[rush] Fix some unrelabile tests in non-CI environments.

common/changes/@microsoft/rush/fix-unreliable-tests_2026-01-06-04-45.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "",
+      "type": "none",
+      "packageName": "@microsoft/rush"
+    }
+  ],
+  "packageName": "@microsoft/rush",
+  "email": "[email protected]"
+}

libraries/rush-lib/src/cli/test/RushCommandLineParser.test.ts

@@ -20,8 +20,8 @@ jest.mock(`@rushstack/package-deps-hash`, () => {
     getGitHashForFiles(filePaths: Iterable<string>): ReadonlyMap<string, string> {
       return new Map(Array.from(filePaths, (filePath: string) => [filePath, filePath]));
     },
-    hashFilesAsync(rootDirectory: string, filePaths: Iterable<string>): ReadonlyMap<string, string> {
-      return new Map(Array.from(filePaths, (filePath: string) => [filePath, filePath]));
+    hashFilesAsync(rootDirectory: string, filePaths: Iterable<string>): Promise<ReadonlyMap<string, string>> {
+      return Promise.resolve(new Map(Array.from(filePaths, (filePath: string) => [filePath, filePath])));
     }
   };
 });
@@ -33,8 +33,13 @@ import { FileSystem, JsonFile, Path } from '@rushstack/node-core-library';
 import type { IDetailedRepoState } from '@rushstack/package-deps-hash';
 import { Autoinstaller } from '../../logic/Autoinstaller';
 import type { ITelemetryData } from '../../logic/Telemetry';
-import { getCommandLineParserInstanceAsync, type SpawnMockArgs, type SpawnMockCall } from './TestUtils';
-import { EnvironmentConfiguration } from '../../api/EnvironmentConfiguration';
+import {
+  getCommandLineParserInstanceAsync,
+  type SpawnMockArgs,
+  type SpawnMockCall,
+  isolateEnvironmentConfigurationForTests,
+  type IEnvironmentConfigIsolation
+} from './TestUtils';
 import { IS_WINDOWS } from '../../utilities/executionUtilities';
 
 // Ordinals into the `mock.calls` array referencing each of the arguments to `spawn`. Note that
@@ -73,13 +78,15 @@ function expectSpawnToMatchRegexp(spawnCall: SpawnMockCall, expectedRegexp: RegE
 
 describe('RushCommandLineParser', () => {
   describe('execute', () => {
+    let _envIsolation: IEnvironmentConfigIsolation;
+
+    beforeEach(() => {
+      _envIsolation = isolateEnvironmentConfigurationForTests();
+    });
+
     afterEach(() => {
       jest.clearAllMocks();
-      EnvironmentConfiguration.reset();
-      jest
-        .spyOn(EnvironmentConfiguration, 'buildCacheOverrideJsonFilePath', 'get')
-        .mockReturnValue(undefined);
-      jest.spyOn(EnvironmentConfiguration, 'buildCacheOverrideJson', 'get').mockReturnValue(undefined);
+      _envIsolation.restore();
     });
 
     describe('in basic repo', () => {

libraries/rush-lib/src/cli/test/RushCommandLineParserFailureCases.test.ts

@@ -13,12 +13,15 @@ jest.mock(`@rushstack/package-deps-hash`, () => {
       return {
         hasSubmodules: false,
         hasUncommittedChanges: false,
-        files: new Map(),
+        files: new Map([['common/config/rush/npm-shrinkwrap.json', 'hash']]),
         symlinks: new Map()
       };
     },
     getRepoChangesAsync(): ReadonlyMap<string, string> {
       return new Map();
+    },
+    hashFilesAsync(rootDirectory: string, filePaths: Iterable<string>): Promise<ReadonlyMap<string, string>> {
+      return Promise.resolve(new Map(Array.from(filePaths, (filePath: string) => [filePath, filePath])));
     }
   };
 });
@@ -28,11 +31,19 @@ import type { IDetailedRepoState } from '@rushstack/package-deps-hash';
 import { Autoinstaller } from '../../logic/Autoinstaller';
 import type { ITelemetryData } from '../../logic/Telemetry';
 import { getCommandLineParserInstanceAsync, setSpawnMock } from './TestUtils';
+import { isolateEnvironmentConfigurationForTests, type IEnvironmentConfigIsolation } from './TestUtils';
 
 describe('RushCommandLineParserFailureCases', () => {
   describe('execute', () => {
+    let _envIsolation: IEnvironmentConfigIsolation;
+
+    beforeEach(() => {
+      _envIsolation = isolateEnvironmentConfigurationForTests({ silenceStderrWrite: true });
+    });
+
     afterEach(() => {
-      jest.clearAllMocks();
+      _envIsolation.restore();
+      jest.restoreAllMocks();
     });
 
     describe('in repo plugin custom flushTelemetry', () => {

libraries/rush-lib/src/cli/test/TestUtils.ts

@@ -6,6 +6,7 @@ import { AlreadyExistsBehavior, FileSystem, PackageJsonLookup } from '@rushstack
 import type { RushCommandLineParser as RushCommandLineParserType } from '../RushCommandLineParser';
 import { FlagFile } from '../../api/FlagFile';
 import { RushConstants } from '../../logic/RushConstants';
+import { EnvironmentConfiguration } from '../../api/EnvironmentConfiguration';
 
 export type SpawnMockArgs = Parameters<typeof import('node:child_process').spawn>;
 export type SpawnMock = jest.Mock<ReturnType<typeof import('node:child_process').spawn>, SpawnMockArgs>;
@@ -37,6 +38,23 @@ export interface IChildProcessModuleMock {
   spawn: jest.Mock;
 }
 
+const DEFAULT_RUSH_ENV_VARS_TO_CLEAR: ReadonlyArray<string> = [
+  'RUSH_BUILD_CACHE_OVERRIDE_JSON',
+  'RUSH_BUILD_CACHE_OVERRIDE_JSON_FILE_PATH',
+  'RUSH_BUILD_CACHE_CREDENTIAL',
+  'RUSH_BUILD_CACHE_ENABLED',
+  'RUSH_BUILD_CACHE_WRITE_ALLOWED'
+];
+
+export interface IWithEnvironmentConfigIsolationOptions {
+  envVarNamesToClear?: ReadonlyArray<string>;
+  silenceStderrWrite?: boolean;
+}
+
+export interface IEnvironmentConfigIsolation {
+  restore(): void;
+}
+
 /**
  * Configure the `child_process` `spawn` mock for these tests. This relies on the mock implementation
  * in `mock_child_process`.
@@ -102,3 +120,81 @@ export async function getCommandLineParserInstanceAsync(
     repoPath
   };
 }
+
+/**
+ * Clears Rush-related environment variables and resets EnvironmentConfiguration for deterministic tests.
+ *
+ * Notes:
+ * - EnvironmentConfiguration caches some values, so we also stub the build-cache override getters.
+ * - Rush treats any stderr output during `rush test` as a warning, which fails the command; some
+ *   tests intentionally simulate failures and may need stderr silenced.
+ */
+export function isolateEnvironmentConfigurationForTests(
+  options: IWithEnvironmentConfigIsolationOptions = {}
+): IEnvironmentConfigIsolation {
+  const envVarNamesToClear: ReadonlyArray<string> =
+    options.envVarNamesToClear ?? DEFAULT_RUSH_ENV_VARS_TO_CLEAR;
+
+  const savedProcessEnv: Record<string, string | undefined> = {};
+  for (const envVarName of envVarNamesToClear) {
+    savedProcessEnv[envVarName] = process.env[envVarName];
+    delete process.env[envVarName];
+  }
+
+  EnvironmentConfiguration.reset();
+
+  const restoreFns: Array<() => void> = [];
+
+  restoreFns.push(() => {
+    for (const envVarName of envVarNamesToClear) {
+      const oldValue: string | undefined = savedProcessEnv[envVarName];
+      if (oldValue === undefined) {
+        delete process.env[envVarName];
+      } else {
+        process.env[envVarName] = oldValue;
+      }
+    }
+  });
+
+  if (options.silenceStderrWrite) {
+    type StderrWrite = typeof process.stderr.write;
+    const silentWrite: unknown = (
+      chunk: string | Uint8Array,
+      encoding?: BufferEncoding | ((err?: Error | null) => void),
+      cb?: (err?: Error | null) => void
+    ): boolean => {
+      if (typeof encoding === 'function') {
+        encoding(null);
+      } else {
+        cb?.(null);
+      }
+      return true;
+    };
+
+    const writeSpy: jest.SpyInstance<ReturnType<StderrWrite>, Parameters<StderrWrite>> = jest
+      .spyOn(process.stderr, 'write')
+      .mockImplementation(silentWrite as StderrWrite);
+
+    restoreFns.push(() => writeSpy.mockRestore());
+  }
+
+  // EnvironmentConfiguration.reset() does not clear cached values for these fields.
+  const overrideJsonFilePathSpy: jest.SpyInstance<string | undefined, []> = jest
+    .spyOn(EnvironmentConfiguration, 'buildCacheOverrideJsonFilePath', 'get')
+    .mockReturnValue(undefined);
+  const overrideJsonSpy: jest.SpyInstance<string | undefined, []> = jest
+    .spyOn(EnvironmentConfiguration, 'buildCacheOverrideJson', 'get')
+    .mockReturnValue(undefined);
+
+  restoreFns.push(() => overrideJsonFilePathSpy.mockRestore());
+  restoreFns.push(() => overrideJsonSpy.mockRestore());
+  restoreFns.push(() => EnvironmentConfiguration.reset());
+
+  return {
+    restore: () => {
+      for (let i: number = restoreFns.length - 1; i >= 0; i--) {
+        restoreFns[i]();
+      }
+    }
+  };
+}