adding check for name change and new tests (#18531)

Co-authored-by: Benjamin Cooke <benjamincooke@Benjamins-MacBook-Pro.local>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/channel.go

@@ -338,6 +338,13 @@ func patchChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if oldChannel.Name == model.DefaultChannelName {
+		if *patch.Name != "" && *patch.Name != oldChannel.Name {
+			c.Err = model.NewAppError("patchChannel", "api.channel.update_channel.tried.app_error", map[string]interface{}{"Channel": model.DefaultChannelName}, "", http.StatusBadRequest)
+			return
+		}
+	}
+
 	rchannel, appErr := c.App.PatchChannel(c.AppContext, oldChannel, patch, c.AppContext.Session().UserId)
 	if appErr != nil {
 		c.Err = appErr