Prevent duplicate invocation of error handler

vertx-web/src/main/java/io/vertx/ext/web/impl/RouterImpl.java

@@ -65,7 +65,8 @@ public void handle(HttpServerRequest request) {
       LOG.trace("Router: " + System.identityHashCode(this) + " accepting request " + request.method() + " " + request.absoluteURI());
     }
 
-    new RoutingContextImpl(null, this, request, state.getRoutes()).next();
+    RoutingContextImpl routingContext = new RoutingContextImpl(null, this, request, state.getRoutes());
+    routingContext.route();
   }
 
   @Override

vertx-web/src/main/java/io/vertx/ext/web/impl/RoutingContextImpl.java

@@ -76,7 +76,9 @@ public RoutingContextImpl(String mountPoint, RouterImpl router, HttpServerReques
     this.router = router;
     this.request = new HttpServerRequestWrapper(request, router.getAllowForward(), this);
     this.body = new RequestBodyImpl(this);
+  }
 
+  void route() {
     final String path = request.path();
     // optimized method which try hard to not allocate
     final boolean hasValidAuthority = ((HttpServerRequestInternal) request).isValidAuthority();
@@ -88,6 +90,8 @@ public RoutingContextImpl(String mountPoint, RouterImpl router, HttpServerReques
     } else if (path.charAt(0) != '/') {
       // For compatiblity we return `Not Found` when a path does not start with `/`
       fail(404);
+    } else {
+      next();
     }
   }
 

vertx-web/src/test/java/io/vertx/ext/web/RouterTest.java

@@ -3655,4 +3655,17 @@ public void testPausedConnection4() {
 
     await();
   }
+
+  @Test
+  public void testErrorHandlerInvokedOnce() throws Exception {
+    AtomicInteger errorHandlerInvocations = new AtomicInteger();
+
+    router.errorHandler(404, rc -> {
+      errorHandlerInvocations.incrementAndGet();
+      rc.response().setStatusCode(404).end();
+    });
+
+    testRequest(HttpMethod.GET, "path-without-slash-prefix", HttpResponseStatus.NOT_FOUND);
+    assertEquals(1, errorHandlerInvocations.get());
+  }
 }