MM-12974: guard app plugins with mutex (#9818)

* guard app plugins with mutex

Shutting down the app could race with a goroutine that uses the plugins environment, since we shut down the plugins first before cleaning up goroutines.

* fix go vet issues

app/plugin_commands.go

@@ -101,12 +101,14 @@ func (a *App) ExecutePluginCommand(args *model.CommandArgs) (*model.Command, *mo
 
 	for _, pc := range a.Srv.pluginCommands {
 		if (pc.Command.TeamId == "" || pc.Command.TeamId == args.TeamId) && pc.Command.Trigger == trigger {
-			pluginHooks, err := a.Srv.Plugins.HooksForPlugin(pc.PluginId)
-			if err != nil {
-				return pc.Command, nil, model.NewAppError("ExecutePluginCommand", "model.plugin_command.error.app_error", nil, "err="+err.Error(), http.StatusInternalServerError)
+			if pluginsEnvironment := a.GetPluginsEnvironment(); pluginsEnvironment != nil {
+				pluginHooks, err := pluginsEnvironment.HooksForPlugin(pc.PluginId)
+				if err != nil {
+					return pc.Command, nil, model.NewAppError("ExecutePluginCommand", "model.plugin_command.error.app_error", nil, "err="+err.Error(), http.StatusInternalServerError)
+				}
+				response, appErr := pluginHooks.ExecuteCommand(&plugin.Context{}, args)
+				return pc.Command, response, appErr
 			}
-			response, appErr := pluginHooks.ExecuteCommand(&plugin.Context{}, args)
-			return pc.Command, response, appErr
 		}
 	}
 	return nil, nil, nil