Fail test in case of double onError/onComplete signals

Author: ktoso

tck/src/main/java/org/reactivestreams/tck/SubscriberWhiteboxVerification.java

@@ -493,6 +493,8 @@ public void run(WhiteboxTestStage stage) throws InterruptedException {
 
         // cumulative pending > Long.MAX_VALUE
         stage.probe.expectErrorWithMessage(IllegalStateException.class, "3.17");
+
+        env.verifyNoAsyncErrors(env.defaultTimeoutMillis());
       }
     });
   }
@@ -626,12 +628,22 @@ public void registerOnNext(T element) {
 
     @Override
     public void registerOnComplete() {
-      elements.complete();
+      try {
+        elements.complete();
+      } catch (IllegalStateException ex) {
+        // "Queue full"
+        env.flop("subscriber::onComplete was called a second time, which is illegal according to Rule 1.7");
+      }
     }
 
     @Override
     public void registerOnError(Throwable cause) {
-      error.complete(cause);
+      try {
+        error.complete(cause);
+      } catch (IllegalStateException ex) {
+        // "Queue full", onError was already called
+        env.flop("subscriber::onError was called a second time, which is illegal according to Rule 1.7");
+      }
     }
 
     public T expectNext() throws InterruptedException {

tck/src/main/java/org/reactivestreams/tck/TestEnvironment.java

@@ -150,7 +150,7 @@ public void verifyNoAsyncErrors() {
   /** If {@code TestEnvironment#printlnDebug} is true, print debug message to std out. */
   public void debug(String msg) {
     if (printlnDebug)
-      System.out.println(msg);
+      System.out.println("[TCK-DEBUG] " + msg);
   }
 
   // ---- classes ----