fix: new instance of FlatESLint should load latest config file version

lib/eslint/flat-eslint.js

@@ -93,6 +93,7 @@ const FLAT_CONFIG_FILENAME = "eslint.config.js";
 const debug = require("debug")("eslint:flat-eslint");
 const removedFormatters = new Set(["table", "codeframe"]);
 const privateMembers = new WeakMap();
+const importedConfigFileModificationTime = new Map();
 
 /**
  * It will calculate the error and warning count for collection of messages per file
@@ -281,7 +282,42 @@ async function loadFlatConfigFile(filePath) {
 
     debug(`Config file URL is ${fileURL}`);
 
-    return (await import(fileURL)).default;
+    const mtime = (await fs.stat(filePath)).mtime.getTime();
+
+    /*
+     * Append a query with the config file's modification time (`mtime`) in order
+     * to import the current version of the config file. Without the query, `import()` would
+     * cache the config file module by the pathname only, and then always return
+     * the same version (the one that was actual when the module was imported for the first time).
+     *
+     * This ensures that the config file module is loaded and executed again
+     * if it has been changed since the last time it was imported.
+     * If it hasn't been changed, `import()` will just return the cached version.
+     *
+     * Note that we should not overuse queries (e.g., by appending the current time
+     * to always reload the config file module) as that could cause memory leaks
+     * because entries are never removed from the import cache.
+     */
+    fileURL.searchParams.append("mtime", mtime);
+
+    /*
+     * With queries, we can bypass the import cache. However, when import-ing a CJS module,
+     * Node.js uses the require infrastructure under the hood. That includes the require cache,
+     * which caches the config file module by its file path (queries have no effect).
+     * Therefore, we also need to clear the require cache before importing the config file module.
+     * In order to get the same behavior with ESM and CJS config files, in particular - to reload
+     * the config file only if it has been changed, we track file modification times and clear
+     * the require cache only if the file has been changed.
+     */
+    if (importedConfigFileModificationTime.get(filePath) !== mtime) {
+        delete require.cache[filePath];
+    }
+
+    const config = (await import(fileURL)).default;
+
+    importedConfigFileModificationTime.set(filePath, mtime);
+
+    return config;
 }
 
 /**

tests/lib/eslint/flat-eslint.js

@@ -11,6 +11,7 @@
 //------------------------------------------------------------------------------
 
 const assert = require("assert");
+const util = require("util");
 const fs = require("fs");
 const fsp = fs.promises;
 const os = require("os");
@@ -41,6 +42,15 @@ function ensureDirectoryExists(dirPath) {
     }
 }
 
+/**
+ * Does nothing for a given time.
+ * @param {number} time Time in ms.
+ * @returns {void}
+ */
+async function sleep(time) {
+    await util.promisify(setTimeout)(time);
+}
+
 //------------------------------------------------------------------------------
 // Tests
 //------------------------------------------------------------------------------
@@ -5256,4 +5266,74 @@ describe("FlatESLint", () => {
         });
     });
 
+    describe("config file", () => {
+
+        it("new instance of FlatESLint should use the latest version of the config file (ESM)", async () => {
+            const cwd = path.join(getFixturePath(), `config_file_${Date.now()}`);
+            const configFileContent = "export default [{ rules: { semi: ['error', 'always'] } }];";
+            const teardown = createCustomTeardown({
+                cwd,
+                files: {
+                    "package.json": '{ "type": "module" }',
+                    "eslint.config.js": configFileContent,
+                    "a.js": "foo\nbar;"
+                }
+            });
+
+            await teardown.prepare();
+
+            let eslint = new FlatESLint({ cwd });
+            let [{ messages }] = await eslint.lintFiles(["a.js"]);
+
+            assert.strictEqual(messages.length, 1);
+            assert.strictEqual(messages[0].ruleId, "semi");
+            assert.strictEqual(messages[0].messageId, "missingSemi");
+            assert.strictEqual(messages[0].line, 1);
+
+            await sleep(100);
+            await fsp.writeFile(path.join(cwd, "eslint.config.js"), configFileContent.replace("always", "never"));
+
+            eslint = new FlatESLint({ cwd });
+            [{ messages }] = await eslint.lintFiles(["a.js"]);
+
+            assert.strictEqual(messages.length, 1);
+            assert.strictEqual(messages[0].ruleId, "semi");
+            assert.strictEqual(messages[0].messageId, "extraSemi");
+            assert.strictEqual(messages[0].line, 2);
+        });
+
+        it("new instance of FlatESLint should use the latest version of the config file (CJS)", async () => {
+            const cwd = path.join(getFixturePath(), `config_file_${Date.now()}`);
+            const configFileContent = "module.exports = [{ rules: { semi: ['error', 'always'] } }];";
+            const teardown = createCustomTeardown({
+                cwd,
+                files: {
+                    "eslint.config.js": configFileContent,
+                    "a.js": "foo\nbar;"
+                }
+            });
+
+            await teardown.prepare();
+
+            let eslint = new FlatESLint({ cwd });
+            let [{ messages }] = await eslint.lintFiles(["a.js"]);
+
+            assert.strictEqual(messages.length, 1);
+            assert.strictEqual(messages[0].ruleId, "semi");
+            assert.strictEqual(messages[0].messageId, "missingSemi");
+            assert.strictEqual(messages[0].line, 1);
+
+            await sleep(100);
+            await fsp.writeFile(path.join(cwd, "eslint.config.js"), configFileContent.replace("always", "never"));
+
+            eslint = new FlatESLint({ cwd });
+            [{ messages }] = await eslint.lintFiles(["a.js"]);
+
+            assert.strictEqual(messages.length, 1);
+            assert.strictEqual(messages[0].ruleId, "semi");
+            assert.strictEqual(messages[0].messageId, "extraSemi");
+            assert.strictEqual(messages[0].line, 2);
+        });
+    });
+
 });