Run with scissors.

Add a useful (if somewhat dangerous) method which will help
replace similarly dangerous code patterns in a few different places.

Change-Id: If1295f7ab9652c906ce718d94eb7914d143e1939

Author: Jeff Brown

core/java/android/os/Handler.java

@@ -411,6 +411,50 @@ public final boolean postAtFrontOfQueue(Runnable r)
         return sendMessageAtFrontOfQueue(getPostMessage(r));
     }
 
+    /**
+     * Runs the specified task synchronously.
+     *
+     * If the current thread is the same as the handler thread, then the runnable
+     * runs immediately without being enqueued.  Otherwise, posts the runnable
+     * to the handler and waits for it to complete before returning.
+     *
+     * This method is dangerous!  Improper use can result in deadlocks.
+     * Never call this method while any locks are held or use it in a
+     * possibly re-entrant manner.
+     *
+     * This method is occasionally useful in situations where a background thread
+     * must synchronously await completion of a task that must run on the
+     * handler's thread.  However, this problem is often a symptom of bad design.
+     * Consider improving the design (if possible) before resorting to this method.
+     *
+     * One example of where you might want to use this method is when you just
+     * set up a Handler thread and need to perform some initialization steps on
+     * it before continuing execution.
+     *
+     * @param r The Runnable that will be executed synchronously.
+     *
+     * @return Returns true if the Runnable was successfully executed.
+     *         Returns false on failure, usually because the
+     *         looper processing the message queue is exiting.
+     *
+     * @hide This method is prone to abuse and should probably not be in the API.
+     * If we ever do make it part of the API, we might want to rename it to something
+     * less funny like runUnsafe().
+     */
+    public final boolean runWithScissors(final Runnable r) {
+        if (r == null) {
+            throw new IllegalArgumentException("runnable must not be null");
+        }
+
+        if (Looper.myLooper() == mLooper) {
+            r.run();
+            return true;
+        }
+
+        BlockingRunnable br = new BlockingRunnable(r);
+        return br.postAndWait(this);
+    }
+
     /**
      * Remove any pending posts of Runnable r that are in the message queue.
      */
@@ -678,4 +722,41 @@ private static void handleCallback(Message message) {
     final Callback mCallback;
     final boolean mAsynchronous;
     IMessenger mMessenger;
+
+    private static final class BlockingRunnable implements Runnable {
+        private final Runnable mTask;
+        private boolean mDone;
+
+        public BlockingRunnable(Runnable task) {
+            mTask = task;
+        }
+
+        @Override
+        public void run() {
+            try {
+                mTask.run();
+            } finally {
+                synchronized (this) {
+                    mDone = true;
+                    notifyAll();
+                }
+            }
+        }
+
+        public boolean postAndWait(Handler handler) {
+            if (!handler.post(this)) {
+                return false;
+            }
+
+            synchronized (this) {
+                while (!mDone) {
+                    try {
+                        wait();
+                    } catch (InterruptedException ex) {
+                    }
+                }
+            }
+            return true;
+        }
+    }
 }