transport: hide Stream string when stored in context

It is not thread-safe to call context.String() on any context with a
stream value since valueCtx will use %#v to access all of the Stream
fields without holding a lock. Instead, use "<nil>" as the GoString.

transport/transport.go

@@ -341,16 +341,22 @@ func (s *Stream) Read(p []byte) (n int, err error) {
 // The key to save transport.Stream in the context.
 type streamKey struct{}
 
+// streamCtxGoStringer implements the GoStringer interface for Stream
+// so that ctx.String() is thread-safe by hiding the stream information.
+type streamCtxGoStringer Stream
+
+func (ss *streamCtxGoStringer) GoString() string { return "<nil>" }
+
 // newContextWithStream creates a new context from ctx and attaches stream
 // to it.
 func newContextWithStream(ctx context.Context, stream *Stream) context.Context {
-	return context.WithValue(ctx, streamKey{}, stream)
+	return context.WithValue(ctx, streamKey{}, (*streamCtxGoStringer)(stream))
 }
 
 // StreamFromContext returns the stream saved in ctx.
-func StreamFromContext(ctx context.Context) (s *Stream, ok bool) {
-	s, ok = ctx.Value(streamKey{}).(*Stream)
-	return
+func StreamFromContext(ctx context.Context) (*Stream, bool) {
+	s, ok := ctx.Value(streamKey{}).(*streamCtxGoStringer)
+	return (*Stream)(s), ok
 }
 
 // state of transport