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_statuses.go

@@ -11,11 +11,12 @@ import (
 
 // GetPluginStatus returns the status for a plugin installed on this server.
 func (a *App) GetPluginStatus(id string) (*model.PluginStatus, *model.AppError) {
-	if a.Srv.Plugins == nil || !*a.Config().PluginSettings.Enable {
+	pluginsEnvironment := a.GetPluginsEnvironment()
+	if pluginsEnvironment == nil {
 		return nil, model.NewAppError("GetPluginStatus", "app.plugin.disabled.app_error", nil, "", http.StatusNotImplemented)
 	}
 
-	pluginStatuses, err := a.Srv.Plugins.Statuses()
+	pluginStatuses, err := pluginsEnvironment.Statuses()
 	if err != nil {
 		return nil, model.NewAppError("GetPluginStatus", "app.plugin.get_statuses.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}
@@ -32,11 +33,12 @@ func (a *App) GetPluginStatus(id string) (*model.PluginStatus, *model.AppError)
 
 // GetPluginStatuses returns the status for plugins installed on this server.
 func (a *App) GetPluginStatuses() (model.PluginStatuses, *model.AppError) {
-	if a.Srv.Plugins == nil || !*a.Config().PluginSettings.Enable {
+	pluginsEnvironment := a.GetPluginsEnvironment()
+	if pluginsEnvironment == nil {
 		return nil, model.NewAppError("GetPluginStatuses", "app.plugin.disabled.app_error", nil, "", http.StatusNotImplemented)
 	}
 
-	pluginStatuses, err := a.Srv.Plugins.Statuses()
+	pluginStatuses, err := pluginsEnvironment.Statuses()
 	if err != nil {
 		return nil, model.NewAppError("GetPluginStatuses", "app.plugin.get_statuses.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}