Make UdpSender lazy to be able to recover from early DNS issues

jaeger-thrift/src/main/java/io/jaegertracing/thrift/internal/senders/UdpSender.java

@@ -27,8 +27,11 @@ public class UdpSender extends ThriftSender {
   public static final String DEFAULT_AGENT_UDP_HOST = "localhost";
   public static final int DEFAULT_AGENT_UDP_COMPACT_PORT = 6831;
 
-  @ToString.Exclude private Agent.Client agentClient;
-  @ToString.Exclude private ThriftUdpTransport udpTransport;
+  private final String host;
+  private final int port;
+
+  @ToString.Exclude private volatile Agent.Client agentClient;
+  @ToString.Exclude private volatile ThriftUdpTransport udpTransport;
 
   /**
    * This constructor expects Jaeger running running on {@value #DEFAULT_AGENT_UDP_HOST}
@@ -54,14 +57,30 @@ public UdpSender(String host, int port, int maxPacketSize) {
       port = DEFAULT_AGENT_UDP_COMPACT_PORT;
     }
 
-    udpTransport = ThriftUdpTransport.newThriftUdpClient(host, port);
-    agentClient = new Agent.Client(protocolFactory.getProtocol(udpTransport));
+    this.host = host;
+    this.port = port;
+  }
+
+  private Agent.Client getAgentClient() {
+    Agent.Client localRef = this.agentClient;
+    if (localRef == null) {
+      synchronized (this) {
+        localRef = this.agentClient;
+        if (localRef == null) {
+          udpTransport = ThriftUdpTransport.newThriftUdpClient(host, port);
+          localRef = new Agent.Client(protocolFactory.getProtocol(udpTransport));
+          this.agentClient = localRef;
+        }
+      }
+    }
+
+    return localRef;
   }
 
   @Override
   public void send(Process process, List<io.jaegertracing.thriftjava.Span> spans) throws SenderException {
     try {
-      agentClient.emitBatch(new Batch(process, spans));
+      getAgentClient().emitBatch(new Batch(process, spans));
     } catch (Exception e) {
       throw new SenderException(String.format("Could not send %d spans", spans.size()), e, spans.size());
     }
@@ -72,7 +91,11 @@ public int close() throws SenderException {
     try {
       return super.close();
     } finally {
-      udpTransport.close();
+      synchronized (this) {
+        if (udpTransport != null) {
+          udpTransport.close();
+        }
+      }
     }
   }
 }

jaeger-thrift/src/test/java/io/jaegertracing/thrift/internal/senders/ThriftSenderFactoryTest.java

@@ -17,7 +17,6 @@
 import static org.junit.Assert.assertTrue;
 
 import io.jaegertracing.Configuration;
-import io.jaegertracing.internal.senders.NoopSender;
 import io.jaegertracing.spi.Sender;
 import org.junit.Before;
 import org.junit.Test;
@@ -47,7 +46,7 @@ public void testSenderWithAgentDataFromEnv() {
     System.setProperty(Configuration.JAEGER_AGENT_HOST, "jaeger-agent");
     System.setProperty(Configuration.JAEGER_AGENT_PORT, "6832");
     Sender sender = Configuration.SenderConfiguration.fromEnv().getSender();
-    assertTrue(sender instanceof NoopSender);
+    assertTrue(sender instanceof UdpSender);
   }
 
   @Test

jaeger-thrift/src/test/java/io/jaegertracing/thrift/internal/senders/UdpLazinessTest.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2020, The Jaeger 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 io.jaegertracing.thrift.internal.senders;
+
+import io.jaegertracing.internal.exceptions.SenderException;
+import org.junit.Test;
+
+import java.net.SocketException;
+import java.util.Collections;
+
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+public class UdpLazinessTest {
+    @Test
+    public void testLazyInitializationOfAgent() {
+        UdpSender udpSender = new UdpSender("agent.acme.test", 55555, 0);
+
+        try {
+            udpSender.send(null, Collections.emptyList());
+            fail("Send should fail!");
+        } catch (SenderException e) {
+            assertTrue("send should throw a socket exception", e.getCause().getCause() instanceof SocketException);
+        }
+    }
+}