feat: implement MCP agent process auto-restart

Modify the MCPAgent to automatically attempt restarting the external\nserver process if it detects the process has exited.\n\n- Replaced sync.Once with mutex-protected checks (a.client == nil) to allow\n  re-initialization.\n- The monitoring goroutine now cleans up agent state (nils client, cmd, stdin)\n  upon process exit, signaling the need for restart.\n- ensureClientInitialized now attempts to start/initialize if the client is nil.\n- GetArtistBiography checks client validity again after locking to handle race\n  conditions where the process might die just after initialization check.

core/agents/mcp/mcp_agent.go

@@ -24,14 +24,16 @@ const (
 	// Hardcoded path for PoC
 	mcpServerPath = "/Users/deluan/Development/navidrome/plugins-mcp/mcp-server"
 	mcpToolName   = "get_artist_biography"
+	// TODO: Add configuration for restart delays
+	initializationTimeout = 10 * time.Second
 )
 
 // MCPAgent interacts with an external MCP server for metadata retrieval.
-// It keeps a single instance of the server process running.
+// It keeps a single instance of the server process running and attempts restart on failure.
 type MCPAgent struct {
-	mu       sync.Mutex
+	mu sync.Mutex
-	initOnce sync.Once
+	// No longer using sync.Once to allow re-initialization
-	initErr  error // Stores initialization error
+	// initErr error // Error state managed implicitly by client being nil
 
 	cmd    *exec.Cmd
 	stdin  io.WriteCloser
@@ -45,7 +47,6 @@ func mcpConstructor(ds model.DataStore) agents.Interface {
 		return nil
 	}
 	log.Info("MCP Agent created, server will be started on first request", "serverPath", mcpServerPath)
-	// Initialize the struct, but don't start the process yet.
 	return &MCPAgent{}
 }
 
@@ -53,84 +54,115 @@ func (a *MCPAgent) AgentName() string {
 	return mcpAgentName
 }
 
-// ensureClientInitialized starts the MCP server process and initializes the client exactly once.
+// ensureClientInitialized starts the MCP server process and initializes the client if needed.
-func (a *MCPAgent) ensureClientInitialized(ctx context.Context) error {
+// It now attempts restart if the client is found to be nil.
-	a.initOnce.Do(func() {
+func (a *MCPAgent) ensureClientInitialized(ctx context.Context) (err error) {
-		log.Info(ctx, "Initializing MCP client and starting server process...", "serverPath", mcpServerPath)
+	a.mu.Lock()
-		// Use background context for the command itself, so it doesn't get cancelled by the request context.
+	// If client is already initialized and valid, we're done.
-		// We manage its lifecycle independently.
+	if a.client != nil {
-		cmd := exec.CommandContext(context.Background(), mcpServerPath)
+		a.mu.Unlock()
+		return nil
+	}
+	// Unlock after the check, as the rest of the function needs the lock.
+	a.mu.Unlock()
 
-		stdin, err := cmd.StdinPipe()
+	// --- Attempt initialization/restart ---
-		if err != nil {
+	log.Info(ctx, "Initializing MCP client and starting/restarting server process...", "serverPath", mcpServerPath)
-			a.initErr = fmt.Errorf("failed to get stdin pipe for MCP server: %w", err)
-			log.Error(ctx, "MCP init failed", "error", a.initErr)
-			return
-		}
 
-		stdout, err := cmd.StdoutPipe()
+	// Use background context for the command itself, so it doesn't get cancelled by the request context.
-		if err != nil {
+	cmd := exec.CommandContext(context.Background(), mcpServerPath)
-			a.initErr = fmt.Errorf("failed to get stdout pipe for MCP server: %w", err)
-			_ = stdin.Close() // Clean up stdin pipe
-			log.Error(ctx, "MCP init failed", "error", a.initErr)
-			return
-		}
 
-		// Capture stderr for debugging
+	var stdin io.WriteCloser
-		var stderr strings.Builder
+	var stdout io.ReadCloser
-		cmd.Stderr = &stderr
+	var stderr strings.Builder
 
-		if err := cmd.Start(); err != nil {
+	stdin, err = cmd.StdinPipe()
-			a.initErr = fmt.Errorf("failed to start MCP server process: %w", err)
+	if err != nil {
-			_ = stdin.Close()
+		err = fmt.Errorf("failed to get stdin pipe for MCP server: %w", err)
-			_ = stdout.Close()
+		log.Error(ctx, "MCP init/restart failed", "error", err)
-			log.Error(ctx, "MCP init failed", "error", a.initErr)
+		return err // Return error directly
-			return
+	}
-		}
 
-		a.cmd = cmd
+	stdout, err = cmd.StdoutPipe()
-		a.stdin = stdin
+	if err != nil {
-		log.Info(ctx, "MCP server process started", "pid", cmd.Process.Pid)
+		err = fmt.Errorf("failed to get stdout pipe for MCP server: %w", err)
+		_ = stdin.Close() // Clean up stdin pipe
+		log.Error(ctx, "MCP init/restart failed", "error", err)
+		return err
+	}
 
-		// Start a goroutine to wait for the process and log if it exits unexpectedly
+	cmd.Stderr = &stderr
-		go func() {
+
-			err := cmd.Wait()
+	if err = cmd.Start(); err != nil {
-			log.Warn("MCP server process exited", "pid", cmd.Process.Pid, "error", err, "stderr", stderr.String())
+		err = fmt.Errorf("failed to start MCP server process: %w", err)
-			// Maybe add logic here to attempt restart or mark the agent as unhealthy?
+		_ = stdin.Close()
-			// For PoC, just log it.
+		_ = stdout.Close()
-			// Ensure pipes are closed if the process exits
+		log.Error(ctx, "MCP init/restart failed", "error", err)
-			a.mu.Lock()
+		return err
+	}
+
+	currentPid := cmd.Process.Pid
+	log.Info(ctx, "MCP server process started/restarted", "pid", currentPid)
+
+	// --- Start monitoring goroutine for *this* process ---
+	go func(processCmd *exec.Cmd, processStderr *strings.Builder, processPid int) {
+		waitErr := processCmd.Wait()
+		// Lock immediately after Wait returns to update state atomically
+		a.mu.Lock()
+		log.Warn("MCP server process exited", "pid", processPid, "error", waitErr, "stderr", processStderr.String())
+		// Clean up state only if this is still the *current* process
+		// (to avoid race condition if a quick restart happened)
+		if a.cmd == processCmd {
 			if a.stdin != nil {
 				_ = a.stdin.Close()
 				a.stdin = nil
 			}
-			// stdout is typically closed by the StdioServerTransport when done reading.
+			a.client = nil // Mark client as unusable, triggering restart on next call
-			a.client = nil // Mark client as unusable
 			a.cmd = nil
-			a.mu.Unlock()
+			log.Info("MCP agent state cleaned up after process exit", "pid", processPid)
-		}()
+		} else {
-
+			log.Debug("MCP agent process exited, but state already updated by newer process", "exitedPid", processPid)
-		// Create and initialize the MCP client
-		transport := stdio.NewStdioServerTransportWithIO(stdout, stdin)
-		client := mcp.NewClient(transport)
-
-		// Use a specific context for initialization, not the potentially short-lived request context
-		initCtx, cancel := context.WithTimeout(context.Background(), 10*time.Second) // Example timeout
-		defer cancel()
-		if _, err := client.Initialize(initCtx); err != nil {
-			a.initErr = fmt.Errorf("failed to initialize MCP client: %w", err)
-			log.Error(ctx, "MCP client initialization failed", "error", a.initErr)
-			// Attempt to kill the process we just started
-			if killErr := cmd.Process.Kill(); killErr != nil {
-				log.Error(ctx, "Failed to kill MCP server process after init failure", "pid", cmd.Process.Pid, "error", killErr)
-			}
-			return
 		}
+		a.mu.Unlock()
+	}(cmd, &stderr, currentPid) // Pass copies/values to the goroutine
 
-		a.client = client
+	// --- Initialize MCP client ---
-		log.Info(ctx, "MCP client initialized successfully")
+	transport := stdio.NewStdioServerTransportWithIO(stdout, stdin) // Use the pipes from this attempt
-	})
+	client := mcp.NewClient(transport)
-	return a.initErr
+
+	initCtx, cancel := context.WithTimeout(context.Background(), initializationTimeout)
+	defer cancel()
+	if _, err = client.Initialize(initCtx); err != nil {
+		err = fmt.Errorf("failed to initialize MCP client: %w", err)
+		log.Error(ctx, "MCP client initialization failed after process start", "pid", currentPid, "error", err)
+		// Attempt to kill the process we just started, as client init failed
+		if killErr := cmd.Process.Kill(); killErr != nil {
+			log.Error(ctx, "Failed to kill MCP server process after init failure", "pid", currentPid, "error", killErr)
+		}
+		return err // Return the initialization error
+	}
+
+	// --- Initialization successful, update agent state ---
+	a.mu.Lock() // Lock again to update agent state
+	// Double-check if another goroutine initialized successfully in the meantime
+	// Although unlikely with the outer lock/check, it's safer.
+	if a.client != nil {
+		a.mu.Unlock()
+		log.Warn(ctx, "MCP client was already initialized by another routine, discarding this attempt", "pid", currentPid)
+		// Kill the redundant process we started
+		if killErr := cmd.Process.Kill(); killErr != nil {
+			log.Error(ctx, "Failed to kill redundant MCP server process", "pid", currentPid, "error", killErr)
+		}
+		return nil // Return success as *a* client is available
+	}
+
+	a.cmd = cmd       // Store the successfully started command
+	a.stdin = stdin   // Store its stdin
+	a.client = client // Store the successfully initialized client
+	a.mu.Unlock()
+
+	log.Info(ctx, "MCP client initialized successfully", "pid", currentPid)
+	return nil // Success
 }
 
 // getArtistBiographyArgs defines the structure for the MCP tool arguments.
@@ -144,23 +176,27 @@ type getArtistBiographyArgs struct {
 
 // GetArtistBiography retrieves the artist biography by calling the external MCP server.
 func (a *MCPAgent) GetArtistBiography(ctx context.Context, id, name, mbid string) (string, error) {
-	// Ensure the client is initialized and the server is running
+	// Ensure the client is initialized and the server is running (attempts restart if needed)
 	if err := a.ensureClientInitialized(ctx); err != nil {
-		log.Error(ctx, "MCP client not initialized, cannot get biography", "error", err)
+		log.Error(ctx, "MCP agent initialization/restart failed, cannot get biography", "error", err)
-		// Don't wrap the initErr, return it directly so callers see the original init problem
+		return "", fmt.Errorf("MCP agent not ready: %w", err)
-		return "", fmt.Errorf("MCP agent not initialized: %w", err)
 	}
 
 	// Lock to ensure only one request uses the client/pipes at a time
 	a.mu.Lock()
-	defer a.mu.Unlock()
 
-	// Check if the client is still valid (it might have been nilled if the process died)
+	// Check if the client is still valid *after* ensuring initialization and acquiring lock.
+	// The monitoring goroutine could have nilled it out if the process died just now.
 	if a.client == nil {
-		log.Error(ctx, "MCP client is not valid (server process likely died)")
+		a.mu.Unlock() // Release lock before returning error
+		log.Error(ctx, "MCP client became invalid after initialization check (server process likely died)")
 		return "", fmt.Errorf("MCP agent process is not running")
 	}
 
+	// Keep a reference to the client while locked
+	currentClient := a.client
+	a.mu.Unlock() // Release lock before making the potentially blocking MCP call
+
 	log.Debug(ctx, "Calling MCP agent GetArtistBiography", "id", id, "name", name, "mbid", mbid)
 
 	// Prepare arguments for the tool call
@@ -170,17 +206,16 @@ func (a *MCPAgent) GetArtistBiography(ctx context.Context, id, name, mbid string
 		Mbid: mbid,
 	}
 
-	// Call the tool using the persistent client
+	// Call the tool using the client reference
 	log.Debug(ctx, "Calling MCP tool", "tool", mcpToolName, "args", args)
-	response, err := a.client.CallTool(ctx, mcpToolName, args)
+	response, err := currentClient.CallTool(ctx, mcpToolName, args) // Use currentClient
 	if err != nil {
 		// Handle potential pipe closures or other communication errors
 		log.Error(ctx, "Failed to call MCP tool", "tool", mcpToolName, "error", err)
 		// Check if the error indicates a broken pipe, suggesting the server died
 		if errors.Is(err, io.ErrClosedPipe) || strings.Contains(err.Error(), "broken pipe") || strings.Contains(err.Error(), "EOF") {
-			log.Warn(ctx, "MCP tool call failed, possibly due to server process exit")
+			log.Warn(ctx, "MCP tool call failed, possibly due to server process exit. State will be reset.")
-			// Attempt to reset for next time? Or just fail?
+			// State reset is handled by the monitoring goroutine, just return error
-			// For now, just return a clear error.
 			return "", fmt.Errorf("MCP agent process communication error: %w", err)
 		}
 		return "", fmt.Errorf("failed to call MCP tool '%s': %w", mcpToolName, err)