Implement in-memory / passthrough transport (#122)

* Implement in-memory / passthrough transport

* Add InMemoryTransport to README

Sources/MCP/Base/Transports/InMemoryTransport.swift

@@ -0,0 +1,197 @@
+import Foundation
+import Logging
+
+/// An in-memory transport implementation for direct communication within the same process.
+///
+/// - Example:
+///   ```swift
+///   // Create a connected pair of transports
+///   let (clientTransport, serverTransport) = await InMemoryTransport.createConnectedPair()
+///
+///   // Use with client and server
+///   let client = Client(name: "MyApp", version: "1.0.0")
+///   let server = Server(name: "MyServer", version: "1.0.0")
+///
+///   try await client.connect(transport: clientTransport)
+///   try await server.connect(transport: serverTransport)
+///   ```
+public actor InMemoryTransport: Transport {
+    /// Logger instance for transport-related events
+    public nonisolated let logger: Logger
+
+    private var isConnected = false
+    private var pairedTransport: InMemoryTransport?
+
+    // Message queues
+    private var incomingMessages: [Data] = []
+    private var messageContinuation: AsyncThrowingStream<Data, Swift.Error>.Continuation?
+
+    /// Creates a new in-memory transport
+    ///
+    /// - Parameter logger: Optional logger instance for transport events
+    public init(logger: Logger? = nil) {
+        self.logger =
+            logger
+            ?? Logger(
+                label: "mcp.transport.in-memory",
+                factory: { _ in SwiftLogNoOpLogHandler() }
+            )
+    }
+
+    /// Creates a connected pair of in-memory transports
+    ///
+    /// This is the recommended way to create transports for client-server communication
+    /// within the same process. The returned transports are already paired and ready
+    /// to be connected.
+    ///
+    /// - Parameter logger: Optional logger instance shared by both transports
+    /// - Returns: A tuple of (clientTransport, serverTransport) ready for use
+    public static func createConnectedPair(
+        logger: Logger? = nil
+    ) async -> (client: InMemoryTransport, server: InMemoryTransport) {
+        let clientLogger: Logger
+        let serverLogger: Logger
+
+        if let providedLogger = logger {
+            // If a logger is provided, use it directly for both transports
+            clientLogger = providedLogger
+            serverLogger = providedLogger
+        } else {
+            // Create default loggers with appropriate labels
+            clientLogger = Logger(
+                label: "mcp.transport.in-memory.client",
+                factory: { _ in SwiftLogNoOpLogHandler() }
+            )
+            serverLogger = Logger(
+                label: "mcp.transport.in-memory.server",
+                factory: { _ in SwiftLogNoOpLogHandler() }
+            )
+        }
+
+        let clientTransport = InMemoryTransport(logger: clientLogger)
+        let serverTransport = InMemoryTransport(logger: serverLogger)
+
+        // Perform pairing
+        await clientTransport.pair(with: serverTransport)
+        await serverTransport.pair(with: clientTransport)
+
+        return (clientTransport, serverTransport)
+    }
+
+    /// Pairs this transport with another for bidirectional communication
+    ///
+    /// - Parameter other: The transport to pair with
+    /// - Important: This method should typically not be called directly.
+    ///   Use `createConnectedPair()` instead.
+    private func pair(with other: InMemoryTransport) {
+        self.pairedTransport = other
+    }
+
+    /// Establishes connection with the transport
+    ///
+    /// For in-memory transports, this validates that the transport is properly
+    /// paired and sets up the message stream.
+    ///
+    /// - Throws: MCPError.internalError if the transport is not paired
+    public func connect() async throws {
+        guard !isConnected else {
+            logger.debug("Transport already connected")
+            return
+        }
+
+        guard pairedTransport != nil else {
+            throw MCPError.internalError(
+                "Transport not paired. Use createConnectedPair() to create paired transports.")
+        }
+
+        isConnected = true
+        logger.info("Transport connected successfully")
+    }
+
+    /// Disconnects from the transport
+    ///
+    /// This closes the message stream and marks the transport as disconnected.
+    public func disconnect() async {
+        guard isConnected else { return }
+
+        isConnected = false
+        messageContinuation?.finish()
+        messageContinuation = nil
+
+        // Notify paired transport of disconnection
+        if let paired = pairedTransport {
+            await paired.handlePeerDisconnection()
+        }
+
+        logger.info("Transport disconnected")
+    }
+
+    /// Handles disconnection from the paired transport
+    private func handlePeerDisconnection() {
+        if isConnected {
+            messageContinuation?.finish(throwing: MCPError.connectionClosed)
+            messageContinuation = nil
+            isConnected = false
+            logger.info("Peer transport disconnected")
+        }
+    }
+
+    /// Sends a message to the paired transport
+    ///
+    /// Messages are delivered directly to the paired transport's receive queue
+    /// without any additional encoding or framing.
+    ///
+    /// - Parameter data: The message data to send
+    /// - Throws: MCPError.internalError if not connected or no paired transport
+    public func send(_ data: Data) async throws {
+        guard isConnected else {
+            throw MCPError.internalError("Transport not connected")
+        }
+
+        guard let paired = pairedTransport else {
+            throw MCPError.internalError("No paired transport")
+        }
+
+        logger.debug("Sending message", metadata: ["size": "\(data.count)"])
+
+        // Deliver message to paired transport
+        await paired.deliverMessage(data)
+    }
+
+    /// Delivers a message from the paired transport
+    private func deliverMessage(_ data: Data) {
+        guard isConnected else {
+            logger.warning("Received message while disconnected")
+            return
+        }
+
+        logger.debug("Message received", metadata: ["size": "\(data.count)"])
+
+        if let continuation = messageContinuation {
+            continuation.yield(data)
+        } else {
+            // Queue message if stream not yet created
+            incomingMessages.append(data)
+        }
+    }
+
+    /// Receives messages from the paired transport
+    ///
+    /// - Returns: An AsyncThrowingStream of Data objects representing messages
+    public func receive() -> AsyncThrowingStream<Data, Swift.Error> {
+        return AsyncThrowingStream<Data, Swift.Error> { continuation in
+            self.messageContinuation = continuation
+
+            // Deliver any queued messages
+            for message in self.incomingMessages {
+                continuation.yield(message)
+            }
+            self.incomingMessages.removeAll()
+
+            // Check if already disconnected
+            if !self.isConnected {
+                continuation.finish()
+            }
+        }
+    }
+}