fix(resilience4j-reactor): release semaphore on cancel (ReactiveX#333)

Author: cschockaert

resilience4j-reactor/src/main/java/io/github/resilience4j/reactor/bulkhead/operator/BulkheadSubscriber.java

@@ -45,6 +45,11 @@ public void hookOnNext(T t) {
         }
     }
 
+    @Override
+    public void hookOnCancel() {
+        releaseBulkhead();
+    }
+
     @Override
     public void hookOnError(Throwable t) {
         if (wasCallPermitted()) {

resilience4j-reactor/src/test/java/io/github/resilience4j/reactor/bulkhead/operator/MonoBulkheadTest.java

@@ -97,4 +97,18 @@ public void shouldEmitBulkheadFullExceptionEvenWhenErrorNotOnSubscribe() {
 
         assertThat(bulkhead.getMetrics().getAvailableConcurrentCalls()).isEqualTo(0);
     }
+
+    @Test
+    public void shouldReleaseBulkheadSemaphoreOnCancel() {
+        assertThat(bulkhead.getMetrics().getAvailableConcurrentCalls()).isEqualTo(1);
+        StepVerifier.create(
+                Mono.just("Event")
+                        .transform(BulkheadOperator.of(bulkhead)))
+                .expectSubscription()
+                .expectNext("Event")
+                .thenCancel()
+                .verify();
+        assertThat(bulkhead.getMetrics().getAvailableConcurrentCalls()).isEqualTo(1);
+    }
+
 }