fix deadlock sendTransport

Author: tosolveit

client/transport/stdio.go

@@ -123,6 +123,8 @@ func (c *Stdio) Start(ctx context.Context) error {
 // If an (optional) cmdFunc custom command factory function was configured, it will be used to construct the subprocess;
 // otherwise, the default behavior uses exec.CommandContext with the merged environment.
 // Initializes stdin, stdout, and stderr pipes for JSON-RPC communication.
+// A background goroutine is also started to wait for the subprocess to exit,
+// ensuring that the done channel is closed automatically if the process terminates unexpectedly.
 func (c *Stdio) spawnCommand(ctx context.Context) error {
 	if c.command == "" {
 		return nil
@@ -163,6 +165,16 @@ func (c *Stdio) spawnCommand(ctx context.Context) error {
 		return fmt.Errorf("failed to start command: %w", err)
 	}
 
+	go func() {
+		_ = cmd.Wait()
+		select {
+		case <-c.done:
+			// Already closed explicitly (via Close), do nothing
+		default:
+			close(c.done) // Automatically signal subprocess exit
+		}
+	}()
+
 	return nil
 }
 
@@ -191,6 +203,16 @@ func (c *Stdio) Close() error {
 	return nil
 }
 
+// IsClosed reports whether the subprocess has exited and the transport is no longer usable.
+func (c *Stdio) IsClosed() bool {
+	select {
+	case <-c.done:
+		return true
+	default:
+		return false
+	}
+}
+
 // GetSessionId returns the session ID of the transport.
 // Since stdio does not maintain a session ID, it returns an empty string.
 func (c *Stdio) GetSessionId() string {
@@ -293,6 +315,11 @@ func (c *Stdio) SendRequest(
 		c.mu.Unlock()
 	}
 
+	if c.IsClosed() {
+		deleteResponseChan()
+		return nil, fmt.Errorf("cannot send request: subprocess is closed")
+	}
+
 	// Send request
 	if _, err := c.stdin.Write(requestBytes); err != nil {
 		deleteResponseChan()
@@ -303,6 +330,9 @@ func (c *Stdio) SendRequest(
 	case <-ctx.Done():
 		deleteResponseChan()
 		return nil, ctx.Err()
+	case <-c.done:
+		deleteResponseChan()
+		return nil, fmt.Errorf("subprocess exited while waiting for response")
 	case response := <-responseChan:
 		return response, nil
 	}

client/transport/stdio_test.go

@@ -384,6 +384,31 @@ func TestStdio(t *testing.T) {
 			t.Errorf("Expected array with 3 items, got %v", result.Params["array"])
 		}
 	})
+
+	t.Run("SendRequestFailsIfSubprocessExited", func(t *testing.T) {
+		// Start a subprocess that exits immediately
+		ctx := context.Background()
+		stdio := NewStdio("sh", nil, "-c", "exit 0")
+
+		err := stdio.Start(ctx)
+		require.NoError(t, err)
+
+		// Wait for subprocess to exit
+		require.Eventually(t, func() bool {
+			return stdio.IsClosed()
+		}, time.Second, 10*time.Millisecond)
+
+		// Try to send a request
+		_, err = stdio.SendRequest(ctx, JSONRPCRequest{
+			JSONRPC: "2.0",
+			ID:      mcp.NewRequestId("dead"),
+			Method:  "noop",
+		})
+
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "subprocess")
+	})
+
 }
 
 func TestStdioErrors(t *testing.T) {
@@ -609,6 +634,32 @@ func TestStdio_SpawnCommand_UsesCommandFunc_Error(t *testing.T) {
 	require.EqualError(t, err, "test error")
 }
 
+func TestStdio_DoneClosedWhenSubcommandExits(t *testing.T) {
+	ctx := context.Background()
+
+	stdio := NewStdioWithOptions(
+		"sh",
+		nil,
+		[]string{"-c", "exit 0"},
+	)
+
+	require.NotNil(t, stdio)
+
+	err := stdio.spawnCommand(ctx)
+	require.NoError(t, err)
+
+	t.Cleanup(func() {
+		if stdio.cmd.Process != nil {
+			_ = stdio.cmd.Process.Kill()
+		}
+	})
+
+	// Wait up to 200ms for the done channel to close
+	require.Eventually(t, func() bool {
+		return stdio.IsClosed()
+	}, 200*time.Millisecond, 10*time.Millisecond, "expected done to be closed after subprocess exited")
+}
+
 func TestStdio_NewStdioWithOptions_AppliesOptions(t *testing.T) {
 	configured := false
 
@@ -620,3 +671,28 @@ func TestStdio_NewStdioWithOptions_AppliesOptions(t *testing.T) {
 	require.NotNil(t, stdio)
 	require.True(t, configured, "option was not applied")
 }
+
+func TestStdio_IsClosed(t *testing.T) {
+	t.Run("returns false before Start", func(t *testing.T) {
+		stdio := NewStdio("sh", nil, "-c", "sleep 1")
+		require.False(t, stdio.IsClosed(), "expected IsClosed to be false before Start")
+	})
+
+	t.Run("returns false after Start", func(t *testing.T) {
+		stdio := NewStdio("sh", nil, "-c", "sleep 1")
+		err := stdio.Start(context.Background())
+		require.NoError(t, err)
+		defer stdio.Close()
+		require.False(t, stdio.IsClosed(), "expected IsClosed to be false right after Start")
+	})
+
+	t.Run("returns true after subprocess exits", func(t *testing.T) {
+		stdio := NewStdio("sh", nil, "-c", "exit 0")
+		err := stdio.Start(context.Background())
+		require.NoError(t, err)
+
+		require.Eventually(t, func() bool {
+			return stdio.IsClosed()
+		}, 200*time.Millisecond, 10*time.Millisecond, "expected IsClosed to return true after subprocess exits")
+	})
+}