fix(plugin): isolate ESLint instance per webpack config in non-threaded mode

Avoid reusing global ESLint instances across multiple webpack configurations.
This fixes incorrect `cwd` and context resolution due to shared state between configurations.
Threaded mode remains broken due to fundamental architectural flaws and requires a complete rewrite to be compatible.

BREAKING CHANGE: threaded mode is broken and now explicitly unsupported
fixes #286

Author: AhNosNimes

src/getESLint.js

@@ -3,7 +3,7 @@ const { cpus } = require('os');
 const { Worker: JestWorker } = require('jest-worker');
 
 // @ts-ignore
-const { setup, lintFiles } = require('./worker');
+const { setup } = require('./worker');
 const { getESLintOptions } = require('./options');
 const { jsonStringifyReplacerSortKeys } = require('./utils');
 const { stringify } = require('flatted');
@@ -25,16 +25,16 @@ const cache = {};
  */
 async function loadESLint(options) {
   const { eslintPath } = options;
-  const eslint = await setup({
+  const eslintPack = await setup({
     eslintPath,
     configType: options.configType,
     eslintOptions: getESLintOptions(options),
   });
 
   return {
     threads: 1,
-    lintFiles,
-    eslint,
+    lintFiles : eslintPack.lintFiles,
+    eslint : eslintPack.eslint,
     // no-op for non-threaded
     cleanup: async () => {},
   };

src/worker.js

@@ -4,16 +4,9 @@
 /** @typedef {{new (arg0: ESLintOptions): ESLint; outputFixes: (arg0: LintResult[]) => any;}} ESLintClass */
 
 Object.assign(module.exports, {
-  lintFiles,
-  setup,
+  setup
 });
 
-/** @type {ESLintClass} */
-let ESLint;
-
-/** @type {ESLint} */
-let eslint;
-
 /** @type {boolean} */
 let fix;
 
@@ -25,48 +18,61 @@ let fix;
  *
  * @param {setupOptions} arg0 - setup worker
  */
-function setup({ eslintPath, configType, eslintOptions }) {
+function setup({eslintPath, configType, eslintOptions}) {
   fix = !!(eslintOptions && eslintOptions.fix);
   const eslintModule = require(eslintPath || 'eslint');
 
   if (eslintModule.ESLint && parseFloat(eslintModule.ESLint.version) >= 9) {
     return eslintModule
-      .loadESLint({ useFlatConfig: configType === 'flat' })
+      .loadESLint({useFlatConfig: configType === 'flat'})
       .then((/** @type {ESLintClass} */ classESLint) => {
-        ESLint = classESLint;
-        eslint = new ESLint(eslintOptions);
-        return eslint;
+        const ESLint = classESLint;
+        const eslint = new ESLint(eslintOptions);
+        const pack = {
+          ESLint,
+          eslint,
+          lintFiles: lintFiles.bind(null, ESLint, eslint),
+        };
+        return pack;
       });
   }
 
   let FlatESLint;
-
+  /** @type {ESLintClass} */
+  let ESLint;
   if (eslintModule.LegacyESLint) {
     ESLint = eslintModule.LegacyESLint;
-    ({ FlatESLint } = eslintModule);
+    ({FlatESLint} = eslintModule);
   } else {
-    ({ ESLint } = eslintModule);
-
+    ({ESLint} = eslintModule);
     if (configType === 'flat') {
       throw new Error(
         "Couldn't find FlatESLint, you might need to set eslintPath to 'eslint/use-at-your-own-risk'",
       );
     }
   }
-
+  /** @type {ESLint} */
+  let eslint;
   if (configType === 'flat') {
     eslint = new FlatESLint(eslintOptions);
   } else {
     eslint = new ESLint(eslintOptions);
   }
+  const pack = {
+    ESLint,
+    eslint,
+    lintFiles: lintFiles.bind(null, ESLint, eslint),
+  };
 
-  return eslint;
+  return pack;
 }
 
 /**
+ * @param {ESLintClass} ESLint
+ * @param {ESLint} eslint
  * @param {string | string[]} files
  */
-async function lintFiles(files) {
+async function lintFiles(ESLint, eslint, files) {
   /** @type {LintResult[]} */
   const result = await eslint.lintFiles(files);
   // if enabled, use eslint autofixing where possible