lib: do not crash using workers with disabled shared array buffers

This allows the repl to function normally while using the
`--no-harmony-sharedarraybuffer` V8 flag.

Fixes: #39717

Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>

Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>

benchmark/worker/atomics-wait.js

@@ -1,6 +1,10 @@
 'use strict';
 /* global SharedArrayBuffer */
 
+if (typeof SharedArrayBuffer === 'undefined') {
+  throw new Error('SharedArrayBuffers must be enabled to run this benchmark');
+}
+
 const common = require('../common.js');
 const bench = common.createBenchmark(main, {
   n: [1e7]

lib/internal/main/worker_thread.js

@@ -9,7 +9,10 @@ const {
   ArrayPrototypeSplice,
   ObjectDefineProperty,
   PromisePrototypeCatch,
-  globalThis: { Atomics },
+  globalThis: {
+    Atomics,
+    SharedArrayBuffer
+  },
 } = primordials;
 
 const {
@@ -135,21 +138,23 @@ port.on('message', (message) => {
 
     require('internal/worker').assignEnvironmentData(environmentData);
 
-    // The counter is only passed to the workers created by the main thread, not
-    // to workers created by other workers.
-    let cachedCwd = '';
-    let lastCounter = -1;
-    const originalCwd = process.cwd;
-
-    process.cwd = function() {
-      const currentCounter = Atomics.load(cwdCounter, 0);
-      if (currentCounter === lastCounter)
+    if (typeof SharedArrayBuffer !== 'undefined') {
+      // The counter is only passed to the workers created by the main thread,
+      // not to workers created by other workers.
+      let cachedCwd = '';
+      let lastCounter = -1;
+      const originalCwd = process.cwd;
+
+      process.cwd = function() {
+        const currentCounter = Atomics.load(cwdCounter, 0);
+        if (currentCounter === lastCounter)
+          return cachedCwd;
+        lastCounter = currentCounter;
+        cachedCwd = originalCwd();
         return cachedCwd;
-      lastCounter = currentCounter;
-      cachedCwd = originalCwd();
-      return cachedCwd;
-    };
-    workerIo.sharedCwdCounter = cwdCounter;
+      };
+      workerIo.sharedCwdCounter = cwdCounter;
+    }
 
     const CJSLoader = require('internal/modules/cjs/loader');
     assert(!CJSLoader.hasLoadedAnyUserCJSModule);

lib/internal/worker.js

@@ -90,7 +90,8 @@ let cwdCounter;
 
 const environmentData = new SafeMap();
 
-if (isMainThread) {
+// SharedArrayBuffers can be disabled with --no-harmony-sharedarraybuffer.
+if (isMainThread && typeof SharedArrayBuffer !== 'undefined') {
   cwdCounter = new Uint32Array(new SharedArrayBuffer(4));
   const originalChdir = process.chdir;
   process.chdir = function(path) {

test/parallel/test-worker-no-sab.js

@@ -0,0 +1,21 @@
+// Flags: --no-harmony-sharedarraybuffer
+
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { Worker } = require('worker_threads');
+
+// Regression test for https://github.com/nodejs/node/issues/39717.
+
+// Do not use isMainThread so that this test itself can be run inside a Worker.
+if (!process.env.HAS_STARTED_WORKER) {
+  process.env.HAS_STARTED_WORKER = 1;
+  const w = new Worker(__filename);
+
+  w.on('exit', common.mustCall((status) => {
+    assert.strictEqual(status, 2);
+  }));
+} else {
+  process.exit(2);
+}