannels.has(channel)) {
+      definedChannels.put(channel, SettableFuture.create());
+    }
+    definedChannels.get(channel).set(Void.TYPE);
+    messageHandlers.put(channel, new HandlerInfo(handler, dartMessengerTaskQueue));
   }
 
   @Override
@@ -189,6 +202,38 @@ public void handleMessageFromDart(
       long messageData) {
     // Called from the ui thread.
     Log.v(TAG, "Received message from Dart over channel '" + channel + "'");
+
+    if (!messageHandlers.has(channel)) {
+      // The channel is not defined when the Dart VM sends a message before the channels are
+      // registered.
+      //
+      // This is possible if the Dart VM starts before channel registration, and if the thread that
+      // registers the channels is busy or slow at registering the channel handlers.
+      //
+      // In such cases, wait for a limited time, and exit with error if the time is exceeded.
+      // This is effectively acting as a lock, so the current thread (Dart UI thread) is blocked
+      // until the lock is released.
+      if (!definedChannels.has(channel)) {
+        definedChannels.put(channel, SettableFuture.create());
+      }
+      final SettableFuture<Boolean> future = definedChannels.get(channel);
+      try {
+        future.get(10, TimeUnit.SECONDS);
+      } catch (ExecutionException | InterruptedException ex) {
+        Log.e(TAG, "Undefined channel " + channel + ".", ex);
+        return;
+      } catch (TimeoutException ex) {
+        Log.e(
+            TAG,
+            "Channel "
+                + channel
+                + " was not defined in time. "
+                + "This likely means that the handler was never defined, or that the thread "
+                + "that defines the handler has been busy for an extended period of time.",
+            ex);
+        return;
+      }
+    }
     @Nullable final HandlerInfo handlerInfo = messageHandlers.get(channel);
     @Nullable
     final DartMessengerTaskQueue taskQueue = (handlerInfo != null) ? handlerInfo.taskQueue : null;