ethstats: avoid concurrent write on websocket, fixes #21403

[holiman]

See #21403 for more details. This PR adds a mutex around the writes

[holiman]

From https://godoc.org/github.com/gorilla/websocket#hdr-Concurrency :

Connections support one concurrent reader and one concurrent writer.

Applications are responsible for ensuring that no more than one goroutine calls the write methods (NextWriter, SetWriteDeadline, WriteMessage, WriteJSON, EnableWriteCompression, SetCompressionLevel) concurrently and that no more than one goroutine calls the read methods (NextReader, SetReadDeadline, ReadMessage, ReadJSON, SetPongHandler, SetPingHandler) concurrently.

The Close and WriteControl methods can be called concurrently with all other methods.

Looks like we need one mutex (one for writes, one for reads)

[holiman]

Fixed by using the same mutex for both reads and writes. The particular optimization to allow concurrent reading/writing is not important for this code.

[fjl]

The locking looks a bit scary. I'd be much happier if we just made a wrapper struct for the connection that exposes ReadJSON, WriteJSON and Close methods. You could then do the locking in ReadJSON and WriteJSON.

[holiman]

The locking looks a bit scary. I'd be much happier if we just made a wrapper struct for the connection that exposes ReadJSON, WriteJSON and Close methods. You could then do the locking in ReadJSON and WriteJSON.

Yup, good call, will fix

[holiman]

Fixed, PTAL