[Fleet] Add xpack.fleet.isAirGapped flag (#174214)

## Summary

Closes https://github.com/elastic/kibana/issues/173125

Adds a new `xpack.fleet.isAirGapped` flag to `kibana.yml` that allow
users in air-gapped environments to inform Fleet that it should "fail
fast" and skip unnecessary requests that won't be possible as Kibana
doesn't have internet access.

This PR also uses the new flag to skip the API request to the
`product_versions` API if the airgapped flag is set to true. There are
probably other places we could use this flag in a similar way, but they
can be handled separately from this PR.

### Checklist

Delete any items that are not applicable to this PR.

- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

docs/settings/fleet-settings.asciidoc

@@ -24,6 +24,8 @@ See the {fleet-guide}/index.html[{fleet}] docs for more information about {fleet
 `xpack.fleet.agents.enabled` {ess-icon}::
 Set to `true` (default) to enable {fleet}.
 
+`xpack.fleet.isAirGapped`::
+Set to `true` to indicate {fleet} is running in an air-gapped environment. Refer to {fleet-guide}/air-gapped.html[Air-gapped environments] for details. Enabling this flag helps Fleet skip needless requests and improve the user experience for air-gapped environments.
 
 [[fleet-data-visualizer-settings]]
 

x-pack/plugins/fleet/common/types/index.ts

@@ -16,6 +16,7 @@ import type {
 
 export interface FleetConfigType {
   enabled: boolean;
+  isAirGapped?: boolean;
   registryUrl?: string;
   registryProxyUrl?: string;
   agents: {

x-pack/plugins/fleet/server/config.ts

@@ -126,6 +126,7 @@ export const config: PluginConfigDescriptor = {
   ],
   schema: schema.object(
     {
+      isAirGapped: schema.maybe(schema.boolean({ defaultValue: false })),
       registryUrl: schema.maybe(schema.uri({ scheme: ['http', 'https'] })),
       registryProxyUrl: schema.maybe(schema.uri({ scheme: ['http', 'https'] })),
       agents: schema.object({

x-pack/plugins/fleet/server/services/agents/versions.test.ts

@@ -8,9 +8,15 @@
 import { readFile } from 'fs/promises';
 
 import fetch from 'node-fetch';
+import type { DeepPartial } from 'utility-types';
+
+import type { FleetConfigType } from '../../../public/plugin';
+
+import { getAvailableVersions } from './versions';
 
 let mockKibanaVersion = '300.0.0';
-let mockConfig = {};
+let mockConfig: DeepPartial<FleetConfigType> = {};
+
 jest.mock('../app_context', () => {
   const { loggerMock } = jest.requireActual('@kbn/logging-mocks');
   return {
@@ -33,8 +39,6 @@ const emptyResponse = {
   text: jest.fn().mockResolvedValue(JSON.stringify({})),
 } as any;
 
-import { getAvailableVersions } from './versions';
-
 describe('getAvailableVersions', () => {
   beforeEach(() => {
     mockedReadFile.mockReset();
@@ -204,4 +208,15 @@ describe('getAvailableVersions', () => {
     // Should sort, uniquify and filter out versions < 7.17
     expect(res).toEqual(['8.1.0', '8.0.0', '7.17.0']);
   });
+
+  it('should not fetch from product versions API when air-gapped config is set', async () => {
+    mockKibanaVersion = '300.0.0';
+    mockedReadFile.mockResolvedValue(`["8.1.0", "8.0.0", "7.17.0", "7.16.0"]`);
+
+    mockConfig = { isAirGapped: true };
+    const res = await getAvailableVersions({ ignoreCache: true });
+
+    expect(res).toEqual(['8.1.0', '8.0.0', '7.17.0']);
+    expect(mockedFetch).not.toBeCalled();
+  });
 });

x-pack/plugins/fleet/server/services/agents/versions.ts

@@ -111,6 +111,11 @@ export const getAvailableVersions = async ({
 };
 
 async function fetchAgentVersionsFromApi() {
+  // If the airgapped flag is set, do not attempt to reach out to the product versions API
+  if (appContextService.getConfig()?.isAirGapped) {
+    return [];
+  }
+
   const logger = appContextService.getLogger();
 
   const options = {