Open
Description
Describe the bug
Came across a runtime crash in NetworkTransport when a MCP client reconnects (mcp-swift-sdk version 0.9.0)
To Reproduce
I haven't yet been able to reliably reproduce it. In my case the host macOS app was running, when I launched Zed where I had the MCP server configured. Zed seems to reconnect, but that caused a runtime crash in the host macOS app.
Logs
macOS crash log snippet:
0x1a76fa1f8 _assertionFailure(_:_:file:line:flags:) + 176
1 libswift_Concurrency.dylib 0x27ecd64e4 CheckedContinuation.resume(throwing:) + 348
2 MyHostApp 0x1003beb94 closure #1 in NetworkTransport.handleReconnection(error:continuation:context:) + 180
Relevant code snippet:
private func handleReconnection(
error: Swift.Error,
continuation: CheckedContinuation<Void, Swift.Error>,
context: String
) async {
if !isStopping,
reconnectionConfig.enabled,
reconnectAttempt < reconnectionConfig.maxAttempts
{
// Try to reconnect with exponential backoff
reconnectAttempt += 1
logger.info(
"Attempting reconnection after \(context) (\(reconnectAttempt)/\(reconnectionConfig.maxAttempts))..."
)
// Calculate backoff delay
let delay = reconnectionConfig.backoffDelay(for: reconnectAttempt)
// Schedule reconnection attempt after delay
Task {
try? await Task.sleep(for: .seconds(delay))
if !isStopping {
// Cancel the current connection before attempting to reconnect.
self.connection.cancel()
// Resume original continuation with error; outer logic or a new call to connect() will handle retry.
continuation.resume(throwing: error)
} else {
continuation.resume(throwing: error) // Stopping, so fail.
}
}
} else {
// Not configured to reconnect, exceeded max attempts, or stopping
self.connection.cancel() // Ensure connection is cancelled
continuation.resume(throwing: error)
}
}Additional context
The crash is triggered when continuation.resume(throwing: ...) gets called multiple times for the same continuation. I haven't yet wrapped my head around the entire flow here and how this is possible, and it's hard to debug as it seems intermittent.
Suggestions appreciated for how to address this.