Addressed next round of review feedback:

- added test to check for close logging, with and without exception
- fixed up close logger formatting
- javadoc fixup for channel exception field
- further refactored remaining code with throwable -> exception in Netty4Transport

Author: schase-es

modules/transport-netty4/src/internalClusterTest/java/org/elasticsearch/transport/netty4/ESLoggingHandlerIT.java

@@ -11,14 +11,21 @@
 
 import org.apache.logging.log4j.Level;
 import org.elasticsearch.ESNetty4IntegTestCase;
+import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.test.ESIntegTestCase;
 import org.elasticsearch.test.MockLog;
 import org.elasticsearch.test.junit.annotations.TestLogging;
+import org.elasticsearch.transport.ConnectionManager;
 import org.elasticsearch.transport.TcpTransport;
+import org.elasticsearch.transport.Transport;
+import org.elasticsearch.transport.TransportConnectionListener;
 import org.elasticsearch.transport.TransportLogger;
+import org.elasticsearch.transport.TransportService;
 
 import java.io.IOException;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
 
 @ESIntegTestCase.ClusterScope(numDataNodes = 2, scope = ESIntegTestCase.Scope.TEST)
 public class ESLoggingHandlerIT extends ESNetty4IntegTestCase {
@@ -96,7 +103,7 @@ public void testConnectionLogging() throws IOException {
                 "close connection log",
                 TcpTransport.class.getCanonicalName(),
                 Level.DEBUG,
-                ".*closed transport connection \\[[1-9][0-9]*\\] to .* with age \\[[0-9]+ms\\].*"
+                ".*closed transport connection \\[[1-9][0-9]*\\] to .* with age \\[[0-9]+ms\\]$"
             )
         );
 
@@ -105,4 +112,46 @@ public void testConnectionLogging() throws IOException {
 
         mockLog.assertAllExpectationsMatched();
     }
+
+    @TestLogging(
+        value = "org.elasticsearch.transport.TcpTransport:DEBUG",
+        reason = "to ensure we log exception disconnect events on DEBUG level"
+    )
+    public void testExceptionalDisconnectLogging() throws Exception {
+        mockLog.addExpectation(
+            new MockLog.PatternSeenEventExpectation(
+                "exceptional close connection log",
+                TcpTransport.class.getCanonicalName(),
+                Level.DEBUG,
+                ".*closed transport connection \\[[1-9][0-9]*\\] to .* with age \\[[0-9]+ms\\], exception:.*"
+            )
+        );
+
+        final String nodeName = internalCluster().startNode();
+
+        final CountDownLatch latch = new CountDownLatch(1);
+        String masterNode = internalCluster().getMasterName();
+        ConnectionManager connManager = internalCluster().getInstance(TransportService.class, masterNode).getConnectionManager();
+        connManager.addListener(new TransportConnectionListener() {
+            @Override
+            public void onConnectionClosed(Transport.Connection conn) {
+                conn.addCloseListener(new ActionListener<>() {
+                    @Override
+                    public void onResponse(Void ignored) {}
+
+                    @Override
+                    public void onFailure(Exception e) {
+                        latch.countDown();
+                    }
+                });
+            }
+        });
+
+        int failAttempts = 0;
+        do {
+            internalCluster().restartNode(nodeName);
+        } while (latch.await(500, TimeUnit.MILLISECONDS) == false && failAttempts++ < 10);
+
+        mockLog.assertAllExpectationsMatched();
+    }
 }

modules/transport-netty4/src/main/java/org/elasticsearch/transport/netty4/Netty4TcpChannel.java

@@ -34,7 +34,9 @@ public class Netty4TcpChannel implements TcpChannel {
     private final ListenableFuture<Void> closeContext = new ListenableFuture<>();
     private final ChannelStats stats = new ChannelStats();
     private final boolean rstOnClose;
-    // Exception causing a close, reported to the closeContext listener
+    /**
+     * Exception causing a close, reported to the {@link #closeContext} listener
+     */
     private volatile Exception closeException = null;
 
     Netty4TcpChannel(Channel channel, boolean isServer, String profile, boolean rstOnClose, ChannelFuture connectFuture) {

modules/transport-netty4/src/main/java/org/elasticsearch/transport/netty4/Netty4Transport.java

@@ -307,7 +307,7 @@ protected void stopInternal() {
         }, serverBootstraps::clear, () -> clientBootstrap = null);
     }
 
-    private Exception exceptionFromThrowable(Throwable cause) {
+    static Exception exceptionFromThrowable(Throwable cause) {
         if (cause instanceof Error) {
             return new Exception(cause);
         } else {
@@ -399,11 +399,7 @@ private static class ServerChannelExceptionHandler extends ChannelInboundHandler
         public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
             ExceptionsHelper.maybeDieOnAnotherThread(cause);
             Netty4TcpServerChannel serverChannel = ctx.channel().attr(SERVER_CHANNEL_KEY).get();
-            if (cause instanceof Error) {
-                onServerException(serverChannel, new Exception(cause));
-            } else {
-                onServerException(serverChannel, (Exception) cause);
-            }
+            onServerException(serverChannel, exceptionFromThrowable(cause));
         }
     }
 }

server/src/main/java/org/elasticsearch/transport/TcpTransport.java

@@ -1208,7 +1208,7 @@ public void onFailure(Exception e) {
             long closeTimeMillis = threadPool.relativeTimeInMillis();
             logger.debug(
                 () -> format(
-                    "closed transport connection [{}] to [{}] with age [{}ms], exception:",
+                    "closed transport connection [%d] to [%s] with age [%dms], exception:",
                     connectionId,
                     node,
                     closeTimeMillis - openTimeMillis