Fix: Check for npmrc in home directory for linux root users (#4047)

**Summary**
Fix for #3920.

In this [commit](https://github.com/wtgtybhertgeghgtwtg/yarn/commit/f4fe7431ed992862a1c1d2357e6521c268bad7a7), `userHome` for linux users running as root was changed to `/usr/local/share`, mainly to allow for other users to run globally added bins. However this introduced a bug where npmrc wasn't being looked in the root directory after checking `/user/local/share`. So this change pushes another location to check in case `userHome` is different from native home directory returned by `os.homedir()`

**Test plan**
I've added a test to generally test the `getPossibleConfigLocations` method, but I haven't been able to properly mock out running it as a root user on linux. Suggestions welcome!

__tests__/registries/npm-registry.js

@@ -1,7 +1,9 @@
 /* @flow */
 
+import {resolve, join as pathJoin} from 'path';
+
 import NpmRegistry from '../../src/registries/npm-registry.js';
-import {resolve} from 'path';
+import {BufferReporter} from '../../src/reporters/index.js';
 import homeDir from '../../src/util/user-home-dir.js';
 
 describe('normalizeConfig', () => {
@@ -50,12 +52,10 @@ function createMocks(): Object {
       getScopedOption: jest.fn(),
     },
   };
-  const mockReporter = jest.fn();
 
   return {
     mockRequestManager,
     mockRegistries,
-    mockReporter,
   };
 }
 
@@ -292,3 +292,22 @@ describe('getScope functional test', () => {
     });
   });
 });
+
+describe('getPossibleConfigLocations', () => {
+  test('searches recursively to home directory', async () => {
+    const testCwd = './project/subdirectory';
+    const {mockRequestManager, mockRegistries} = createMocks();
+    const reporter = new BufferReporter({verbose: true});
+    const npmRegistry = new NpmRegistry(testCwd, mockRegistries, mockRequestManager, reporter);
+    await npmRegistry.getPossibleConfigLocations('npmrc', reporter);
+
+    const logs = reporter.getBuffer().map(logItem => logItem.data);
+    expect(logs).toEqual(
+      expect.arrayContaining([
+        expect.stringContaining(JSON.stringify(pathJoin('project', 'subdirectory', '.npmrc'))),
+        expect.stringContaining(JSON.stringify(pathJoin('project', '.npmrc'))),
+        expect.stringContaining(JSON.stringify(pathJoin(homeDir, '.npmrc'))),
+      ]),
+    );
+  });
+});

src/registries/npm-registry.js

@@ -12,12 +12,11 @@ import envReplace from '../util/env-replace.js';
 import Registry from './base-registry.js';
 import {addSuffix} from '../util/misc';
 import {getPosixPath, resolveWithHome} from '../util/path';
-
-const normalizeUrl = require('normalize-url');
-const userHome = require('../util/user-home-dir').default;
-const path = require('path');
-const url = require('url');
-const ini = require('ini');
+import normalizeUrl from 'normalize-url';
+import {default as userHome, home} from '../util/user-home-dir';
+import path from 'path';
+import url from 'url';
+import ini from 'ini';
 
 const DEFAULT_REGISTRY = 'https://registry.npmjs.org/';
 const REGEX_REGISTRY_PREFIX = /^https?:/;
@@ -178,6 +177,12 @@ export default class NpmRegistry extends Registry {
       [false, path.join(getGlobalPrefix(), 'etc', filename)],
     ];
 
+    // When home directory for global install is different from where $HOME/npmrc is stored,
+    // E.g. /usr/local/share vs /root on linux machines, check the additional location
+    if (home !== userHome) {
+      possibles.push([true, path.join(home, localfile)]);
+    }
+
     // npmrc --> ../.npmrc, ../../.npmrc, etc.
     const foldersFromRootToCwd = getPosixPath(this.cwd).split('/');
     while (foldersFromRootToCwd.length > 1) {

src/util/user-home-dir.js

@@ -4,7 +4,8 @@ import ROOT_USER from './root-user.js';
 
 const path = require('path');
 
-const userHomeDir =
-  process.platform === 'linux' && ROOT_USER ? path.resolve('/usr/local/share') : require('os').homedir();
+export const home = require('os').homedir();
+
+const userHomeDir = ROOT_USER ? path.resolve('/usr/local/share') : home;
 
 export default userHomeDir;