MM-13185: Fix Message Export config special case. (#9898)

It should only apply when setting through the System Console, to avoid
messing up externally managed config files.

api4/system.go

@@ -130,6 +130,20 @@ func updateConfig(c *Context, w http.ResponseWriter, r *http.Request) {
 	// Do not allow plugin uploads to be toggled through the API
 	cfg.PluginSettings.EnableUploads = c.App.GetConfig().PluginSettings.EnableUploads
 
+	// If the Message Export feature has been toggled in the System Console, rewrite the ExportFromTimestamp field to an
+	// appropriate value. The rewriting occurs here to ensure it doesn't affect values written to the config file
+	// directly and not through the System Console UI.
+	if *cfg.MessageExportSettings.EnableExport != *c.App.GetConfig().MessageExportSettings.EnableExport {
+		if *cfg.MessageExportSettings.EnableExport && *cfg.MessageExportSettings.ExportFromTimestamp == int64(0) {
+			// When the feature is toggled on, use the current timestamp as the start time for future exports.
+			cfg.MessageExportSettings.ExportFromTimestamp = model.NewInt64(model.GetMillis())
+		} else if !*cfg.MessageExportSettings.EnableExport {
+			// When the feature is disabled, reset the timestamp so that the timestamp will be set if
+			// the feature is re-enabled from the System Console in future.
+			cfg.MessageExportSettings.ExportFromTimestamp = model.NewInt64(0)
+		}
+	}
+
 	err := c.App.SaveConfig(cfg, true)
 	if err != nil {
 		c.Err = err