test/bufconn: add Listener.DialContext(context.Context) (#4763)

test/bufconn/bufconn.go

@@ -21,6 +21,7 @@
 package bufconn
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"net"
@@ -86,8 +87,17 @@ func (l *Listener) Addr() net.Addr { return addr{} }
 // providing it the server half of the connection, and returns the client half
 // of the connection.
 func (l *Listener) Dial() (net.Conn, error) {
+	return l.DialContext(context.Background())
+}
+
+// DialContext creates an in-memory full-duplex network connection, unblocks Accept by
+// providing it the server half of the connection, and returns the client half
+// of the connection.  If ctx is Done, returns ctx.Err()
+func (l *Listener) DialContext(ctx context.Context) (net.Conn, error) {
 	p1, p2 := newPipe(l.sz), newPipe(l.sz)
 	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
 	case <-l.done:
 		return nil, errClosed
 	case l.ch <- &conn{p1, p2}: