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fix: avoid dropped errors when transport is closed or uninitialized (#995)

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Signed-off-by: Dariusz Jędrzejczyk <2554306+chemicL@users.noreply.github.com>
This commit is contained in:
Dariusz Jędrzejczyk 2026-06-04 14:11:32 +02:00 • committed by GitHub
commit df75857fb1
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8 changed files with 109 additions and 47 deletions
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74
mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
View file

@ -38,6 +38,7 @@ import io.modelcontextprotocol.spec.McpClientTransport;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpTransportException;
import io.modelcontextprotocol.spec.McpTransportSession;
import io.modelcontextprotocol.spec.McpTransportSessionClosedException;
import io.modelcontextprotocol.spec.McpTransportSessionNotFoundException;
import io.modelcontextprotocol.spec.McpTransportStream;
import io.modelcontextprotocol.spec.ProtocolVersions;
@ -189,14 +190,6 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
return new DefaultMcpTransportSession(onClose);
}
private McpTransportSession<Disposable> createClosedSession(McpTransportSession<Disposable> existingSession) {
var existingSessionId = Optional.ofNullable(existingSession)
.filter(session -> !(session instanceof ClosedMcpTransportSession<Disposable>))
.flatMap(McpTransportSession::sessionId)
.orElse(null);
return new ClosedMcpTransportSession<>(existingSessionId);
}
private Publisher<Void> createDelete(String sessionId) {
var uri = Utils.resolveUri(this.baseUri, this.endpoint);
@ -240,7 +233,8 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
public Mono<Void> closeGracefully() {
return Mono.defer(() -> {
logger.debug("Graceful close triggered");
McpTransportSession<Disposable> currentSession = this.activeSession.getAndUpdate(this::createClosedSession);
McpTransportSession<Disposable> currentSession = this.activeSession
.getAndSet(ClosedMcpTransportSession.INSTANCE);
if (currentSession != null) {
return Mono.from(currentSession.closeGracefully());
}
@ -250,6 +244,19 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
private Mono<Disposable> reconnect(McpTransportStream<Disposable> stream) {
return Mono.deferContextual(ctx -> {
var rh = this.handler.get();
if (rh == null) {
logger.warn("Transport has no request handler registered. Remember to call connect!");
}
final Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchema.JSONRPCMessage>> requestHandler = rh != null
? rh : msg -> Mono.error(new IllegalStateException("No request handler"));
final McpTransportSession<Disposable> transportSession = this.activeSession.get();
if (ClosedMcpTransportSession.INSTANCE.equals(transportSession)) {
throw new McpTransportSessionClosedException();
}
if (stream != null) {
logger.debug("Reconnecting stream {} with lastId {}", stream.streamId(), stream.lastId());
@ -259,7 +266,7 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
}
final AtomicReference<Disposable> disposableRef = new AtomicReference<>();
final McpTransportSession<Disposable> transportSession = this.activeSession.get();
var uri = Utils.resolveUri(this.baseUri, this.endpoint);
Disposable connection = Mono.deferContextual(connectionCtx -> {
@ -389,18 +396,18 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
"Received unrecognized SSE event type: " + sseResponseEvent.sseEvent().event()));
})
.retryWhen(authorizationErrorRetrySpec())
.flatMap(jsonrpcMessage -> this.handler.get().apply(Mono.just(jsonrpcMessage)))
.flatMap(jsonrpcMessage -> requestHandler.apply(Mono.just(jsonrpcMessage)))
.onErrorMap(CompletionException.class, t -> t.getCause())
.onErrorComplete(t -> {
this.handleException(t);
return true;
})
.doFinally(s -> {
Disposable ref = disposableRef.getAndSet(null);
if (ref != null) {
transportSession.removeConnection(ref);
}
}))
.onErrorComplete(t -> {
this.handleException(t);
return true;
})
.contextWrite(ctx)
.subscribe();
@ -467,10 +474,23 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
return Mono.create(deliveredSink -> {
var rh = this.handler.get();
if (rh == null) {
logger.warn("Transport has no request handler registered. Remember to call connect!");
}
final Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchema.JSONRPCMessage>> requestHandler = rh != null
? rh : msg -> Mono.error(new IllegalStateException("No request handler"));
var transportSession = this.activeSession.get();
if (ClosedMcpTransportSession.INSTANCE.equals(transportSession)) {
throw new McpTransportSessionClosedException();
}
logger.debug("Sending message {}", sentMessage);
final AtomicReference<Disposable> disposableRef = new AtomicReference<>();
final McpTransportSession<Disposable> transportSession = this.activeSession.get();
var uri = Utils.resolveUri(this.baseUri, this.endpoint);
String jsonBody = this.toString(sentMessage);
@ -643,22 +663,26 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
new RuntimeException("Failed to send message: " + responseEvent));
})
.retryWhen(authorizationErrorRetrySpec())
.flatMap(jsonRpcMessage -> this.handler.get().apply(Mono.just(jsonRpcMessage)))
.flatMap(jsonRpcMessage -> requestHandler.apply(Mono.just(jsonRpcMessage)))
.onErrorMap(CompletionException.class, t -> t.getCause())
.onErrorComplete(t -> {
// handle the error first
this.handleException(t);
// inform the caller of sendMessage
deliveredSink.error(t);
return true;
})
.doFinally(s -> {
logger.debug("SendMessage finally: {}", s);
Disposable ref = disposableRef.getAndSet(null);
if (ref != null) {
transportSession.removeConnection(ref);
}
})).contextWrite(deliveredSink.contextView()).subscribe();
})).onErrorComplete(t -> {
// handle the error first
try {
this.handleException(t);
}
catch (Exception e) {
logger.error("Error handling exception {}", t.getMessage(), e);
}
// inform the caller of sendMessage
deliveredSink.error(t);
return true;
}).contextWrite(deliveredSink.contextView()).subscribe();
disposableRef.set(connection);
transportSession.addConnection(connection);

26
mcp-core/src/main/java/io/modelcontextprotocol/spec/ClosedMcpTransportSession.java
View file

@ -6,43 +6,41 @@ package io.modelcontextprotocol.spec;
import java.util.Optional;
import org.reactivestreams.Publisher;
import reactor.core.Disposable;
import reactor.core.publisher.Mono;
import reactor.util.annotation.Nullable;
/**
* Represents a closed MCP session, which may not be reused. All calls will throw a
* {@link McpTransportSessionClosedException}.
* Represents a closed MCP session, which may not be reused.
*
* @param <CONNECTION> the resource representing the connection that the transport
* manages.
* @author Daniel Garnier-Moiroux
* @author Dariusz Jędrzejczyk
*/
public class ClosedMcpTransportSession<CONNECTION> implements McpTransportSession<CONNECTION> {
public final class ClosedMcpTransportSession implements McpTransportSession<Disposable> {
private final String sessionId;
public static final ClosedMcpTransportSession INSTANCE = new ClosedMcpTransportSession();
public ClosedMcpTransportSession(@Nullable String sessionId) {
this.sessionId = sessionId;
private ClosedMcpTransportSession() {
}
@Override
public Optional<String> sessionId() {
throw new McpTransportSessionClosedException(sessionId);
return Optional.empty();
}
@Override
public boolean markInitialized(String sessionId) {
throw new McpTransportSessionClosedException(sessionId);
throw new IllegalStateException("MCP Session is already closed");
}
@Override
public void addConnection(CONNECTION connection) {
throw new McpTransportSessionClosedException(sessionId);
public void addConnection(Disposable connection) {
throw new IllegalStateException("MCP Session is already closed");
}
@Override
public void removeConnection(CONNECTION connection) {
throw new McpTransportSessionClosedException(sessionId);
public void removeConnection(Disposable connection) {
throw new IllegalStateException("MCP Session is already closed");
}
@Override

13
mcp-core/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java
View file

@ -119,7 +119,8 @@ public class McpClientSession implements McpSession {
this.requestHandlers.putAll(requestHandlers);
this.notificationHandlers.putAll(notificationHandlers);
this.transport.connect(mono -> mono.doOnNext(this::handle)).transform(connectHook).subscribe();
this.transport.connect(mono -> mono.doOnNext(this::handle)).transform(connectHook).subscribe(ignored -> {
}, error -> logger.warn("Client failed during connect", error));
}
private void dismissPendingResponses() {
@ -160,7 +161,15 @@ public class McpClientSession implements McpSession {
var errorResponse = McpSchema.JSONRPCResponse.error(request.id(), jsonRpcError);
return Mono.just(errorResponse);
}).flatMap(this.transport::sendMessage).onErrorComplete(t -> {
logger.warn("Issue sending response to the client, ", t);
if (t instanceof McpTransportSessionClosedException) {
logger.debug("Can't send response to request {} when the transport is closed", request.id());
}
else if (McpTransport.isPeerClosed(t)) {
logger.debug("Can't send response to request {}: connection closed by peer", request.id(), t);
}
else {
logger.warn("Failed to send response to the server", t);
}
return true;
}).subscribe();
}

23
mcp-core/src/main/java/io/modelcontextprotocol/spec/McpTransport.java
View file

@ -8,6 +8,8 @@ import java.util.List;
import io.modelcontextprotocol.spec.McpSchema.JSONRPCMessage;
import io.modelcontextprotocol.json.TypeRef;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import reactor.core.publisher.Mono;
/**
@ -39,6 +41,8 @@ import reactor.core.publisher.Mono;
*/
public interface McpTransport {
Logger logger = LoggerFactory.getLogger(McpTransport.class);
/**
* Closes the transport connection and releases any associated resources.
*
@ -48,7 +52,24 @@ public interface McpTransport {
* </p>
*/
default void close() {
this.closeGracefully().subscribe();
this.closeGracefully().subscribe(ignored -> {
}, error -> {
if (isPeerClosed(error)) {
logger.debug("Error during asynchronous close", error);
}
else {
logger.warn("Error during asynchronous close", error);
}
});
}
static boolean isPeerClosed(Throwable t) {
for (Throwable c = t; c != null; c = c.getCause()) {
if (c instanceof java.io.EOFException) {
return true;
}
}
return false;
}
/**

6
mcp-core/src/main/java/io/modelcontextprotocol/spec/McpTransportSessionClosedException.java
View file

@ -13,8 +13,14 @@ import reactor.util.annotation.Nullable;
* @see ClosedMcpTransportSession
* @author Daniel Garnier-Moiroux
*/
public class McpTransportSessionClosedException extends RuntimeException {
public McpTransportSessionClosedException() {
super("Transport has already been closed.");
}
@Deprecated(forRemoval = true)
public McpTransportSessionClosedException(@Nullable String sessionId) {
super(sessionId != null ? "MCP session with ID %s has been closed".formatted(sessionId)
: "MCP session has been closed");

4
mcp-test/src/main/java/io/modelcontextprotocol/client/AbstractMcpAsyncClientTests.java
View file

@ -464,7 +464,7 @@ public abstract class AbstractMcpAsyncClientTests {
void testInitializeWithRootsListProviders() {
withClient(createMcpTransport(),
builder -> builder.roots(Root.builder("file:///test/path").name("test-root").build()), client -> {
StepVerifier.create(client.initialize().then(client.closeGracefully())).verifyComplete();
StepVerifier.create(client.initialize()).expectNextCount(1).verifyComplete();
});
}
@ -728,8 +728,6 @@ public abstract class AbstractMcpAsyncClientTests {
builder -> builder.loggingConsumer(notification -> Mono.fromRunnable(() -> logReceived.set(true))),
client -> {
StepVerifier.create(client.initialize()).expectNextMatches(Objects::nonNull).verifyComplete();
StepVerifier.create(client.closeGracefully()).verifyComplete();
});
}

9
mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportTest.java
View file

@ -8,11 +8,14 @@ import io.modelcontextprotocol.client.transport.customizer.McpAsyncHttpClientReq
import io.modelcontextprotocol.client.transport.customizer.McpSyncHttpClientRequestCustomizer;
import io.modelcontextprotocol.common.McpTransportContext;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpTransportSessionClosedException;
import io.modelcontextprotocol.spec.ProtocolVersions;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.Map;
import java.util.function.Consumer;
import java.util.function.Function;
import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;
@ -139,13 +142,14 @@ class HttpClientStreamableHttpTransportTest {
var testMessage = new McpSchema.JSONRPCRequest(McpSchema.METHOD_INITIALIZE, "test-id", initializeRequest);
StepVerifier.create(transport.sendMessage(testMessage))
.expectErrorMessage("MCP session has been closed")
.expectErrorMessage("Transport has already been closed.")
.verify();
}
@Test
void testCloseInitialized() {
var transport = HttpClientStreamableHttpTransport.builder(host).build();
transport.connect(Function.identity()).block();
var initializeRequest = McpSchema.InitializeRequest
.builder(ProtocolVersions.MCP_2025_11_25, McpSchema.ClientCapabilities.builder().roots(true).build(),
@ -157,7 +161,8 @@ class HttpClientStreamableHttpTransportTest {
StepVerifier.create(transport.closeGracefully()).verifyComplete();
StepVerifier.create(transport.sendMessage(testMessage))
.expectErrorMatches(err -> err.getMessage().matches("MCP session with ID [a-zA-Z0-9-]* has been closed"))
.expectErrorMatches(err -> err instanceof McpTransportSessionClosedException
&& err.getMessage().contains("Transport has already been closed"))
.verify();
}

1
mcp-test/src/test/java/io/modelcontextprotocol/server/transport/ServerTransportSecurityIntegrationTests.java
View file

@ -81,6 +81,7 @@ class ServerTransportSecurityIntegrationTests {
@AfterEach
void tearDown() {
requestCustomizer.reset();
mcpClient.close();
}