Loki language provider: don't cache empty array while querying (#92092)

* Loki language provider: don't cache empty array while querying

* Prettier

* Unfocus test

* chore: add extra assertion

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

@@ -268,6 +268,24 @@ describe('Language completion provider', () => {
         end: expect.any(Number),
       });
     });
+
+    it('should use a single promise to resolve values', async () => {
+      const datasource = setup({ testkey: ['label1_val1', 'label1_val2'], label2: [] });
+      const provider = await getLanguageProvider(datasource);
+      const requestSpy = jest.spyOn(provider, 'request');
+      const promise1 = provider.fetchLabelValues('testkey');
+      const promise2 = provider.fetchLabelValues('testkey');
+      const promise3 = provider.fetchLabelValues('testkeyNOPE');
+      expect(requestSpy).toHaveBeenCalledTimes(2);
+
+      const values1 = await promise1;
+      const values2 = await promise2;
+      const values3 = await promise3;
+
+      expect(values1).toStrictEqual(values2);
+      expect(values2).not.toStrictEqual(values3);
+      expect(requestSpy).toHaveBeenCalledTimes(2);
+    });
   });
 
   describe('fetchLabels', () => {

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

@@ -31,6 +31,7 @@ export default class LokiLanguageProvider extends LanguageProvider {
    */
   private seriesCache = new LRUCache<string, Record<string, string[]>>({ max: 10 });
   private labelsCache = new LRUCache<string, string[]>({ max: 10 });
+  private labelsPromisesCache = new LRUCache<string, Promise<string[]>>({ max: 10 });
 
   constructor(datasource: LokiDatasource, initialValues?: any) {
     super();
@@ -272,18 +273,34 @@ export default class LokiLanguageProvider extends LanguageProvider {
 
     const cacheKey = this.generateCacheKey(url, start, end, paramCacheKey);
 
-    let labelValues = this.labelsCache.get(cacheKey);
-    if (!labelValues) {
-      // Clear value when requesting new one. Empty object being truthy also makes sure we don't request twice.
-      this.labelsCache.set(cacheKey, []);
-      const res = await this.request(url, params);
-      if (Array.isArray(res)) {
-        labelValues = res.slice().sort();
-        this.labelsCache.set(cacheKey, labelValues);
-      }
+    // Values in cache, return
+    const labelValues = this.labelsCache.get(cacheKey);
+    if (labelValues) {
+      return labelValues;
     }
 
-    return labelValues ?? [];
+    // Promise in cache, return
+    let labelValuesPromise = this.labelsPromisesCache.get(cacheKey);
+    if (labelValuesPromise) {
+      return labelValuesPromise;
+    }
+
+    labelValuesPromise = new Promise(async (resolve) => {
+      try {
+        const data = await this.request(url, params);
+        if (Array.isArray(data)) {
+          const labelValues = data.slice().sort();
+          this.labelsCache.set(cacheKey, labelValues);
+          this.labelsPromisesCache.delete(cacheKey);
+          resolve(labelValues);
+        }
+      } catch (error) {
+        console.error(error);
+        resolve([]);
+      }
+    });
+    this.labelsPromisesCache.set(cacheKey, labelValuesPromise);
+    return labelValuesPromise;
   }
 
   /**