[SIEM] Fixes Signals count spinner (#56797)

## Fixes an issue where the Signals count spinner can spin forever

Per the animated gif below, in `7.6` `BC 4`, the `Signals count` spinner on the Overview page spins forever until the signals index is created (in the current Kibana space):

![signals-count-loading-spinner](https://user-images.githubusercontent.com/4459398/73785251-2ca42000-4754-11ea-8671-daa81f351c9b.gif)

The `Signals count` spinner will spin forever until the user clicks the `Detections` tab, which-in turn creates the signals index (if it doesn't exist), per the animated gif below:

![create-signals-index](https://user-images.githubusercontent.com/4459398/73785319-4ba2b200-4754-11ea-9bb0-a745a8b2be5d.gif)

This behavior is an issue because:

- When a fresh deployment is created on Elastic Cloud, a user won't understand why the `Signals count` widget is always spinning on the `Overview` page. (The user must click the `Detections` page to resolve this.)
- In deployments where authentication is disabled, or, for _reasons_, a Detections index will never be created, the `Signals count` spinner on the Overview page will always spin.

To reproduce:

1. Spin up a new `7.6` `BC 4` deployment on Elastic Cloud
2. Login to Kibana for the first time
3. Navigate to the SIEM app

**Expected result**
- All histograms on the Overview page eventually stop displaying their respective loading spinners

**Actual result**
- The `Signals count` widget spinner spins forever. (The user must click the `Detections` tab to create the signals index.)

## Deleting the signals index

To reproduce the issue above when a signals index has already been created (by clicking on the Detections tab), run the following from the Kibana `Dev Tools` `Console`:

```
DELETE /.siem-signals-default-000001
```

It is also possible to reproduce this issue by creating a new space, because it won't have a signals index.

elastic/siem-team#514

x-pack/legacy/plugins/siem/public/pages/detection_engine/components/signals_histogram_panel/helpers.test.tsx

@@ -0,0 +1,35 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+import { showInitialLoadingSpinner } from './helpers';
+
+describe('helpers', () => {
+  describe('showInitialLoadingSpinner', () => {
+    test('it should (only) show the spinner during initial loading, while we are fetching data', () => {
+      expect(showInitialLoadingSpinner({ isInitialLoading: true, isLoadingSignals: true })).toBe(
+        true
+      );
+    });
+
+    test('it should STOP showing the spinner (during initial loading) when the first data fetch completes', () => {
+      expect(showInitialLoadingSpinner({ isInitialLoading: true, isLoadingSignals: false })).toBe(
+        false
+      );
+    });
+
+    test('it should NOT show the spinner after initial loading has completed, even if the user requests more data (e.g. by clicking Refresh)', () => {
+      expect(showInitialLoadingSpinner({ isInitialLoading: false, isLoadingSignals: true })).toBe(
+        false
+      );
+    });
+
+    test('it should NOT show the spinner after initial loading has completed', () => {
+      expect(showInitialLoadingSpinner({ isInitialLoading: false, isLoadingSignals: false })).toBe(
+        false
+      );
+    });
+  });
+});

x-pack/legacy/plugins/siem/public/pages/detection_engine/components/signals_histogram_panel/helpers.tsx

@@ -76,3 +76,17 @@ export const getSignalsHistogramQuery = (
     },
   },
 });
+
+/**
+ * Returns `true` when the signals histogram initial loading spinner should be shown
+ *
+ * @param isInitialLoading The loading spinner will only be displayed if this value is `true`, because after initial load, a different, non-spinner loading indicator is displayed
+ * @param isLoadingSignals When `true`, IO is being performed to request signals (for rendering in the histogram)
+ */
+export const showInitialLoadingSpinner = ({
+  isInitialLoading,
+  isLoadingSignals,
+}: {
+  isInitialLoading: boolean;
+  isLoadingSignals: boolean;
+}): boolean => isInitialLoading && isLoadingSignals;

x-pack/legacy/plugins/siem/public/pages/detection_engine/components/signals_histogram_panel/index.tsx

@@ -23,7 +23,7 @@ import { InspectButtonContainer } from '../../../../components/inspect';
 import { useQuerySignals } from '../../../../containers/detection_engine/signals/use_query';
 import { MatrixLoader } from '../../../../components/matrix_histogram/matrix_loader';
 
-import { formatSignalsData, getSignalsHistogramQuery } from './helpers';
+import { formatSignalsData, getSignalsHistogramQuery, showInitialLoadingSpinner } from './helpers';
 import * as i18n from './translations';
 
 const DEFAULT_PANEL_HEIGHT = 300;
@@ -54,7 +54,6 @@ interface SignalsHistogramPanelProps {
   from: number;
   query?: Query;
   legendPosition?: Position;
-  loadingInitial?: boolean;
   panelHeight?: number;
   signalIndexName: string | null;
   setQuery: (params: RegisterQuery) => void;
@@ -75,7 +74,6 @@ export const SignalsHistogramPanel = memo<SignalsHistogramPanelProps>(
     query,
     from,
     legendPosition = 'right',
-    loadingInitial = false,
     panelHeight = DEFAULT_PANEL_HEIGHT,
     setQuery,
     signalIndexName,
@@ -86,7 +84,7 @@ export const SignalsHistogramPanel = memo<SignalsHistogramPanelProps>(
     title = i18n.HISTOGRAM_HEADER,
     updateDateRange,
   }) => {
-    const [isInitialLoading, setIsInitialLoading] = useState(loadingInitial || true);
+    const [isInitialLoading, setIsInitialLoading] = useState(true);
     const [defaultNumberFormat] = useUiSetting$<string>(DEFAULT_NUMBER_FORMAT);
     const [totalSignalsObj, setTotalSignalsObj] = useState<SignalsTotal>(defaultTotalSignalsObj);
     const [selectedStackByOption, setSelectedStackByOption] = useState<SignalsHistogramOption>(
@@ -124,10 +122,16 @@ export const SignalsHistogramPanel = memo<SignalsHistogramPanelProps>(
     const formattedSignalsData = useMemo(() => formatSignalsData(signalsData), [signalsData]);
 
     useEffect(() => {
-      if (!loadingInitial && isInitialLoading && !isLoadingSignals && signalsData) {
+      let canceled = false;
+
+      if (!canceled && !showInitialLoadingSpinner({ isInitialLoading, isLoadingSignals })) {
         setIsInitialLoading(false);
       }
-    }, [loadingInitial, isLoadingSignals, signalsData]);
+
+      return () => {
+        canceled = true; // prevent long running data fetches from updating state after unmounting
+      };
+    }, [isInitialLoading, isLoadingSignals, setIsInitialLoading]);
 
     useEffect(() => {
       return () => {

x-pack/legacy/plugins/siem/public/pages/detection_engine/detection_engine.tsx

@@ -177,7 +177,6 @@ const DetectionEnginePageComponent: React.FC<DetectionEnginePageComponentProps>
                             deleteQuery={deleteQuery}
                             filters={filters}
                             from={from}
-                            loadingInitial={loading}
                             query={query}
                             setQuery={setQuery}
                             showTotalSignalsCount={true}