Don't log "expected" websocket errors, like the socket being closed

app/api_topology.go

@@ -90,7 +90,9 @@ func handleWebsocket(
 	go func(c *websocket.Conn) {
 		for { // just discard everything the browser sends
 			if _, _, err := c.NextReader(); err != nil {
-				log.Println("err:", err)
+				if !xfer.IsExpectedWSCloseError(err) {
+					log.Println("err:", err)
+				}
 				close(quit)
 				break
 			}
@@ -111,7 +113,9 @@ func handleWebsocket(
 		previousTopo = newTopo
 
 		if err := conn.SetWriteDeadline(time.Now().Add(websocketTimeout)); err != nil {
-			log.Println("err:", err)
+			if !xfer.IsExpectedWSCloseError(err) {
+				log.Println("err:", err)
+			}
 			return
 		}
 

common/xfer/websocket.go

@@ -7,6 +7,17 @@ import (
 	"github.com/ugorji/go/codec"
 )
 
+// IsExpectedWSCloseError returns boolean indicating whether the error is a
+// clean disconnection.
+func IsExpectedWSCloseError(err error) bool {
+	return websocket.IsCloseError(
+		err,
+		websocket.CloseNormalClosure,
+		websocket.CloseGoingAway,
+		websocket.CloseNoStatusReceived,
+	)
+}
+
 // WriteJSONtoWS writes the JSON encoding of v to the connection.
 func WriteJSONtoWS(c *websocket.Conn, v interface{}) error {
 	w, err := c.NextWriter(websocket.TextMessage)