feat(yarn): set http proxy config from environment variables

lib/modules/manager/npm/post-update/yarn.spec.ts

@@ -46,6 +46,8 @@ describe('modules/manager/npm/post-update/yarn', () => {
 
   beforeEach(() => {
     delete process.env.BUILDPACK;
+    delete process.env.HTTP_PROXY;
+    delete process.env.HTTPS_PROXY;
     Fixtures.reset();
     GlobalConfig.set({ localDir: '.', cacheDir: '/tmp/cache' });
     removeDockerContainer.mockResolvedValue();
@@ -147,6 +149,38 @@ describe('modules/manager/npm/post-update/yarn', () => {
     expect(fixSnapshots(execSnapshots)).toMatchSnapshot();
   });
 
+  it('sets http proxy', async () => {
+    process.env.HTTP_PROXY = 'http://proxy';
+    process.env.HTTPS_PROXY = 'http://proxy';
+    GlobalConfig.set({
+      localDir: '.',
+      allowScripts: true,
+      cacheDir: '/tmp/cache',
+    });
+    Fixtures.mock(
+      {
+        'yarn.lock': 'package-lock-contents',
+      },
+      'some-dir',
+    );
+    const execSnapshots = mockExecAll({
+      stdout: '3.0.0',
+      stderr: '',
+    });
+    const config = {
+      constraints: {
+        yarn: '3.0.0',
+      },
+    };
+    const res = await yarnHelper.generateLockFile('some-dir', {}, config);
+    expect(res.lockFile).toBe('package-lock-contents');
+    expect(fixSnapshots(execSnapshots)).toMatchObject([
+      { cmd: 'yarn config set --home httpProxy http://proxy' },
+      { cmd: 'yarn config set --home httpsProxy http://proxy' },
+      {},
+    ]);
+  });
+
   it('does not use global cache if zero install is detected', async () => {
     Fixtures.mock(
       {

lib/modules/manager/npm/post-update/yarn.ts

@@ -210,6 +210,17 @@ export async function generateLockFile(
       commands.push(`yarn set version ${quote(yarnUpdate.newValue!)}`);
     }
 
+    if (process.env.HTTP_PROXY && !isYarn1) {
+      commands.push(
+        `yarn config set --home httpProxy ${quote(process.env.HTTP_PROXY)}`,
+      );
+    }
+    if (process.env.HTTPS_PROXY && !isYarn1) {
+      commands.push(
+        `yarn config set --home httpsProxy ${quote(process.env.HTTPS_PROXY)}`,
+      );
+    }
+
     // This command updates the lock file based on package.json
     commands.push(`yarn install${cmdOptions}`);
 

lib/modules/manager/npm/readme.md

@@ -7,3 +7,15 @@ The following `depTypes` are currently supported by the npm manager :
 - `engines` : Renovate will update any `node`, `npm` and `yarn` version specified under `engines`.
 - `volta` : Renovate will update any `node`, `npm`, `pnpm` and `yarn` version specified under `volta`.
 - `packageManager`
+
+### Yarn
+
+#### Version Selection / Installation
+
+If Renovate detects a `packageManager` setting for Yarn in `package.json` then it will use Corepack to install Yarn.
+
+#### HTTP Proxy Support
+
+Yarn itself does not natively recognize/support the `HTTP_PROXY` and `HTTPS_PROXY` environment variables.
+If Renovate detects Yarn 2+, and one or both of those variables are present, then it will run commands like `yarn config set --home httpProxy http://proxy` prior to executing `yarn install`.
+This will result in the `~/.yarnrc.yml` file being created or modified with these settings, and the settings are not removed afterwards.