vmalert: fix API to return non-nil values (#4222)

Properly return empty slices instead of nil for `/api/v1/rules` and `/api/v1/alerts` API handlers.
This improves compatibility with Grafana.

 https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4221

Signed-off-by: hagen1778 <roman@victoriametrics.com>

app/vmalert/web.go

@@ -211,7 +211,7 @@ func (rh *requestHandler) groups() []APIGroup {
 	rh.m.groupsMu.RLock()
 	defer rh.m.groupsMu.RUnlock()
 
-	var groups []APIGroup
+	groups := make([]APIGroup, 0)
 	for _, g := range rh.m.groups {
 		groups = append(groups, g.toAPI())
 	}
@@ -276,6 +276,7 @@ func (rh *requestHandler) listAlerts() ([]byte, error) {
 	defer rh.m.groupsMu.RUnlock()
 
 	lr := listAlertsResponse{Status: "success"}
+	lr.Data.Alerts = make([]*APIAlert, 0)
 	for _, g := range rh.m.groups {
 		for _, r := range g.Rules {
 			a, ok := r.(*AlertingRule)

app/vmalert/web_test.go

@@ -144,5 +144,59 @@ func TestHandler(t *testing.T) {
 	t.Run("/api/v1/1/0/status", func(t *testing.T) {
 		getResp(ts.URL+"/api/v1/1/0/status", nil, 404)
 	})
-
+}
+
+func TestEmptyResponse(t *testing.T) {
+	rh := &requestHandler{m: &manager{groups: make(map[uint64]*Group)}}
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { rh.handler(w, r) }))
+	defer ts.Close()
+
+	getResp := func(url string, to interface{}, code int) {
+		t.Helper()
+		resp, err := http.Get(url)
+		if err != nil {
+			t.Fatalf("unexpected err %s", err)
+		}
+		if code != resp.StatusCode {
+			t.Errorf("unexpected status code %d want %d", resp.StatusCode, code)
+		}
+		defer func() {
+			if err := resp.Body.Close(); err != nil {
+				t.Errorf("err closing body %s", err)
+			}
+		}()
+		if to != nil {
+			if err = json.NewDecoder(resp.Body).Decode(to); err != nil {
+				t.Errorf("unexpected err %s", err)
+			}
+		}
+	}
+
+	t.Run("/api/v1/alerts", func(t *testing.T) {
+		lr := listAlertsResponse{}
+		getResp(ts.URL+"/api/v1/alerts", &lr, 200)
+		if lr.Data.Alerts == nil {
+			t.Errorf("expected /api/v1/alerts response to have non-nil data")
+		}
+
+		lr = listAlertsResponse{}
+		getResp(ts.URL+"/vmalert/api/v1/alerts", &lr, 200)
+		if lr.Data.Alerts == nil {
+			t.Errorf("expected /api/v1/alerts response to have non-nil data")
+		}
+	})
+
+	t.Run("/api/v1/rules", func(t *testing.T) {
+		lr := listGroupsResponse{}
+		getResp(ts.URL+"/api/v1/rules", &lr, 200)
+		if lr.Data.Groups == nil {
+			t.Errorf("expected /api/v1/rules response to have non-nil data")
+		}
+
+		lr = listGroupsResponse{}
+		getResp(ts.URL+"/vmalert/api/v1/rules", &lr, 200)
+		if lr.Data.Groups == nil {
+			t.Errorf("expected /api/v1/rules response to have non-nil data")
+		}
+	})
 }

docs/CHANGELOG.md

@@ -21,6 +21,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): fix a panic when the duration in the query contains uppercase `M` suffix. Such a suffix isn't allowed to use in durations, since it clashes with `a million` suffix, e.g. it isn't clear whether `rate(metric[5M])` means rate over 5 minutes, 5 months or 5 million seconds. See [this](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3589) and [this](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4120) issues.
 * BUGFIX: [VictoriaMetrics cluster](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html): prevent from possible panic when the number of vmstorage nodes increases when [automatic vmstorage discovery](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#automatic-vmstorage-discovery) is enabled.
 * BUGFIX: properly limit the number of [OpenTSDB HTTP](https://docs.victoriametrics.com/#sending-opentsdb-data-via-http-apiput-requests) concurrent requests specified via `-maxConcurrentInserts` command-line flag. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4204). Thanks to @zouxiang1993 for [the fix](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/4208).
+* BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert.html): properly return empty slices instead of nil for `/api/v1/rules` and `/api/v1/alerts` API handlers. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4221).
 * BUGFIX: [stream aggregation](https://docs.victoriametrics.com/stream-aggregation.html): suppress `series after dedup` error message in logs when `-remoteWrite.streamAggr.dedupInterval` command-line flag is set at [vmagent](https://docs.victoriametrics.com/vmgent.html) or when `-streamAggr.dedupInterval` command-line flag is set at [single-node VictoriaMetrics](https://docs.victoriametrics.com/).
 
 ## [v1.87.5](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.87.5)