workers: fix spawning from preload scripts

Fix spawning nested worker threads from preload scripts and
warn about doing so.

Signed-off-by: James M Snell <jasnell@gmail.com>
Fixes: #36531

PR-URL: #37481
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/worker_threads.md

@@ -1127,6 +1127,17 @@ Calling `unref()` on a worker allows the thread to exit if this is the only
 active handle in the event system. If the worker is already `unref()`ed calling
 `unref()` again has no effect.
 
+## Notes
+
+### Launching worker threads from preload scripts
+
+Take care when launching worker threads from preload scripts (scripts loaded
+and run using the `-r` command line flag). Unless the `execArgv` option is
+explicitly set, new Worker threads automatically inherit the command line flags
+from the running process and will preload the same preload scripts as the main
+thread. If the preload script unconditionally launches a worker thread, every
+thread spawned will spawn another until the application crashes.
+
 [Addons worker support]: addons.md#addons_worker_support
 [ECMAScript module loader]: esm.md#esm_data_imports
 [HTML structured clone algorithm]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm

lib/internal/main/worker_thread.js

@@ -121,10 +121,6 @@ port.on('message', (message) => {
     initializeCJSLoader();
     initializeESMLoader();
 
-    const CJSLoader = require('internal/modules/cjs/loader');
-    assert(!CJSLoader.hasLoadedAnyUserCJSModule);
-    loadPreloadModules();
-    initializeFrozenIntrinsics();
     if (argv !== undefined) {
       ArrayPrototypePushApply(process.argv, argv);
     }
@@ -147,6 +143,11 @@ port.on('message', (message) => {
     };
     workerIo.sharedCwdCounter = cwdCounter;
 
+    const CJSLoader = require('internal/modules/cjs/loader');
+    assert(!CJSLoader.hasLoadedAnyUserCJSModule);
+    loadPreloadModules();
+    initializeFrozenIntrinsics();
+
     if (!hasStdin)
       process.stdin.push(null);
 

test/fixtures/worker-preload.js

@@ -0,0 +1,9 @@
+const {
+  Worker,
+  workerData,
+  threadId
+} = require('worker_threads');
+
+if (threadId < 2) {
+  new Worker('1 + 1', { eval: true });
+}

test/parallel/test-preload-worker.js

@@ -0,0 +1,10 @@
+'use strict';
+
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const worker = fixtures.path('worker-preload.js');
+const { exec } = require('child_process');
+const kNodeBinary = process.argv[0];
+
+
+exec(`"${kNodeBinary}" -r "${worker}" -pe "1+1"`, common.mustSucceed());