jsonrpc2: initialize Connection.async before calling binder.Bind

If Connection.async is not initialized before calling binder.Bind()
and a goroutine is spawned in binder.Bind() to monitor the end of the
connection using conn.Wait(), this conn.Wait() can wait forever:

    func (s *MyServer) Bind(
        ctx context.Context,
        conn *jsonrpc2.Connection,
    ) (jsonrpc2.ConnectionOptions, error) {
        // XXX register conn in *MyServer

        // monitor the end of the connection
        go func() {
            conn.Wait()
            conn.Close()
            // YYY do some cleanup in *MyServer
        }()
        return jsonrpc2.ConnectionOptions{}, nil
    }

The "// YYY do some cleanup in *MyServer" should be reached for each
end of connection.

If Connection.async is initialized too late, after conn.Wait() is
called in the Bind() spawned goroutine, conn.Wait() never returns as
it reads on a nil conn.async.ready channel.

Fixes #56131.

Change-Id: I3c6e69d7dcc860479c4518ebd18e2431606322bc
GitHub-Last-Rev: a7fce45
GitHub-Pull-Request: #45
Reviewed-on: https://go-review.googlesource.com/c/exp/+/436888
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Joedian Reid <[email protected]>

Author: maxatome

jsonrpc2/conn.go

@@ -85,6 +85,7 @@ func newConnection(ctx context.Context, rwc io.ReadWriteCloser, binder Binder) (
 		outgoingBox: make(chan map[ID]chan<- *Response, 1),
 		incomingBox: make(chan map[ID]*incoming, 1),
 	}
+	c.async.init()
 
 	options, err := binder.Bind(ctx, c)
 	if err != nil {
@@ -101,7 +102,6 @@ func newConnection(ctx context.Context, rwc io.ReadWriteCloser, binder Binder) (
 	}
 	c.outgoingBox <- make(map[ID]chan<- *Response)
 	c.incomingBox <- make(map[ID]*incoming)
-	c.async.init()
 	// the goroutines started here will continue until the underlying stream is closed
 	reader := options.Framer.Reader(rwc)
 	readToQueue := make(chan *incoming)