Close channel on handshake error with old version (#56989) (#57025)

Today a transport response uses the same wire format version as the
corresponding request. This mostly works since we mostly know we are
communicating with a node with a compatible version. TCP handshakes don't have
this guarantee since they use `Version.CURRENT.minimumCompatibilityVersion()`
to let us handshake with older nodes. This results in the strange situation of
a node of major version `N` responding to a node of major version `N-1` using a
wire format of version `N-2`.

We put extra effort into the longer BWC requirements for successful responses,
but we do not offer the same guarantees for error responses since they may be
rather complicated to serialize. This can result in the request sender
misinterpreting the response which may have unpredictable consequences.

Rather than strengthening the guarantees in this area, this commit simply logs
the exception and closes the connection on a handshake error with a node that
uses an incompatible wire format.

Closes #54337

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

@@ -42,7 +42,7 @@ public class Header {
     // These are directly set by tests
     String actionName;
     Tuple<Map<String, String>, Map<String, Set<String>>> headers;
-    private Set<String> features;
+    Set<String> features;
 
     Header(int networkMessageSize, long requestId, byte status, Version version) {
         this.networkMessageSize = networkMessageSize;

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

@@ -72,7 +72,7 @@ public int internalDecode(ReleasableBytesReference reference, Consumer<Object> f
                 } else {
                     totalNetworkSize = messageLength + TcpHeader.BYTES_REQUIRED_FOR_MESSAGE_SIZE;
 
-                    Header header = readHeader(messageLength, reference);
+                    Header header = readHeader(version, messageLength, reference);
                     bytesConsumed += headerBytesToRead;
                     if (header.isCompressed()) {
                         decompressor = new TransportDecompressor(recycler);
@@ -166,7 +166,8 @@ private int headerBytesToRead(BytesReference reference) {
         }
     }
 
-    private Header readHeader(int networkMessageSize, BytesReference bytesReference) throws IOException {
+    // exposed for use in tests
+    static Header readHeader(Version version, int networkMessageSize, BytesReference bytesReference) throws IOException {
         try (StreamInput streamInput = bytesReference.streamInput()) {
             streamInput.skip(TcpHeader.BYTES_REQUIRED_FOR_MESSAGE_SIZE);
             long requestId = streamInput.readLong();

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

@@ -143,7 +143,14 @@ private <T extends TransportRequest> void handleRequest(TcpChannel channel, Head
             try {
                 handshaker.handleHandshake(transportChannel, requestId, stream);
             } catch (Exception e) {
-                sendErrorResponse(action, transportChannel, e);
+                if (Version.CURRENT.isCompatible(header.getVersion())) {
+                    sendErrorResponse(action, transportChannel, e);
+                } else {
+                    logger.warn(new ParameterizedMessage(
+                        "could not send error response to handshake received on [{}] using wire format version [{}], closing channel",
+                        channel, header.getVersion()), e);
+                    channel.close();
+                }
             }
         } else {
             final TransportChannel transportChannel = new TcpTransportChannel(outboundHandler, channel, action, requestId, version,

server/src/test/java/org/elasticsearch/transport/InboundHandlerTests.java

@@ -19,25 +19,37 @@
 
 package org.elasticsearch.transport;
 
+import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.Version;
+import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.bytes.ReleasableBytesReference;
+import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
+import org.elasticsearch.common.io.stream.InputStreamStreamInput;
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
 import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.BigArrays;
 import org.elasticsearch.tasks.TaskManager;
 import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.test.MockLogAppender;
+import org.elasticsearch.test.VersionUtils;
 import org.elasticsearch.threadpool.TestThreadPool;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.junit.After;
 import org.junit.Before;
 
 import java.io.IOException;
+import java.io.InputStream;
 import java.util.Collections;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 
 import static org.hamcrest.Matchers.instanceOf;
@@ -165,4 +177,84 @@ public TestResponse read(StreamInput in) throws IOException {
             assertEquals(responseValue, responseCaptor.get().value);
         }
     }
+
+    public void testSendsErrorResponseToHandshakeFromCompatibleVersion() throws Exception {
+        // Nodes use their minimum compatibility version for the TCP handshake, so a node from v(major-1).x will report its version as
+        // v(major-2).last in the TCP handshake, with which we are not really compatible. We put extra effort into making sure that if
+        // successful we can respond correctly in a format this old, but we do not guarantee that we can respond correctly with an error
+        // response. However if the two nodes are from the same major version then we do guarantee compatibility of error responses.
+
+        final Version remoteVersion = VersionUtils.randomCompatibleVersion(random(), version);
+        final long requestId = randomNonNegativeLong();
+        final Header requestHeader = new Header(between(0, 100), requestId,
+            TransportStatus.setRequest(TransportStatus.setHandshake((byte) 0)), remoteVersion);
+        final InboundMessage requestMessage = unreadableInboundHandshake(remoteVersion, requestHeader);
+        requestHeader.actionName = TransportHandshaker.HANDSHAKE_ACTION_NAME;
+        requestHeader.headers = Tuple.tuple(org.elasticsearch.common.collect.Map.of(), org.elasticsearch.common.collect.Map.of());
+        requestHeader.features = org.elasticsearch.common.collect.Set.of();
+        handler.inboundMessage(channel, requestMessage);
+
+        final BytesReference responseBytesReference = channel.getMessageCaptor().get();
+        final Header responseHeader = InboundDecoder.readHeader(remoteVersion, responseBytesReference.length(), responseBytesReference);
+        assertTrue(responseHeader.isResponse());
+        assertTrue(responseHeader.isError());
+    }
+
+
+    public void testClosesChannelOnErrorInHandshakeWithIncompatibleVersion() throws Exception {
+        // Nodes use their minimum compatibility version for the TCP handshake, so a node from v(major-1).x will report its version as
+        // v(major-2).last in the TCP handshake, with which we are not really compatible. We put extra effort into making sure that if
+        // successful we can respond correctly in a format this old, but we do not guarantee that we can respond correctly with an error
+        // response so we must just close the connection on an error. To avoid the failure disappearing into a black hole we at least log
+        // it.
+
+        final MockLogAppender mockAppender = new MockLogAppender();
+        mockAppender.start();
+        mockAppender.addExpectation(
+            new MockLogAppender.SeenEventExpectation(
+                "expected message",
+                InboundHandler.class.getCanonicalName(),
+                Level.WARN,
+                "could not send error response to handshake"));
+        final Logger inboundHandlerLogger = LogManager.getLogger(InboundHandler.class);
+        Loggers.addAppender(inboundHandlerLogger, mockAppender);
+
+        try {
+            final AtomicBoolean isClosed = new AtomicBoolean();
+            channel.addCloseListener(ActionListener.wrap(() -> assertTrue(isClosed.compareAndSet(false, true))));
+
+            final Version remoteVersion = Version.fromId(randomIntBetween(0, version.minimumCompatibilityVersion().id - 1));
+            final long requestId = randomNonNegativeLong();
+            final Header requestHeader = new Header(between(0, 100), requestId,
+                TransportStatus.setRequest(TransportStatus.setHandshake((byte) 0)), remoteVersion);
+            final InboundMessage requestMessage = unreadableInboundHandshake(remoteVersion, requestHeader);
+            requestHeader.actionName = TransportHandshaker.HANDSHAKE_ACTION_NAME;
+            requestHeader.headers = Tuple.tuple(org.elasticsearch.common.collect.Map.of(), org.elasticsearch.common.collect.Map.of());
+            requestHeader.features = org.elasticsearch.common.collect.Set.of();
+            handler.inboundMessage(channel, requestMessage);
+            assertTrue(isClosed.get());
+            assertNull(channel.getMessageCaptor().get());
+            mockAppender.assertAllExpectationsMatched();
+        } finally {
+            Loggers.removeAppender(inboundHandlerLogger, mockAppender);
+            mockAppender.stop();
+        }
+    }
+
+    private static InboundMessage unreadableInboundHandshake(Version remoteVersion, Header requestHeader) {
+        return new InboundMessage(requestHeader, ReleasableBytesReference.wrap(BytesArray.EMPTY), () -> { }) {
+            @Override
+            public StreamInput openOrGetStreamInput() {
+                final StreamInput streamInput = new InputStreamStreamInput(new InputStream() {
+                    @Override
+                    public int read() {
+                        throw new ElasticsearchException("unreadable handshake");
+                    }
+                });
+                streamInput.setVersion(remoteVersion);
+                return streamInput;
+            }
+        };
+    }
+
 }