#4737 Error when tracking exception with unknown cause (#4740)

Author: pyricau

src/main/java/rx/exceptions/AssemblyStackTraceException.java

@@ -52,7 +52,15 @@ public void attachTo(Throwable exception) {
 
         for (;;) {
             if (exception.getCause() == null) {
-                exception.initCause(this);
+                try {
+                    exception.initCause(this);
+                } catch (IllegalStateException e) {
+                    RxJavaHooks.onError(new RuntimeException(
+                        "Received an exception with a cause set to null, instead of being unset."
+                            + " To fix this, look down the chain of causes. The last exception had"
+                            + " a cause explicitly set to null. It should be unset instead.",
+                        exception));
+                }
                 return;
             }
 

src/test/java/rx/plugins/RxJavaHooksTest.java

@@ -23,8 +23,10 @@
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicInteger;
 
+import java.util.concurrent.atomic.AtomicReference;
 import org.junit.Test;
 
+import org.mockito.Mockito;
 import rx.*;
 import rx.Observable;
 import rx.Scheduler.Worker;
@@ -39,6 +41,12 @@
 
 public class RxJavaHooksTest {
 
+    public static class TestExceptionWithUnknownCause extends RuntimeException {
+        TestExceptionWithUnknownCause() {
+            super((Throwable) null);
+        }
+    }
+
     static Observable<Integer> createObservable() {
         return Observable.range(1, 5).map(new Func1<Integer, Integer>() {
             @Override
@@ -48,6 +56,15 @@ public Integer call(Integer t) {
         });
     }
 
+    static Observable<Integer> createObservableThrowingUnknownCause() {
+        return Observable.range(1, 5).map(new Func1<Integer, Integer>() {
+            @Override
+            public Integer call(Integer t) {
+                throw new TestExceptionWithUnknownCause();
+            }
+        });
+    }
+
     @Test
     public void constructorShouldBePrivate() {
         TestUtil.checkUtilityClass(RxJavaHooks.class);
@@ -89,6 +106,31 @@ public void assemblyTrackingObservable() {
         }
     }
 
+    @Test
+    public void assemblyTrackingObservableUnknownCause() {
+        RxJavaHooks.enableAssemblyTracking();
+        try {
+            final AtomicReference<Throwable> onErrorThrowableRef = new AtomicReference<Throwable>();
+            RxJavaHooks.setOnError(new Action1<Throwable>() {
+                @Override
+                public void call(Throwable throwable) {
+                    onErrorThrowableRef.set(throwable);
+                }
+            });
+            TestSubscriber<Integer> ts = TestSubscriber.create();
+
+            createObservableThrowingUnknownCause().subscribe(ts);
+
+            ts.assertError(TestExceptionWithUnknownCause.class);
+
+            Throwable receivedError = onErrorThrowableRef.get();
+            assertNotNull(receivedError);
+            assertTrue(receivedError.getMessage().contains("cause set to null"));
+        } finally {
+            RxJavaHooks.reset();
+        }
+    }
+
     static Single<Integer> createSingle() {
         return Single.just(1).map(new Func1<Integer, Integer>() {
             @Override