Bug 889881 - Use IdleHandler to make MessageQueue.next() not block; r=blassey

mobile/android/base/GeckoAppShell.java

@@ -56,6 +56,7 @@ import android.net.ConnectivityManager;
 import android.net.NetworkInfo;
 import android.net.Uri;
 import android.os.Build;
+import android.os.Handler;
 import android.os.Looper;
 import android.os.Message;
 import android.os.MessageQueue;
@@ -293,6 +294,19 @@ public class GeckoAppShell
     }
 
     public static void runGecko(String apkPath, String args, String url, String type) {
+        // Preparation for pumpMessageLoop()
+        MessageQueue.IdleHandler idleHandler = new MessageQueue.IdleHandler() {
+            @Override public boolean queueIdle() {
+                Handler geckoHandler = ThreadUtils.getGeckoHandler();
+                Message idleMsg = Message.obtain(geckoHandler);
+                // Use |Message.obj == GeckoHandler| to identify our "queue is empty" message
+                idleMsg.obj = geckoHandler;
+                geckoHandler.sendMessageAtFrontOfQueue(idleMsg);
+                // Keep this IdleHandler
+                return true;
+            }
+        };
+        Looper.myQueue().addIdleHandler(idleHandler);
 
         // run gecko -- it will spawn its own thread
         GeckoAppShell.nativeInit();
@@ -321,6 +335,9 @@ public class GeckoAppShell
 
         // and go
         GeckoLoader.nativeRun(combinedArgs);
+
+        // Remove pumpMessageLoop() idle handler
+        Looper.myQueue().removeIdleHandler(idleHandler);
     }
 
     // Called on the UI thread after Gecko loads.
@@ -2463,10 +2480,16 @@ public class GeckoAppShell
     }
 
     public static boolean pumpMessageLoop() {
+        Handler geckoHandler = ThreadUtils.getGeckoHandler();
         MessageQueue mq = Looper.myQueue();
         Message msg = getNextMessageFromQueue(mq); 
         if (msg == null)
             return false;
+        if (msg.getTarget() == geckoHandler && msg.obj == geckoHandler) {
+            // Our "queue is empty" message; see runGecko()
+            msg.recycle();
+            return false;
+        }
         if (msg.getTarget() == null) 
             Looper.myLooper().quit();
         else

widget/android/AndroidJNI.cpp

@@ -803,21 +803,14 @@ NS_EXPORT jobject JNICALL
 Java_org_mozilla_gecko_GeckoAppShell_getNextMessageFromQueue(JNIEnv* jenv, jclass, jobject queue)
 {
     static jclass jMessageQueueCls = nullptr;
-    static jfieldID jMessagesField;
     static jmethodID jNextMethod;
     if (!jMessageQueueCls) {
         jMessageQueueCls = (jclass) jenv->NewGlobalRef(jenv->FindClass("android/os/MessageQueue"));
-        jMessagesField = jenv->GetFieldID(jMessageQueueCls, "mMessages", "Landroid/os/Message;");
         jNextMethod = jenv->GetMethodID(jMessageQueueCls, "next", "()Landroid/os/Message;");
     }
-    if (!jMessageQueueCls || !jMessagesField || !jNextMethod)
+    if (!jMessageQueueCls || !jNextMethod)
         return NULL;
-    jobject msg = jenv->GetObjectField(queue, jMessagesField);
-    // if queue.mMessages is null, queue.next() will block, which we don't want
-    if (!msg)
-        return msg;
-    msg = jenv->CallObjectMethod(queue, jNextMethod);
-    return msg;
+    return jenv->CallObjectMethod(queue, jNextMethod);
 }
 
 NS_EXPORT void JNICALL