Have SnowflakeClientConn implement io.WriterTo.

By forwarding the method to the inner smux.Stream. This is to prevent
io.Copy in the top-level proxy function from allocating a buffer per
client.

The smux.Stream WriteTo method returns io.EOF on success, contrary to
the contract of io.Copy that says it should return nil. Ignore io.EOF in
the proxy loop to avoid a log message.

/anti-censorship/pluggable-transports/snowflake/-/issues/40177

server/server.go

@@ -57,7 +57,7 @@ func proxy(local *net.TCPConn, conn net.Conn) {
 		wg.Done()
 	}()
 	go func() {
-		if _, err := io.Copy(local, conn); err != nil && !errors.Is(err, io.ErrClosedPipe) {
+		if _, err := io.Copy(local, conn); err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, io.ErrClosedPipe) {
 			log.Printf("error copying WebSocket to ORPort %v", err)
 		}
 		local.CloseWrite()