child_process: validate shell option in normalizeExecArgs()

- narrow validation type to string (previously de facto not validated)
- ensure empty string is coerced to true
- add test cases for options.shell

Author: Renegade334

lib/child_process.js

@@ -196,7 +196,12 @@ function normalizeExecArgs(command, options, callback) {
 
   // Make a shallow copy so we don't clobber the user's options object.
   options = { __proto__: null, ...options };
-  options.shell = typeof options.shell === 'string' ? options.shell : true;
+
+  // Validate the shell, if present, and ensure a truthy value.
+  if (options.shell != null) {
+    validateString(options.shell, 'options.shell');
+  }
+  options.shell ||= true;
 
   return {
     file: command,

test/parallel/test-child-process-exec-any-shells-windows.js

@@ -33,7 +33,7 @@ const testCopy = (shellName, shellPath) => {
 const system32 = `${process.env.SystemRoot}\\System32`;
 
 // Test CMD
-test(true);
+test();
 test('cmd');
 testCopy('cmd.exe', `${system32}\\cmd.exe`);
 test('cmd.exe');

test/parallel/test-child-process-exec-enforce-shell.js

@@ -0,0 +1,23 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const { exec, execSync } = require('child_process');
+
+const invalidArgTypeError = {
+  code: 'ERR_INVALID_ARG_TYPE',
+  name: 'TypeError'
+};
+
+exec('echo should-be-passed-as-argument', { shell: '' }, common.mustSucceed((stdout, stderr) => {
+  assert.match(stdout, /should-be-passed-as-argument/);
+  assert.ok(!stderr);
+}));
+
+{
+  const ret = execSync('echo should-be-passed-as-argument', { encoding: 'utf-8', shell: '' });
+  assert.match(ret, /should-be-passed-as-argument/);
+}
+
+for (const fn of [exec, execSync]) {
+  assert.throws(() => fn('should-throw-on-boolean-shell-option', { shell: false }), invalidArgTypeError);
+}