Protect TcpNetConnectionSupport calls

5.0 introduced the `TcpNetConnectionSupport` user hook to create connection objects.

If a user-supplied instance threw an exception, the server socket would remain open
but the server `accept()` thread is gone.

- wrap the connection initialization code in try/catch
- close the server socket if necessary on an exception

spring-integration-ip/src/test/java/org/springframework/integration/ip/tcp/connection/TcpNetConnectionSupportTests.java

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * 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 org.springframework.integration.ip.tcp.connection;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.net.Socket;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+
+import javax.net.SocketFactory;
+
+import org.junit.Test;
+
+import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.messaging.Message;
+
+/**
+ * @author Gary Russell
+ * @since 5.0.3
+ *
+ */
+public class TcpNetConnectionSupportTests {
+
+	@Test
+	public void testBadCode() throws Exception {
+		TcpNetServerConnectionFactory server = new TcpNetServerConnectionFactory(0);
+		AtomicReference<Message<?>> message = new AtomicReference<>();
+		CountDownLatch latch1 = new CountDownLatch(1);
+		server.registerListener(m -> {
+			message.set(m);
+			latch1.countDown();
+			return false;
+		});
+		AtomicBoolean firstTime = new AtomicBoolean(true);
+		server.setTcpNetConnectionSupport(new DefaultTcpNetConnectionSupport() {
+
+			@Override
+			public TcpNetConnection createNewConnection(Socket socket, boolean server, boolean lookupHost,
+					ApplicationEventPublisher applicationEventPublisher, String connectionFactoryName)
+					throws Exception {
+				if (firstTime.getAndSet(false)) {
+					throw new RuntimeException("intended");
+				}
+				return super.createNewConnection(socket, server, lookupHost, applicationEventPublisher, connectionFactoryName);
+			}
+
+		});
+		CountDownLatch latch2 = new CountDownLatch(1);
+		server.setApplicationEventPublisher(e -> {
+			if (e instanceof TcpConnectionServerListeningEvent) {
+			latch2.countDown();
+		}
+		});
+		server.afterPropertiesSet();
+		server.start();
+		assertThat(latch2.await(10, TimeUnit.SECONDS)).isTrue();
+		Socket socket = SocketFactory.getDefault().createSocket("localhost", server.getPort());
+		socket.close();
+		socket = SocketFactory.getDefault().createSocket("localhost", server.getPort());
+		socket.getOutputStream().write("foo\r\n".getBytes());
+		socket.close();
+		assertThat(latch1.await(10, TimeUnit.SECONDS)).isTrue();
+		assertThat(message.get()).isNotNull();
+		server.stop();
+	}
+
+}