Fix a race condition that may make OperatorMaterialize emit too many terminal notifications (#5850)

* Fix a race condition that may make OperatorMaterialize emit the
terminal notification more than once

The guards in `OperatorMaterialize.ParentSubscriber#drain` were never
working, because `busy` was actually never set to true.
Therefore it was possible that the `drain` loop was executed by
more than one thread concurrently, which could led to undefined
behavior.

This fix sets `busy` to true at the entry of `drain`.

* Add unit test for race in OperatorMaterialize

* Set sudo required in travis config

Author: Piotr Kołaczkowski

.travis.yml

@@ -1,7 +1,7 @@
 language: java
 jdk:
 - oraclejdk8
-sudo: false
+sudo: required
 # as per http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
 
 git:

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

@@ -134,6 +134,7 @@ private void drain() {
                     missed = true;
                     return;
                 }
+                busy = true;
             }
             // drain loop
             final AtomicLong localRequested = this.requested;

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

@@ -29,9 +29,12 @@
 import rx.Notification;
 import rx.Observable;
 import rx.Subscriber;
+import rx.TestUtil;
+import rx.functions.Action0;
 import rx.functions.Action1;
 import rx.observers.TestSubscriber;
 import rx.schedulers.Schedulers;
+import rx.subjects.PublishSubject;
 
 public class OperatorMaterializeTest {
 
@@ -201,6 +204,33 @@ public void testUnsubscribeJustBeforeCompletionNotificationShouldPreventThatNoti
         ts.assertUnsubscribed();
     }
 
+    @Test
+    public void testConcurrency() {
+        for (int i = 0; i < 1000; i++) {
+            final TestSubscriber<Notification<Integer>> ts = TestSubscriber.create(0);
+            final PublishSubject<Integer> ps = PublishSubject.create();
+            Action0 publishAction = new Action0() {
+                @Override
+                public void call() {
+                    ps.onCompleted();
+                }
+            };
+
+            Action0 requestAction = new Action0() {
+                @Override
+                public void call() {
+                    ts.requestMore(1);
+                }
+            };
+
+            ps.materialize().subscribe(ts);
+            TestUtil.race(publishAction, requestAction);
+            ts.assertValueCount(1);
+            ts.assertTerminalEvent();
+            ts.assertNoErrors();
+        }
+    }
+
     private static class TestObserver extends Subscriber<Notification<String>> {
 
         boolean onCompleted;