Loki: Fix error when empty template variables response (#69373)

* Loki: Fix error when empty template variables

* Update

* Add test

* Add test for statsMetadataRequest

public/app/plugins/datasource/loki/datasource.test.ts

@@ -32,7 +32,7 @@ import { LokiDatasource, REF_ID_DATA_SAMPLES } from './datasource';
 import { createLokiDatasource, createMetadataRequest } from './mocks';
 import { runSplitQuery } from './querySplitting';
 import { parseToNodeNamesArray } from './queryUtils';
-import { LokiOptions, LokiQuery, LokiQueryType, LokiVariableQueryType, SupportingQueryType } from './types';
+import { LokiOptions, LokiQuery, LokiQueryType, LokiVariableQueryType, QueryStats, SupportingQueryType } from './types';
 import { LokiVariableSupport } from './variables';
 
 jest.mock('@grafana/runtime', () => {
@@ -1139,6 +1139,24 @@ describe('LokiDatasource', () => {
       });
     });
   });
+
+  describe('getQueryStats', () => {
+    it('uses statsMetadataRequest', async () => {
+      const ds = createLokiDatasource(templateSrvStub);
+      const spy = jest.spyOn(ds, 'statsMetadataRequest').mockResolvedValue({} as QueryStats);
+      ds.getQueryStats('{foo="bar"}');
+      expect(spy).toHaveBeenCalled();
+    });
+  });
+
+  describe('statsMetadataRequest', () => {
+    it('throws error if url starts with /', () => {
+      const ds = createLokiDatasource();
+      expect(async () => {
+        await ds.statsMetadataRequest('/index');
+      }).rejects.toThrow('invalid metadata request url: /index');
+    });
+  });
 });
 
 describe('applyTemplateVariables', () => {

public/app/plugins/datasource/loki/datasource.ts

@@ -422,7 +422,21 @@ export class LokiDatasource
     }
 
     const res = await this.getResource(url, params, options);
-    return res.data ?? (res || []);
+    return res.data || [];
+  }
+
+  // We need a specific metadata method for stats endpoint as it does not return res.data,
+  // but it returns stats directly in res object.
+  async statsMetadataRequest(
+    url: string,
+    params?: Record<string, string | number>,
+    options?: Partial<BackendSrvRequest>
+  ): Promise<QueryStats> {
+    if (url.startsWith('/')) {
+      throw new Error(`invalid metadata request url: ${url}`);
+    }
+
+    return await this.getResource(url, params, options);
   }
 
   async getQueryStats(query: string): Promise<QueryStats | undefined> {
@@ -438,7 +452,7 @@ export class LokiDatasource
 
     for (const labelMatcher of labelMatchers) {
       try {
-        const data = await this.metadataRequest(
+        const data = await this.statsMetadataRequest(
           'index/stats',
           { query: labelMatcher, start, end },
           { showErrorAlert: false }