Attempt to start a stopped server will fail with IllegalStateException (

#2439)

Signed-off-by: Tomas Langer <tomas.langer@oracle.com>

webserver/webserver/src/main/java/io/helidon/webserver/NettyWebServer.java

@@ -193,6 +193,11 @@ public MessageBodyWriterContext writerContext() {
 
     @Override
     public synchronized Single<WebServer> start() {
+        if (shutdownThreadGroupsInitiated.get() || (startFuture.isDone() && shutdownFuture.isDone())) {
+            // if we are shutting down, or shutdown - restart is not an option
+            throw new IllegalStateException("WebServer cannot be restarted once it has been shutdown, or it failed to start.");
+        }
+
         if (!started) {
 
             channelsUpFuture.thenAccept(this::started)

webserver/webserver/src/main/java/io/helidon/webserver/WebServer.java

@@ -69,6 +69,8 @@ public interface WebServer {
 
     /**
      * Starts the server. Has no effect if server is running.
+     * The start will fail on a server that is shut down, or that failed to start.
+     * In such cases, create a new instance of Web Server.
      *
      * @return a single to react on startup process
      */

webserver/webserver/src/test/java/io/helidon/webserver/Gh377.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2020 Oracle and/or its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.helidon.webserver;
+
+import java.util.concurrent.TimeUnit;
+
+import org.junit.jupiter.api.Test;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.greaterThan;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+/**
+ * Tests correct behavior when webserver is shutdown and an attempt is made to start it again.
+ * Github issue #377.
+ */
+class Gh377 {
+    @Test
+    void testRestart() {
+        WebServer webServer = WebServer.builder()
+                .routing(Routing.builder()
+                                 .get("/", (req, res) -> res.send("Hello World"))
+                                 .build())
+                .build()
+                .start()
+                .await(10, TimeUnit.SECONDS);
+
+        assertThat(webServer.port(), greaterThan(0));
+
+        // shutdown
+        webServer.shutdown()
+                .await(10, TimeUnit.SECONDS);
+
+        assertThat(webServer.port(), is(-1));
+
+        // attempt to start again
+        assertThrows(IllegalStateException.class, () -> webServer.start()
+                .await(10, TimeUnit.SECONDS));
+
+        assertThat(webServer.port(), is(-1));
+    }
+}