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fix: resolve CheckedContinuation deadlock in NetworkTransport on actor dealloc (Swift 6.3) #213

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fix: resolve CheckedContinuation deadlock in NetworkTransport on actor dealloc (Swift 6.3) #213

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a201799
direnv for git
Mar 22, 2026
5d50718
fix: use LockedValue for continuation properties (Swift 6.3 compat) (…
doozMen Mar 18, 2026
70ea8de
fix continuation (#9)
doozMen Mar 17, 2026
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1 change: 1 addition & 0 deletions .envrc
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@@ -0,0 +1 @@
export GIT_DIR=$PWD/.jj/repo/store/git
4 changes: 3 additions & 1 deletion .gitignore
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Expand Up @@ -6,4 +6,6 @@ DerivedData/
.swiftpm/configuration/registries.json
.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
.netrc
.index-build/
.index-build/
.claude/settings.local.json
.swift-version
120 changes: 109 additions & 11 deletions Sources/MCP/Base/Transports/NetworkTransport.swift
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Expand Up @@ -459,7 +459,14 @@ import Logging
content: Heartbeat().data,
contentContext: .defaultMessage,
isComplete: true,
completion: .contentProcessed { [weak self] error in
completion: .contentProcessed { [weak self, continuation] error in
guard let self = self else {
continuation.resume(
throwing: MCPError.internalError(
"Transport deallocated during heartbeat"))
return
}

if let error = error {
continuation.resume(throwing: error)
} else {
Expand Down Expand Up @@ -511,9 +518,6 @@ import Logging
var messageWithNewline = message
messageWithNewline.append(UInt8(ascii: "\n"))

// Use a local actor-isolated variable to track continuation state
var sendContinuationResumed = false

try await withCheckedThrowingContinuation {
[weak self] (continuation: CheckedContinuation<Void, Swift.Error>) in
guard let self = self else {
Expand All @@ -528,6 +532,26 @@ import Logging
completion: .contentProcessed { [weak self] error in
guard let self = self else { return }

if let error = error {
self.logger.error("Send error: \(error)")

// Schedule reconnection check on a separate task
Task { [weak self] in
guard let self = self else { return }
let isStopping = await self.isStopping
if !isStopping && self.reconnectionConfig.enabled {
let isConnected = await self.isConnected
if isConnected && error.isConnectionLost {
self.logger.warning(
"Connection appears broken, will attempt to reconnect..."
)
await self.setIsConnected(false)
try? await Task.sleep(for: .milliseconds(500))

let currentIsStopping = await self.isStopping
if !currentIsStopping {
self.connection.cancel()
try? await self.connect()
Task { @MainActor in
if !sendContinuationResumed {
sendContinuationResumed = true
Expand Down Expand Up @@ -560,15 +584,63 @@ import Logging
}
}
}
if let error = error {
self.logger.error("Send error: \(error)")

// Schedule reconnection check on a separate task
Task { [weak self] in
guard let self = self else { return }
let isStopping = await self.isStopping
if !isStopping && self.reconnectionConfig.enabled {
let isConnected = await self.isConnected
if isConnected && error.isConnectionLost {
self.logger.warning(
"Connection appears broken, will attempt to reconnect..."
)
await self.setIsConnected(false)
try? await Task.sleep(for: .milliseconds(500))

let currentIsStopping = await self.isStopping
if !currentIsStopping {
self.connection.cancel()
try? await self.connect()
completion: .contentProcessed { [weak self, continuation] error in
guard let self = self else {
continuation.resume(
throwing: MCPError.internalError(
"Transport deallocated during send"))
return
}

if let error = error {
self.logger.error("Send error: \(error)")

// Schedule reconnection check on a separate task
Task { [weak self] in
guard let self = self else { return }
let isStopping = await self.isStopping
if !isStopping && self.reconnectionConfig.enabled {
let isConnected = await self.isConnected
if isConnected && error.isConnectionLost {
self.logger.warning(
"Connection appears broken, will attempt to reconnect..."
)
await self.setIsConnected(false)
try? await Task.sleep(for: .milliseconds(500))

let currentIsStopping = await self.isStopping
if !currentIsStopping {
self.connection.cancel()
try? await self.connect()
}
}

continuation.resume(
throwing: MCPError.internalError("Send error: \(error)"))
} else {
continuation.resume()
}
}

continuation.resume(
throwing: MCPError.internalError("Send error: \(error)"))
} else {
continuation.resume()
}
})
}
Expand Down Expand Up @@ -747,8 +819,6 @@ import Logging
/// - Returns: The received data chunk
/// - Throws: Network errors or transport failures
private func receiveData() async throws -> Data {
var receiveContinuationResumed = false

return try await withCheckedThrowingContinuation {
[weak self] (continuation: CheckedContinuation<Data, Swift.Error>) in
guard let self = self else {
Expand All @@ -759,6 +829,15 @@ import Logging
let maxLength = bufferConfig.maxReceiveBufferSize ?? Int.max
connection.receive(minimumIncompleteLength: 1, maximumLength: maxLength) {
content, _, isComplete, error in
if let error = error {
continuation.resume(throwing: MCPError.transportError(error))
} else if let content = content {
continuation.resume(returning: content)
} else if isComplete {
self.logger.trace("Connection completed by peer")
continuation.resume(throwing: MCPError.connectionClosed)
} else {
continuation.resume(returning: Data())
Task { @MainActor in
if !receiveContinuationResumed {
receiveContinuationResumed = true
Expand All @@ -774,6 +853,25 @@ import Logging
continuation.resume(returning: Data())
}
}
if let error = error {
continuation.resume(throwing: MCPError.transportError(error))
} else if let content = content {
continuation.resume(returning: content)
} else if isComplete {
self.logger.trace("Connection completed by peer")
continuation.resume(throwing: MCPError.connectionClosed)
} else {
continuation.resume(returning: Data())
[weak self, continuation] content, _, isComplete, error in
if let error = error {
continuation.resume(throwing: MCPError.transportError(error))
} else if let content = content {
continuation.resume(returning: content)
} else if isComplete {
self?.logger.trace("Connection completed by peer")
continuation.resume(throwing: MCPError.connectionClosed)
} else {
continuation.resume(returning: Data())
}
}
}
Expand Down
12 changes: 8 additions & 4 deletions Sources/MCP/Client/Client.swift
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Expand Up @@ -592,24 +592,27 @@ public actor Client {
/// Use this object to add requests to the batch.
/// - Throws: `MCPError.internalError` if the client is not connected.
/// Can also rethrow errors from the `body` closure or from sending the batch request.
public func withBatch(body: @escaping @Sendable (Batch) async throws -> Void) async throws {
@discardableResult
public func withBatch<T: Sendable>(
body: @escaping @Sendable (Batch) async throws -> T
) async throws -> T {
guard let connection = connection else {
throw MCPError.internalError("Client connection not initialized")
}

// Create Batch actor, passing self (Client)
let batch = Batch(client: self)

// Populate the batch actor by calling the user's closure.
try await body(batch)
// Populate the batch actor by calling the user's closure and capture result.
let result = try await body(batch)

// Get the collected requests from the batch actor
let requests = await batch.requests

// Check if there are any requests to send
guard !requests.isEmpty else {
await logger?.debug("Batch requested but no requests were added.")
return // Nothing to send
return result // Return result even if no requests
}

await logger?.debug(
Expand All @@ -620,6 +623,7 @@ public actor Client {
try await connection.send(data)

// Responses will be handled asynchronously by the message loop and handleBatchResponse/handleResponse.
return result
}

// MARK: - Lifecycle
Expand Down
72 changes: 54 additions & 18 deletions Tests/MCPTests/ClientTests.swift
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Expand Up @@ -345,12 +345,10 @@ struct ClientTests {

let request1 = Ping.request()
let request2 = Ping.request()
nonisolated(unsafe) var resultTask1: Task<Ping.Result, Swift.Error>?
nonisolated(unsafe) var resultTask2: Task<Ping.Result, Swift.Error>?

try await client.withBatch { batch in
resultTask1 = try await batch.addRequest(request1)
resultTask2 = try await batch.addRequest(request2)
let (resultTask1, resultTask2) = try await client.withBatch { batch in
let task1 = try await batch.addRequest(request1)
let task2 = try await batch.addRequest(request2)
return (task1, task2)
}

// Check if batch message was sent (after initialize and initialized notification)
Expand Down Expand Up @@ -381,13 +379,8 @@ struct ClientTests {
try await transport.queue(batch: [anyResponse1, anyResponse2])

// Wait for results and verify
guard let task1 = resultTask1, let task2 = resultTask2 else {
#expect(Bool(false), "Result tasks not created")
return
}

_ = try await task1.value // Should succeed
_ = try await task2.value // Should succeed
_ = try await resultTask1.value // Should succeed
_ = try await resultTask2.value // Should succeed

#expect(Bool(true)) // Reaching here means success

Expand Down Expand Up @@ -426,11 +419,11 @@ struct ClientTests {
let request1 = Ping.request() // Success
let request2 = Ping.request() // Error

nonisolated(unsafe) var resultTasks: [Task<Ping.Result, Swift.Error>] = []

try await client.withBatch { batch in
resultTasks.append(try await batch.addRequest(request1))
resultTasks.append(try await batch.addRequest(request2))
let resultTasks = try await client.withBatch { batch in
[
try await batch.addRequest(request1),
try await batch.addRequest(request2),
]
}

// Check if batch message was sent (after initialize and initialized notification)
Expand Down Expand Up @@ -514,6 +507,49 @@ struct ClientTests {
await client.disconnect()
}

@Test("Batch request - empty with non-Void return")
func testBatchRequestEmptyNonVoid() async throws {
let transport = MockTransport()
let client = Client(name: "TestClient", version: "1.0")

// Set up a task to handle the initialize response
let initTask = Task {
try await Task.sleep(for: .milliseconds(10))
if let lastMessage = await transport.sentMessages.last,
let data = lastMessage.data(using: .utf8),
let request = try? JSONDecoder().decode(Request<Initialize>.self, from: data)
{
let response = Initialize.response(
id: request.id,
result: .init(
protocolVersion: Version.latest,
capabilities: .init(),
serverInfo: .init(name: "TestServer", version: "1.0"),
instructions: nil
)
)
try await transport.queue(response: response)
}
}

try await client.connect(transport: transport)
try await Task.sleep(for: .milliseconds(10))
initTask.cancel()

// Call withBatch with non-Void return but don't add any requests
let result: Int = try await client.withBatch { _ in
42
}

// Verify the closure's return value is passed through
#expect(result == 42)

// Check that only initialize message and initialized notification were sent
#expect(await transport.sentMessages.count == 2)

await client.disconnect()
}

@Test("Notify method sends notifications")
func testClientNotify() async throws {
let transport = MockTransport()
Expand Down