rules: fix silly mistakes in the rules API

Fixes thanos-io#2938 and is a follow up
to thanos-io#2925.

Changes:
* Copying all fields properly to the group-level as well
* Set TZ to UTC explicitly in other `time.Time` fields to avoid a panic
* Make sure that `RuleGroup` always has an empty array in the `rules`
field because that's customary and that is what the new React UI
expects.

Manual testing for now.

Signed-off-by: Giedrius Statkevičius <giedriuswork@gmail.com>

pkg/rules/manager.go

@@ -39,10 +39,14 @@ type Group struct {
 
 func (g Group) toProto() *rulespb.RuleGroup {
 	ret := &rulespb.RuleGroup{
-		Name:                    g.Name(),
-		File:                    g.OriginalFile,
-		Interval:                g.Interval().Seconds(),
-		PartialResponseStrategy: g.PartialResponseStrategy,
+		Name:                              g.Name(),
+		File:                              g.OriginalFile,
+		Interval:                          g.Interval().Seconds(),
+		PartialResponseStrategy:           g.PartialResponseStrategy,
+		DeprecatedPartialResponseStrategy: g.PartialResponseStrategy,
+		// https://github.com/gogo/protobuf/issues/519
+		LastEvaluation:            g.GetEvaluationTimestamp().UTC(),
+		EvaluationDurationSeconds: g.GetEvaluationDuration().Seconds(),
 	}
 
 	for _, r := range g.Rules() {
@@ -65,7 +69,8 @@ func (g Group) toProto() *rulespb.RuleGroup {
 					Health:                    string(rule.Health()),
 					LastError:                 lastError,
 					EvaluationDurationSeconds: rule.GetEvaluationDuration().Seconds(),
-					LastEvaluation:            rule.GetEvaluationTimestamp(),
+					// https://github.com/gogo/protobuf/issues/519
+					LastEvaluation: rule.GetEvaluationTimestamp().UTC(),
 				}}})
 		case *rules.RecordingRule:
 			ret.Rules = append(ret.Rules, &rulespb.Rule{
@@ -76,7 +81,8 @@ func (g Group) toProto() *rulespb.RuleGroup {
 					Health:                    string(rule.Health()),
 					LastError:                 lastError,
 					EvaluationDurationSeconds: rule.GetEvaluationDuration().Seconds(),
-					LastEvaluation:            rule.GetEvaluationTimestamp(),
+					// https://github.com/gogo/protobuf/issues/519
+					LastEvaluation: rule.GetEvaluationTimestamp().UTC(),
 				}}})
 		default:
 			// We cannot do much, let's panic, API will recover.

pkg/rules/rulespb/custom.go

@@ -241,6 +241,15 @@ func (m *Rule) MarshalJSON() ([]byte, error) {
 	})
 }
 
+func (r *RuleGroup) MarshalJSON() ([]byte, error) {
+	if r.Rules == nil {
+		// Ensure that empty slices are marshaled as '[]' and not 'null'.
+		r.Rules = make([]*Rule, 0)
+	}
+	type plain RuleGroup
+	return json.Marshal((*plain)(r))
+}
+
 func (x *AlertState) UnmarshalJSON(entry []byte) error {
 	fieldStr, err := strconv.Unquote(string(entry))
 	if err != nil {