BlockingUtils should not assume the cause of ExecutionException is no…

…t null (#873)

__Motivation__

BlockingUtils should not assume the cause of ExecutionException is not null.

__Modifications__

Add method `BlockingUtils.executionExceptionCause` that returns nonnull
cause or original throwable.

__Result__

Fixes #870.

servicetalk-http-api/src/main/java/io/servicetalk/http/api/BlockingUtils.java

@@ -97,7 +97,7 @@ static <T> T futureGetCancelOnInterrupt(Future<T> future) throws Exception {
             future.cancel(false);
             throw e;
         } catch (ExecutionException e) {
-            return throwException(e.getCause());
+            return throwException(executionExceptionCause(e));
         }
     }
 
@@ -150,7 +150,7 @@ static <T> T blockingInvocation(Single<T> source) throws Exception {
         try {
             return source.toFuture().get();
         } catch (final ExecutionException e) {
-            return throwException(e.getCause());
+            return throwException(executionExceptionCause(e));
         }
     }
 
@@ -160,7 +160,11 @@ static void blockingInvocation(Completable source) throws Exception {
         try {
             source.toFuture().get();
         } catch (final ExecutionException e) {
-            throwException(e.getCause());
+            throwException(executionExceptionCause(e));
         }
     }
+
+    private static Throwable executionExceptionCause(ExecutionException original) {
+        return (original.getCause() != null) ? original.getCause() : original;
+    }
 }