doc: require --vfs-load in a worker's execArgv

The worker paragraph for --vfs-load said only that a worker created with
its own execArgv "has to be given --vfs-load again", which reads as a
remark rather than as the requirement it is: such a worker inherits none
of the parent's options, so it does not mount the source at all, and a
script in the mount cannot be its entry point.

Say that a worker given its own execArgv must carry the same --vfs-load
to run a script from the mount, and that a worker whose script comes
from elsewhere needs nothing added. Cover the requirement with a test:
the same worker that runs with the flag fails to load without it.

Signed-off-by: Philipp Dunkel <pip@pipobscure.com>

Author: pipobscure

doc/api/cli.md

@@ -3822,10 +3822,16 @@ provider claims the source, Node.js exits with an error.
 In worker threads `--vfs-load` mounts but does not load: a worker inherits the
 mount and runs its own entry point, which may itself live in the mount.
 
-The source is mounted at the same reserved mount point in every thread,
-whatever else that thread mounts, so a path into it stays valid in a worker -
-including one created with its own `execArgv`, which does not inherit the
-parent's options and has to be given `--vfs-load` again.
+The source is mounted at the same reserved mount point in every thread that
+mounts it, whatever else that thread mounts, so a path into the mount means the
+same thing in all of them.
+
+A worker created with its own `execArgv` inherits none of the parent's options,
+and so does not mount the source at all. Such a worker must be given the same
+`--vfs-load` to run a script from the mount; without it, that thread has no
+mount for the script to come from, and the worker fails to load it. A worker
+whose script comes from anywhere else, such as the real file system, needs
+nothing added.
 
 `--vfs-load` is not permitted in [`NODE_OPTIONS`][]: which entry point runs is
 the command line's decision, and the environment must not be able to redirect

doc/node.1

@@ -1913,10 +1913,15 @@ reverse registration order, and may claim directories as well as files. If no
 provider claims the source, Node.js exits with an error.
 In worker threads \fB--vfs-load\fR mounts but does not load: a worker inherits the
 mount and runs its own entry point, which may itself live in the mount.
-The source is mounted at the same reserved mount point in every thread,
-whatever else that thread mounts, so a path into it stays valid in a worker -
-including one created with its own \fBexecArgv\fR, which does not inherit the
-parent's options and has to be given \fB--vfs-load\fR again.
+The source is mounted at the same reserved mount point in every thread that
+mounts it, whatever else that thread mounts, so a path into the mount means the
+same thing in all of them.
+A worker created with its own \fBexecArgv\fR inherits none of the parent's options,
+and so does not mount the source at all. Such a worker must be given the same
+\fB--vfs-load\fR to run a script from the mount; without it, that thread has no
+mount for the script to come from, and the worker fails to load it. A worker
+whose script comes from anywhere else, such as the real file system, needs
+nothing added.
 \fB--vfs-load\fR is not permitted in \fBNODE_OPTIONS\fR: which entry point runs is
 the command line's decision, and the environment must not be able to redirect
 it.

test/parallel/test-vfs-load.js

@@ -227,6 +227,29 @@ require('node:vfs').create().mount();
   assert.strictEqual(seen.size, 1, [...seen].join());
 }
 
+// Without that flag the worker has no mount to load from, so a script in the
+// mount cannot be its entry point.
+{
+  const dir = fixture('worker-execargv-missing');
+  fs.mkdirSync(dir, { recursive: true });
+  fs.writeFileSync(path.join(dir, 'index.js'), `
+'use strict';
+const path = require('path');
+const { Worker } = require('worker_threads');
+const w = new Worker(path.join(__dirname, 'worker.js'), { execArgv: [] });
+w.on('message', (m) => { console.log('ran:' + m); process.exit(0); });
+w.on('error', (e) => { console.log('failed:' + e.code); process.exit(0); });
+`);
+  fs.writeFileSync(path.join(dir, 'worker.js'), `
+'use strict';
+require('worker_threads').parentPort.postMessage('unexpected');
+`);
+  const res = run([`--vfs-load=${dir}`]);
+  assert.strictEqual(res.status, 0, res.stderr);
+  assert.match(res.stdout, /failed:/);
+  assert.doesNotMatch(res.stdout, /ran:/);
+}
+
 // --vfs-load names the source it loads, so it always takes a value.
 {
   const res = run(['--vfs-load']);