Fix a race condition in OperatorMerge.InnerSubscriber#onError (#5851)

* Fix a race condition in OperatorMerge.InnerSubscriber#onError

Inner subscriber must queue the error first before setting done,
so that after emitLoop() removes the subscriber,
emitLoop is guaranteed to notice the error.
Otherwise it would be possible that inner subscribers count was 0,
and at the same time the error queue was empty.

* Add unit test for OperatorMerge.InnerSubscriber#onError race

Author: Piotr Kołaczkowski

src/main/java/rx/internal/operators/OperatorMerge.java

@@ -847,8 +847,11 @@ public void onNext(T t) {
         }
         @Override
         public void onError(Throwable e) {
-            done = true;
+            // Need to queue the error first before setting done, so that after emitLoop() removes the subscriber,
+            // it is guaranteed to notice the error. Otherwise it would be possible that inner subscribers count was 0,
+            // and at the same time the error queue was empty.
             parent.getOrCreateErrorQueue().offer(e);
+            done = true;
             parent.emit();
         }
         @Override

src/test/java/rx/internal/operators/OperatorMergeTest.java

@@ -1205,6 +1205,34 @@ public void onNext(Integer t) {
         assertTrue(latch.await(10, TimeUnit.SECONDS));
     }
 
+    @Test
+    public void testConcurrentMergeInnerError() {
+        for (int i = 0; i < 1000; i++) {
+            final TestSubscriber<Integer> ts = TestSubscriber.create();
+            final PublishSubject<Integer> ps1 = PublishSubject.create();
+            final PublishSubject<Integer> ps2 = PublishSubject.create();
+            final Exception error = new NullPointerException();
+            Action0 action1 = new Action0() {
+                @Override
+                public void call() {
+                    ps1.onNext(1);
+                    ps1.onCompleted();
+                }
+            };
+            Action0 action2 = new Action0() {
+                @Override
+                public void call() {
+                    ps2.onError(error);
+                }
+            };
+
+            Observable.mergeDelayError(ps1, ps2).subscribe(ts);
+            TestUtil.race(action1, action2);
+            ts.assertTerminalEvent();
+            ts.assertError(error);
+        }
+    }
+
     private static Action1<Integer> printCount() {
         return new Action1<Integer>() {
             long count;