Fix plugin deprecations (#29737) (#29985)

* [deprecations] convert flattened settings to nested settings

* [deprecations] reuse rename method for elasticsearch.url to elasticsearch.hosts

* newline

* add known issues to docs

src/server/config/complete.js

@@ -17,7 +17,7 @@
  * under the License.
  */
 
-import { difference } from 'lodash';
+import { difference, get, set } from 'lodash';
 import { transformDeprecations } from './transform_deprecations';
 import { unset, formatListAsProse, getFlattenedObject } from '../../utils';
 import { getTransform } from '../../deprecation';
@@ -39,9 +39,13 @@ async function getUnusedConfigKeys(
     const { spec } = plugins[i];
     const transform = await getTransform(spec);
     const prefix = spec.getConfigPrefix();
-    const pluginSettings = settings[prefix];
+
+    // nested plugin prefixes (a.b) translate to nested objects
+    const pluginSettings = get(settings, prefix);
     if (pluginSettings) {
-      settings[prefix] = transform(pluginSettings);
+      // flattened settings are expected to be converted to nested objects
+      // a.b = true => { a: { b: true }}
+      set(settings, prefix, transform(pluginSettings));
     }
   }
 

src/server/config/complete.test.js

@@ -252,6 +252,41 @@ describe('server/config completeMixin()', function () {
 
       await expect(callCompleteMixin()).resolves.toBe(undefined);
     });
+
+    it('should transform deeply nested deprecated plugin settings', async () => {
+      const { callCompleteMixin } = setup({
+        settings: {
+          xpack: {
+            monitoring: {
+              elasticsearch: {
+                url: 'http://localhost:9200'
+              }
+            }
+          }
+        },
+        configValues: {
+          xpack: {
+            monitoring: {
+              elasticsearch: {
+                hosts: 'http://localhost:9200'
+              }
+            }
+          }
+        },
+        plugins: [
+          {
+            spec: {
+              getDeprecationsProvider() {
+                return async ({ rename }) => [rename('elasticsearch.url', 'elasticsearch.hosts')];
+              },
+              getConfigPrefix: () => 'xpack.monitoring'
+            }
+          }
+        ],
+      });
+
+      await expect(callCompleteMixin()).resolves.toBe(undefined);
+    });
   });
 
   describe('disabled plugins', () => {