Fix receiver name checking in deep sub-routes

Fixes https://github.com/prometheus/alertmanager/issues/1759

Signed-off-by: Julius Volz <julius.volz@gmail.com>

config/config.go

@@ -435,17 +435,17 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 // checkReceiver returns an error if a node in the routing tree
 // references a receiver not in the given map.
 func checkReceiver(r *Route, receivers map[string]struct{}) error {
+	for _, sr := range r.Routes {
+		if err := checkReceiver(sr, receivers); err != nil {
+			return err
+		}
+	}
 	if r.Receiver == "" {
 		return nil
 	}
 	if _, ok := receivers[r.Receiver]; !ok {
 		return fmt.Errorf("undefined receiver %q used in route", r.Receiver)
 	}
-	for _, sr := range r.Routes {
-		if err := checkReceiver(sr, receivers); err != nil {
-			return err
-		}
-	}
 	return nil
 }
 

config/config_test.go

@@ -103,6 +103,34 @@ receivers:
 
 }
 
+func TestReceiverExistsForDeepSubRoute(t *testing.T) {
+	in := `
+route:
+    receiver: team-X
+    routes:
+      - match:
+          foo: bar
+        routes:
+        - match:
+            foo: bar
+          receiver: nonexistent
+
+receivers:
+- name: 'team-X'
+`
+	_, err := Load(in)
+
+	expected := "undefined receiver \"nonexistent\" used in route"
+
+	if err == nil {
+		t.Fatalf("no error returned, expected:\n%q", expected)
+	}
+	if err.Error() != expected {
+		t.Errorf("\nexpected:\n%q\ngot:\n%q", expected, err.Error())
+	}
+
+}
+
 func TestReceiverHasName(t *testing.T) {
 	in := `
 route: