worker: fix TOCTOU race in CWD caching

lib/internal/worker.js

@@ -112,8 +112,8 @@ if (isMainThread) {
   cwdCounter = new Uint32Array(constructSharedArrayBuffer(4));
   const originalChdir = process.chdir;
   process.chdir = function(path) {
-    AtomicsAdd(cwdCounter, 0, 1);
     originalChdir(path);
+    AtomicsAdd(cwdCounter, 0, 1);
   };
 }
 

test/parallel/test-worker-cwd-no-stale-cache.js

@@ -0,0 +1,73 @@
+'use strict';
+
+// This test verifies that worker threads do not cache stale CWD values
+// after process.chdir() has completed in the main thread.
+//
+// Regression test for a TOCTOU race condition where the atomic counter
+// was incremented before chdir() completed, allowing workers to cache
+// stale directory paths.
+
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const { Worker, isMainThread, parentPort } = require('worker_threads');
+
+if (!isMainThread) {
+  // Worker: respond to 'check' messages with current cwd
+  parentPort.on('message', (msg) => {
+    if (msg.type === 'check') {
+      parentPort.postMessage({
+        type: 'cwd',
+        cwd: process.cwd(),
+        expected: msg.expected,
+      });
+    }
+  });
+  return;
+}
+
+// Main thread
+const testDir = __dirname;
+const parentDir = path.dirname(testDir);
+
+// Ensure we start in a known directory
+process.chdir(testDir);
+
+const worker = new Worker(__filename);
+
+let checksCompleted = 0;
+const totalChecks = 100;
+
+worker.on('message', common.mustCall((msg) => {
+  if (msg.type === 'cwd') {
+    // After chdir() has returned in the main thread, the worker
+    // must see the new directory, not a stale cached value
+    assert.strictEqual(
+      msg.cwd,
+      msg.expected,
+      `Worker returned stale CWD: got "${msg.cwd}", expected "${msg.expected}"`
+    );
+    checksCompleted++;
+
+    if (checksCompleted < totalChecks) {
+      // Alternate between directories
+      const newDir = checksCompleted % 2 === 0 ? testDir : parentDir;
+      process.chdir(newDir);
+      // Immediately after chdir returns, ask worker for cwd
+      worker.postMessage({ type: 'check', expected: newDir });
+    } else {
+      worker.terminate();
+    }
+  }
+}, totalChecks));
+
+worker.on('online', common.mustCall(() => {
+  // Start the test cycle
+  process.chdir(parentDir);
+  worker.postMessage({ type: 'check', expected: parentDir });
+}));
+
+worker.on('exit', common.mustCall((code) => {
+  assert.strictEqual(code, 1); // terminated
+  assert.strictEqual(checksCompleted, totalChecks);
+}));