Fix potential nil pointer dereference

server/events/sse.go

@@ -95,16 +95,22 @@ func (b *broker) prepareMessage(ctx context.Context, event Event) message {
 
 var errWriteTimeOut = errors.New("write timeout")
 
-// writeEvent Write to the ResponseWriter, Server Sent Events compatible
+// writeEvent Write to the ResponseWriter, Server Sent Events compatible, and sends it
+// right away, by flushing the writer (if it is a Flusher). It waits for the message to be flushed
+// or times out after the specified timeout
 func writeEvent(w io.Writer, event message, timeout time.Duration) (err error) {
-	flusher, _ := w.(http.Flusher)
 	complete := make(chan struct{}, 1)
-	go func() {
-		_, err = fmt.Fprintf(w, "id: %d\nevent: %s\ndata: %s\n\n", event.id, event.event, event.data)
-		// Flush the data immediately instead of buffering it for later.
-		flusher.Flush()
+	flusher, ok := w.(http.Flusher)
+	if ok {
+		go func() {
+			_, err = fmt.Fprintf(w, "id: %d\nevent: %s\ndata: %s\n\n", event.id, event.event, event.data)
+			// Flush the data immediately instead of buffering it for later.
+			flusher.Flush()
+			complete <- struct{}{}
+		}()
+	} else {
 		complete <- struct{}{}
-	}()
+	}
 	select {
 	case <-complete:
 		return