Add routing validation

The root route in the configuration must not exclude any alerts,
i.e. not have any matchers.
All receivers referenced in the routing tree are validated to exist.

config/config.go

@@ -207,9 +207,38 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 		}
 		names[rcv.Name] = struct{}{}
 	}
+
+	// The root route must not have any matchers as it is the fallback node
+	// for all alerts.
+    if c.Route == nil {
+        return fmt.Errorf("No routes provided")
+    }
+	if len(c.Route.Match) > 0 || len(c.Route.MatchRE) > 0 {
+		return fmt.Errorf("Root route must not have any matchers")
+	}
+
+	// Validate that all receivers used in the routing tree are defined.
+	if err := checkReceiver(c.Route, names); err != nil {
+		return err
+	}
+
 	return checkOverflow(c.XXX, "config")
 }
 
+// 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 {
+	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
+}
+
 // DefaultGlobalConfig provides global default values.
 var DefaultGlobalConfig = GlobalConfig{
 	ResolveTimeout: model.Duration(5 * time.Minute),