test: add test for Module._stat

Module._stat landed in nodejs#44537 without
a test, so this change adds one.

Signed-off-by: Darshan Sen <raisinten@gmail.com>

test/parallel/test-vfs.js

@@ -0,0 +1,85 @@
+'use strict';
+const common = require('../common');
+
+// This tests the creation of a vfs by monkey-patching fs and Module._stat.
+
+const Module = require('module');
+const fs = require('fs');
+const tmpdir = require('../common/tmpdir');
+const { deepStrictEqual, match, ok, strictEqual, throws } = require('assert');
+const { join } = require('path');
+
+const directory = join(tmpdir.path, 'directory');
+const doesNotExist = join(tmpdir.path, 'does-not-exist');
+const file = join(tmpdir.path, 'file.js');
+
+tmpdir.refresh();
+fs.writeFileSync(file, "module.exports = { a: 'b' }");
+fs.mkdirSync(directory);
+
+strictEqual(Module._stat(directory), 1);
+ok(Module._stat(doesNotExist) < 0);
+strictEqual(Module._stat(file), 0);
+
+const vfsDirectory = join(process.execPath, 'directory');
+const vfsDoesNotExist = join(process.execPath, 'does-not-exist');
+const vfsFile = join(process.execPath, 'file.js');
+
+ok(Module._stat(vfsDirectory) < 0);
+ok(Module._stat(vfsDoesNotExist) < 0);
+ok(Module._stat(vfsFile) < 0);
+
+deepStrictEqual(require(file), { a: 'b' });
+throws(() => require(vfsFile), { code: 'MODULE_NOT_FOUND' });
+
+process.on('warning', common.mustCall((warning) => {
+  match(warning.message, /is an experimental feature/);
+}, 1));
+
+const originalStat = Module._stat;
+Module._stat = function(filename) {
+  if (!filename.startsWith(process.execPath)) {
+    return originalStat(filename);
+  }
+
+  if (filename === process.execPath) {
+    return 1;
+  }
+
+  switch (filename) {
+    case vfsDirectory:
+      return 1;
+    case vfsDoesNotExist:
+      return -2;
+    case vfsFile:
+      return 0;
+  }
+};
+
+const originalReadFileSync = fs.readFileSync;
+fs.readFileSync = function readFileSync(pathArgument, options) {
+  if (!pathArgument.startsWith(process.execPath)) {
+    return originalReadFileSync.apply(this, arguments);
+  }
+  if (pathArgument === vfsFile) {
+    return "module.exports = { x: 'y' };";
+  }
+  throw new Error();
+};
+
+fs.realpathSync = function realpathSync(pathArgument, options) {
+  return pathArgument;
+};
+
+strictEqual(Module._stat(directory), 1);
+ok(Module._stat(doesNotExist) < 0);
+strictEqual(Module._stat(file), 0);
+
+strictEqual(Module._stat(vfsDirectory), 1);
+ok(Module._stat(vfsDoesNotExist) < 0);
+strictEqual(Module._stat(vfsFile), 0);
+
+strictEqual(Module._stat(process.execPath), 1);
+
+deepStrictEqual(require(file), { a: 'b' });
+deepStrictEqual(require(vfsFile), { x: 'y' });