[bugfix] Fix occasionally streaming empty messages (#3456)

internal/api/client/streaming/stream.go

@@ -35,6 +35,8 @@ import (
 	"github.com/gorilla/websocket"
 )
 
+var pingMsg = []byte("ping!")
+
 // StreamGETHandler swagger:operation GET /api/v1/streaming streamGet
 //
 // Initiate a websocket connection for live streaming of statuses and notifications.
@@ -389,40 +391,54 @@ func (m *Module) writeToWSConn(
 ) {
 	for {
 		// Wrap context with timeout to send a ping.
-		pingctx, cncl := context.WithTimeout(ctx, ping)
+		pingCtx, cncl := context.WithTimeout(ctx, ping)
 
-		// Block on receipt of msg.
+		// Block and wait for
-		msg, ok := stream.Recv(pingctx)
+		// one of the following:
+		//
+		// - receipt of msg
+		// - timeout of pingCtx
+		// - stream closed.
+		msg, gotMsg := stream.Recv(pingCtx)
 
-		// Check if cancel because ping.
+		// If ping context has timed
-		pinged := (pingctx.Err() != nil)
+		// out, we should send a ping.
+		//
+		// In any case cancel pingCtx
+		// as we're done with it.
+		shouldPing := (pingCtx.Err() != nil)
 		cncl()
 
 		switch {
-		case !ok && pinged:
-			// The ping context timed out!
-			l.Trace("writing websocket ping")
 
-			// Wrapped context time-out, send a keep-alive "ping".
+		// We have a message to stream.
-			if err := wsConn.WriteControl(websocket.PingMessage, nil, time.Time{}); err != nil {
+		case gotMsg:
+			l.Tracef("writing websocket message: %+v", msg)
+			if err := wsConn.WriteJSON(msg); err != nil {
+				// If there's an error writing then drop the
+				// connection, as client may have disappeared
+				// suddenly; they can reconnect if necessary.
+				l.Debugf("error writing websocket message: %v", err)
+				break
+			}
+
+		// We have no message but we
+		// need to send a keep-alive ping.
+		case shouldPing:
+			l.Trace("writing websocket ping")
+			if err := wsConn.WriteControl(websocket.PingMessage, pingMsg, time.Time{}); err != nil {
+				// If there's an error writing then drop the
+				// connection, as client may have disappeared
+				// suddenly; they can reconnect if necessary.
 				l.Debugf("error writing websocket ping: %v", err)
 				break
 			}
 
-		case !ok:
+		// We have no message and we shouldn't
-			// Stream was
+		// send a ping; this means the stream
-			// closed.
+		// has been closed from the client's end.
+		default:
 			return
 		}
-
-		l.Trace("writing websocket message: %+v", msg)
-
-		// Received a new message from the processor.
-		if err := wsConn.WriteJSON(msg); err != nil {
-			l.Debugf("error writing websocket message: %v", err)
-			break
-		}
 	}
-
-	l.Debug("finished websocket write")
 }