MM-8607 Add ability to turn off non-critical services when under load (#13212)

* MM-8607: add ability to turn off non-critical services under load

* server busy invalid param unit tests

* MM-8607: rename server busy endpoints

* MM-8607: handle case where App not initialized

* MM-8607: additional unit test cases per feedback.

* MM-8607: use decorator to check isbusy when adding endpoint route

* MM-8607: rename endpoints, use struct for json

* Update api4/system.go

Fix misspelled log output

Co-Authored-By: Saturnino Abril <saturnino.abril@gmail.com>

* MM-8607: fix i18n order; max seconds for server busy expiry

web/context.go

@@ -165,6 +165,10 @@ func (c *Context) SetInvalidUrlParam(parameter string) {
 	c.Err = NewInvalidUrlParamError(parameter)
 }
 
+func (c *Context) SetServerBusyError() {
+	c.Err = NewServerBusyError()
+}
+
 func (c *Context) HandleEtag(etag string, routeName string, w http.ResponseWriter, r *http.Request) bool {
 	metrics := c.App.Metrics
 	if et := r.Header.Get(model.HEADER_ETAG_CLIENT); len(etag) > 0 {
@@ -193,6 +197,10 @@ func NewInvalidUrlParamError(parameter string) *model.AppError {
 	err := model.NewAppError("Context", "api.context.invalid_url_param.app_error", map[string]interface{}{"Name": parameter}, "", http.StatusBadRequest)
 	return err
 }
+func NewServerBusyError() *model.AppError {
+	err := model.NewAppError("Context", "api.context.server_busy.app_error", nil, "", http.StatusServiceUnavailable)
+	return err
+}
 
 func (c *Context) SetPermissionError(permission *model.Permission) {
 	c.Err = c.App.MakePermissionError(permission)

web/handlers.go

@@ -66,6 +66,7 @@ type Handler struct {
 	TrustRequester      bool
 	RequireMfa          bool
 	IsStatic            bool
+	DisableWhenBusy     bool
 
 	cspShaDirective string
 }
@@ -159,6 +160,10 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		c.MfaRequired()
 	}
 
+	if c.Err == nil && h.DisableWhenBusy && c.App.Srv.Busy.IsBusy() {
+		c.SetServerBusyError()
+	}
+
 	if c.Err == nil {
 		h.HandleFunc(c, w, r)
 	}