feat: allow templating of the extends field (#27955)

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>
Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

docs/usage/config-presets.md

@@ -234,6 +234,40 @@ Parameters are supported similar to other methods:
 }
 ```
 
+## Templating presets
+
+You can use [Handlebars](https://handlebarsjs.com/) templates to be flexible with your presets.
+This can be handy when you want to include presets conditionally.
+
+<!-- prettier-ignore -->
+!!! note
+    The template only supports a small subset of options, but you can extend them via `customEnvVariables`.
+
+Read the [templates](./templates.md) section to learn more.
+
+### Example use-case
+
+The following example shows a self-hosted Renovate preset located in a GitLab repository called `renovate/presets`.
+
+```json
+{
+  "extends": ["local>renovate/presets"]
+}
+```
+
+Usually you want to validate the preset before you put it in your Renovate configuration
+Here is an example of how you can use templating to validate and load the preset on a branch level:
+
+```javascript
+// config.js
+module.exports = {
+  customEnvVariables: {
+    GITLAB_REF: process.env.CI_COMMIT_REF_NAME || 'main',
+  },
+  extends: ['local>renovate/presets#{{ env.GITLAB_REF }}'],
+};
+```
+
 ## Contributing to presets
 
 Have you configured a rule that could help others?

lib/config/presets/index.spec.ts

@@ -336,6 +336,21 @@ describe('config/presets/index', () => {
       expect(res).toMatchSnapshot();
     });
 
+    it('resolves self-hosted preset with templating', async () => {
+      GlobalConfig.set({ customEnvVariables: { GIT_REF: 'abc123' } });
+      config.extends = ['local>username/preset-repo#{{ env.GIT_REF }}'];
+      local.getPreset.mockImplementationOnce(({ tag }) =>
+        tag === 'abc123'
+          ? Promise.resolve({ labels: ['self-hosted with template resolved'] })
+          : Promise.reject(new Error('Failed to resolve self-hosted preset')),
+      );
+
+      const res = await presets.resolveConfigPresets(config);
+
+      expect(res.labels).toEqual(['self-hosted with template resolved']);
+      expect(local.getPreset).toHaveBeenCalledOnce();
+    });
+
     it('resolves self-hosted transitive presets without baseConfig', async () => {
       config.platform = 'gitlab';
       config.endpoint = 'https://dummy.example.com/api/v4';

lib/config/presets/index.ts

@@ -10,6 +10,7 @@ import * as packageCache from '../../util/cache/package';
 import { getTtlOverride } from '../../util/cache/package/decorator';
 import { clone } from '../../util/clone';
 import { regEx } from '../../util/regex';
+import * as template from '../../util/template';
 import { GlobalConfig } from '../global';
 import * as massage from '../massage';
 import * as migration from '../migration';
@@ -320,6 +321,10 @@ export async function resolveConfigPresets(
   let config: AllConfig = {};
   // First, merge all the preset configs from left to right
   if (inputConfig.extends?.length) {
+    // Compile templates
+    inputConfig.extends = inputConfig.extends.map((tmpl) =>
+      template.compile(tmpl, {}),
+    );
     for (const preset of inputConfig.extends) {
       if (shouldResolvePreset(preset, existingPresets, ignorePresets)) {
         logger.trace(`Resolving preset "${preset}"`);