child_process: handle undefined/null for fork() args

PR-URL: #22416
Fixes: #20749
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>

lib/child_process.js

@@ -69,6 +69,11 @@ exports.fork = function fork(modulePath /* , args, options */) {
     args = arguments[pos++];
   }
 
+  if (pos < arguments.length &&
+      (arguments[pos] === undefined || arguments[pos] === null)) {
+    pos++;
+  }
+
   if (pos < arguments.length && arguments[pos] != null) {
     if (typeof arguments[pos] !== 'object') {
       throw new ERR_INVALID_ARG_VALUE(`arguments[${pos}]`, arguments[pos]);

test/fixtures/child-process-echo-options.js

@@ -0,0 +1 @@
+process.send({ env: process.env });

test/parallel/test-child-process-fork-options.js

@@ -0,0 +1,37 @@
+'use strict';
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+
+// This test ensures that fork should parse options
+// correctly if args is undefined or null
+
+const assert = require('assert');
+const { fork } = require('child_process');
+
+const expectedEnv = { foo: 'bar' };
+
+{
+  const cp = fork(fixtures.path('child-process-echo-options.js'), undefined,
+                  { env: Object.assign({}, process.env, expectedEnv) });
+
+  cp.on('message', common.mustCall(({ env }) => {
+    assert.strictEqual(env.foo, expectedEnv.foo);
+  }));
+
+  cp.on('exit', common.mustCall((code) => {
+    assert.strictEqual(code, 0);
+  }));
+}
+
+{
+  const cp = fork(fixtures.path('child-process-echo-options.js'), null,
+                  { env: Object.assign({}, process.env, expectedEnv) });
+
+  cp.on('message', common.mustCall(({ env }) => {
+    assert.strictEqual(env.foo, expectedEnv.foo);
+  }));
+
+  cp.on('exit', common.mustCall((code) => {
+    assert.strictEqual(code, 0);
+  }));
+}