Alerting: Modify configuration apply and save semantics - v2 (#34143)

* Save default configuration to the database and copy over secure settings

pkg/services/ngalert/api/tooling/definitions/alertmanager.go

@@ -447,6 +447,21 @@ func (c *PostableApiAlertingConfig) validate() error {
 		return fmt.Errorf("cannot mix Alertmanager & Grafana receiver types")
 	}
 
+	if hasGrafReceivers {
+		// Taken from https://github.com/prometheus/alertmanager/blob/master/config/config.go#L170-L191
+		// Check if we have a root route. We cannot check for it in the
+		// UnmarshalYAML method because it won't be called if the input is empty
+		// (e.g. the config file is empty or only contains whitespace).
+		if c.Route == nil {
+			return fmt.Errorf("no route provided in config")
+		}
+
+		// Check if continue in root route.
+		if c.Route.Continue {
+			return fmt.Errorf("cannot have continue in root route")
+		}
+	}
+
 	for _, receiver := range AllReceivers(c.Route) {
 		_, ok := receivers[receiver]
 		if !ok {
@@ -475,9 +490,14 @@ func (c *PostableApiAlertingConfig) ReceiverType() ReceiverType {
 // AllReceivers will recursively walk a routing tree and return a list of all the
 // referenced receiver names.
 func AllReceivers(route *config.Route) (res []string) {
+	if route == nil {
+		return res
+	}
+
 	if route.Receiver != "" {
 		res = append(res, route.Receiver)
 	}
+
 	for _, subRoute := range route.Routes {
 		res = append(res, AllReceivers(subRoute)...)
 	}