Extract Route validation from serialization methods so it can be re-used (#47649)

* Extract validation and reject invalid policies

* Validation in dedicated file

* Tests for validation

* Extract root route validation

* Update call and drop TODO

* empty commit to kick actions

* Normalization should be idempotent

* Cleaner representation of validation errors, chain errors properly

* Make internal validate unexported

* Fix missed rename

* Genericize error message

* Improve method names

* Rebase, fix

* Update asserts

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

@@ -0,0 +1,67 @@
+package definitions
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/prometheus/common/model"
+)
+
+// Validate normalizes a possibly nested Route r, and returns errors if r is invalid.
+func (r *Route) validateChild() error {
+	r.GroupBy = nil
+	r.GroupByAll = false
+	for _, l := range r.GroupByStr {
+		if l == "..." {
+			r.GroupByAll = true
+		} else {
+			r.GroupBy = append(r.GroupBy, model.LabelName(l))
+		}
+	}
+
+	if len(r.GroupBy) > 0 && r.GroupByAll {
+		return fmt.Errorf("cannot have wildcard group_by (`...`) and other other labels at the same time")
+	}
+
+	groupBy := map[model.LabelName]struct{}{}
+
+	for _, ln := range r.GroupBy {
+		if _, ok := groupBy[ln]; ok {
+			return fmt.Errorf("duplicated label %q in group_by, %s %s", ln, r.Receiver, r.GroupBy)
+		}
+		groupBy[ln] = struct{}{}
+	}
+
+	if r.GroupInterval != nil && time.Duration(*r.GroupInterval) == time.Duration(0) {
+		return fmt.Errorf("group_interval cannot be zero")
+	}
+	if r.RepeatInterval != nil && time.Duration(*r.RepeatInterval) == time.Duration(0) {
+		return fmt.Errorf("repeat_interval cannot be zero")
+	}
+
+	// Routes are a self-referential structure.
+	if r.Routes != nil {
+		for _, child := range r.Routes {
+			err := child.validateChild()
+			if err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}
+
+// Validate normalizes a Route r, and returns errors if r is an invalid root route. Root routes must satisfy a few additional conditions.
+func (r *Route) Validate() error {
+	if len(r.Receiver) == 0 {
+		return fmt.Errorf("root route must specify a default receiver")
+	}
+	if len(r.Match) > 0 || len(r.MatchRE) > 0 {
+		return fmt.Errorf("root route must not have any matchers")
+	}
+	if len(r.MuteTimeIntervals) > 0 {
+		return fmt.Errorf("root route must not have any mute time intervals")
+	}
+	return r.validateChild()
+}