fix(linter): check for flat config correctly in @nx/eslint:lint executor (#26350)

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## Current Behavior
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## Expected Behavior
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Fixes #22575

packages/eslint/src/executors/lint/lint.impl.ts

@@ -1,11 +1,11 @@
-import { ExecutorContext, joinPathFragments, workspaceRoot } from '@nx/devkit';
-import { ESLint } from 'eslint';
-import { existsSync, mkdirSync, writeFileSync } from 'fs';
-import { dirname, resolve } from 'path';
-
+import { joinPathFragments, type ExecutorContext } from '@nx/devkit';
+import type { ESLint } from 'eslint';
+import { mkdirSync, writeFileSync } from 'fs';
+import { interpolate } from 'nx/src/tasks-runner/utils';
+import { dirname, posix, resolve } from 'path';
+import { findFlatConfigFile } from '../../utils/config-file';
 import type { Schema } from './schema';
 import { resolveAndInstantiateESLint } from './utility/eslint-utils';
-import { interpolate } from 'nx/src/tasks-runner/utils';
 
 export default async function run(
   options: Schema,
@@ -37,22 +37,18 @@ export default async function run(
   const { printConfig, errorOnUnmatchedPattern, ...normalizedOptions } =
     options;
 
-  /**
-   * Until ESLint v9 is released and the new so called flat config is the default
-   * we only want to support it if the user has explicitly opted into it by converting
-   * their root ESLint config to use eslint.config.js
-   */
-  const hasFlatConfig = existsSync(
-    joinPathFragments(workspaceRoot, 'eslint.config.js')
-  );
+  // locate the flat config file if it exists starting from the project root
+  const flatConfigFilePath = findFlatConfigFile(projectRoot, context.root);
+  const hasFlatConfig = flatConfigFilePath !== null;
 
   // while standard eslint uses by default closest config to the file, if otherwise not specified,
-  // the flat config would always use the root config, so we need to explicitly set it to the local one
+  // the flat config would be resolved starting from the cwd, which we changed to the workspace root
+  // so we explicitly set the config path to the flat config file path we previously found
   if (hasFlatConfig && !normalizedOptions.eslintConfig) {
-    const eslintConfigPath = joinPathFragments(projectRoot, 'eslint.config.js');
-    if (existsSync(eslintConfigPath)) {
-      normalizedOptions.eslintConfig = eslintConfigPath;
-    }
+    normalizedOptions.eslintConfig = posix.relative(
+      systemRoot,
+      flatConfigFilePath
+    );
   }
 
   /**

packages/eslint/src/executors/lint/utility/eslint-utils.ts

@@ -1,4 +1,5 @@
 import type { ESLint } from 'eslint';
+import { isFlatConfig } from '../../../utils/config-file';
 import { resolveESLintClass } from '../../../utils/resolve-eslint-class';
 import type { Schema } from '../schema';
 
@@ -7,11 +8,7 @@ export async function resolveAndInstantiateESLint(
   options: Schema,
   useFlatConfig = false
 ) {
-  if (
-    useFlatConfig &&
-    eslintConfigPath &&
-    !eslintConfigPath?.endsWith('eslint.config.js')
-  ) {
+  if (useFlatConfig && eslintConfigPath && !isFlatConfig(eslintConfigPath)) {
     throw new Error(
       'When using the new Flat Config with ESLint, all configs must be named eslint.config.js and .eslintrc files may not be used. See https://eslint.org/docs/latest/use/configure/configuration-files-new'
     );

packages/eslint/src/utils/config-file.ts

@@ -1,19 +1,93 @@
-import { joinPathFragments } from '@nx/devkit';
-import { existsSync } from 'fs';
+import { existsSync, statSync } from 'fs';
+import { basename, dirname, join, resolve } from 'path';
 
-export const ESLINT_CONFIG_FILENAMES = [
+// TODO(leo): add support for eslint.config.mjs and eslint.config.cjs
+export const ESLINT_FLAT_CONFIG_FILENAMES = ['eslint.config.js'];
+
+export const ESLINT_OLD_CONFIG_FILENAMES = [
   '.eslintrc',
   '.eslintrc.js',
   '.eslintrc.cjs',
   '.eslintrc.yaml',
   '.eslintrc.yml',
   '.eslintrc.json',
-  'eslint.config.js',
+];
+
+export const ESLINT_CONFIG_FILENAMES = [
+  ...ESLINT_OLD_CONFIG_FILENAMES,
+  ...ESLINT_FLAT_CONFIG_FILENAMES,
 ];
 
 export const baseEsLintConfigFile = '.eslintrc.base.json';
 export const baseEsLintFlatConfigFile = 'eslint.base.config.js';
 
 export function isFlatConfig(configFilePath: string): boolean {
-  return configFilePath.endsWith('.config.js');
+  const configFileName = basename(configFilePath);
+
+  return ESLINT_FLAT_CONFIG_FILENAMES.includes(configFileName);
+}
+
+// https://eslint.org/docs/latest/use/configure/configuration-files#configuration-file-resolution
+export function findFlatConfigFile(
+  directory: string,
+  workspaceRoot: string
+): string | null {
+  let currentDir = resolve(workspaceRoot, directory);
+
+  if (currentDir === workspaceRoot) {
+    return getConfigFileInDirectory(currentDir, ESLINT_FLAT_CONFIG_FILENAMES);
+  }
+
+  while (currentDir !== workspaceRoot) {
+    const configFilePath = getConfigFileInDirectory(
+      currentDir,
+      ESLINT_FLAT_CONFIG_FILENAMES
+    );
+    if (configFilePath) {
+      return configFilePath;
+    }
+    currentDir = dirname(currentDir);
+  }
+
+  return null;
+}
+
+export function findOldConfigFile(
+  filePathOrDirectory: string,
+  workspaceRoot: string
+): string | null {
+  let currentDir = statSync(filePathOrDirectory).isDirectory()
+    ? filePathOrDirectory
+    : dirname(filePathOrDirectory);
+
+  if (currentDir === workspaceRoot) {
+    return getConfigFileInDirectory(currentDir, ESLINT_OLD_CONFIG_FILENAMES);
+  }
+
+  while (currentDir !== workspaceRoot) {
+    const configFilePath = getConfigFileInDirectory(
+      currentDir,
+      ESLINT_OLD_CONFIG_FILENAMES
+    );
+    if (configFilePath) {
+      return configFilePath;
+    }
+    currentDir = dirname(currentDir);
+  }
+
+  return null;
+}
+
+function getConfigFileInDirectory(
+  directory: string,
+  candidateFileNames: string[]
+): string | null {
+  for (const filename of candidateFileNames) {
+    const filePath = join(directory, filename);
+    if (existsSync(filePath)) {
+      return filePath;
+    }
+  }
+
+  return null;
 }