A server connection using stdio transport cannot be closed gracefully

[chriscasola]

Describe the bug
When calling Connect on a server using mcp.NewStdioTransport and then subsequently calling Close() on the server session, the Close() call never returns.

To Reproduce

Run this mainprog:

func main() {
	server := mcp.NewServer("test-server", "1.0.0", nil)

	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
	defer stop()

	serverSession, err := server.Connect(ctx, mcp.NewStdioTransport())
	if err != nil {
		panic(err)
	}
	fmt.Println("started")

	<-ctx.Done()
	fmt.Println("stopping")
	if err := serverSession.Close(); err != nil {
		panic(err)
	}

	fmt.Println("stopped")
}

After observing the "started" message, stop the program with CTRL-C.

The "stopping" message will be logged, but "stopped" is not logged and the program does not exit.

Expected behavior
The program should exit.

Logs
NA

Additional context
I think this is happening because jsonrpc2.Connection expects the reader.Read to return here after Close() is called but it does not. See this comment.

I think this is because the reader is a json.Decoder which will not return when the underlying reader is closed.

If this explanation makes sense, I can work on a fix, just let me know.

[findleyr]

Thank you, @chriscasola for both the report and diagnosis. Yes, a fix would be appreciated. If you'd prefer, I can fix it.

There is a test in mcp/cmd_test.go that tests closing of the client, but not the server. In order to write a test based on your repro, you can probably modify the runServer function to close the server session on interrupt.

[chriscasola]

@findleyr I realized as I was "fixing" this that my reproducer was just wrong. I was signaling with ctrl-c which is syscall.SIGTERM not os.Interrupt.

I added some tests anyway and modified server.Run to also close the server connection when it's context is cancelled.

#111