[Discover] Emit an error when Lens fails so that Discover doesn't dis…

…play 'no hits'

src/plugins/unified_histogram/kibana.json

@@ -11,5 +11,5 @@
   "ui": true,
   "requiredPlugins": [],
   "optionalPlugins": [],
-  "requiredBundles": ["data", "dataViews", "embeddable", "kibanaUtils"]
+  "requiredBundles": ["data", "dataViews", "embeddable", "kibanaUtils", "inspector"]
 }

src/plugins/unified_histogram/public/chart/histogram.test.tsx

@@ -17,6 +17,7 @@ import { REQUEST_DEBOUNCE_MS } from './consts';
 import { act } from 'react-dom/test-utils';
 import * as buildBucketInterval from './build_bucket_interval';
 import * as useTimeRange from './use_time_range';
+import { RequestStatus } from '@kbn/inspector-plugin/public';
 
 const mockBucketInterval = { description: '1 minute', scale: undefined, scaled: false };
 jest.spyOn(buildBucketInterval, 'buildBucketInterval').mockReturnValue(mockBucketInterval);
@@ -183,6 +184,47 @@ describe('Histogram', () => {
     );
   });
 
+  it('should execute onLoad correctly when the request has a failure status', async () => {
+    const { component, props } = mountComponent();
+    const embeddable = unifiedHistogramServicesMock.lens.EmbeddableComponent;
+    const onLoad = component.find(embeddable).props().onLoad;
+    const adapters = createDefaultInspectorAdapters();
+    jest
+      .spyOn(adapters.requests, 'getRequests')
+      .mockReturnValue([{ status: RequestStatus.ERROR } as any]);
+    onLoad(false, adapters);
+    expect(props.onTotalHitsChange).toHaveBeenLastCalledWith(
+      UnifiedHistogramFetchStatus.error,
+      undefined
+    );
+    expect(props.onChartLoad).toHaveBeenLastCalledWith({ complete: false, adapters });
+  });
+
+  it('should execute onLoad correctly when the response has shard failures', async () => {
+    const { component, props } = mountComponent();
+    const embeddable = unifiedHistogramServicesMock.lens.EmbeddableComponent;
+    const onLoad = component.find(embeddable).props().onLoad;
+    const adapters = createDefaultInspectorAdapters();
+    const rawResponse = {
+      _shards: {
+        total: 1,
+        successful: 0,
+        skipped: 0,
+        failed: 1,
+        failures: [],
+      },
+    };
+    jest
+      .spyOn(adapters.requests, 'getRequests')
+      .mockReturnValue([{ response: { json: { rawResponse } } } as any]);
+    onLoad(false, adapters);
+    expect(props.onTotalHitsChange).toHaveBeenLastCalledWith(
+      UnifiedHistogramFetchStatus.error,
+      undefined
+    );
+    expect(props.onChartLoad).toHaveBeenLastCalledWith({ complete: false, adapters });
+  });
+
   it('should not recreate onLoad in debounced lens props when hits.total changes', async () => {
     const { component, props } = mountComponent();
     const embeddable = unifiedHistogramServicesMock.lens.EmbeddableComponent;

src/plugins/unified_histogram/public/chart/histogram.tsx

@@ -17,6 +17,7 @@ import type { TimeRange } from '@kbn/es-query';
 import useDebounce from 'react-use/lib/useDebounce';
 import { ViewMode } from '@kbn/embeddable-plugin/public';
 import type { TypedLensByValueInput } from '@kbn/lens-plugin/public';
+import { RequestStatus } from '@kbn/inspector-plugin/public';
 import {
   UnifiedHistogramBucketInterval,
   UnifiedHistogramChartContext,
@@ -73,17 +74,29 @@ export function Histogram({
 
   const onLoad = useCallback(
     (isLoading: boolean, adapters: Partial<DefaultInspectorAdapters> | undefined) => {
+      const lensRequest = adapters?.requests?.getRequests()[0];
+      const requestFailed = lensRequest?.status === RequestStatus.ERROR;
+      const json = lensRequest?.response?.json as
+        | IKibanaSearchResponse<estypes.SearchResponse>
+        | undefined;
+      const response = json?.rawResponse;
+
+      // Lens will swallow shard failures and return `isLoading: false` because it displays
+      // its own errors, but this causes us to emit onTotalHitsChange(UnifiedHistogramFetchStatus.complete, 0).
+      // This is incorrect, so we check for request failures and shard failures here, and emit an error instead.
+      if (requestFailed || response?._shards.failed) {
+        onTotalHitsChange?.(UnifiedHistogramFetchStatus.error, undefined);
+        onChartLoad?.({ complete: false, adapters: adapters ?? {} });
+        return;
+      }
+
       const totalHits = adapters?.tables?.tables?.unifiedHistogram?.meta?.statistics?.totalCount;
 
       onTotalHitsChange?.(
         isLoading ? UnifiedHistogramFetchStatus.loading : UnifiedHistogramFetchStatus.complete,
         totalHits ?? previousHits.current
       );
 
-      const lensRequest = adapters?.requests?.getRequests()[0];
-      const json = lensRequest?.response?.json as IKibanaSearchResponse<estypes.SearchResponse>;
-      const response = json?.rawResponse;
-
       if (response) {
         const newBucketInterval = buildBucketInterval({
           data,