Fix async early exceptions

Author: ehmicky

index.js

@@ -12,6 +12,7 @@ const mergeStream = require('merge-stream');
 const pFinally = require('p-finally');
 const onExit = require('signal-exit');
 const stdio = require('./lib/stdio');
+const mergePrototypes = require('./lib/merge');
 
 const TEN_MEGABYTES = 1000 * 1000 * 10;
 
@@ -247,13 +248,16 @@ const execa = (file, args, options) => {
 	try {
 		spawned = childProcess.spawn(parsed.file, parsed.args, parsed.options);
 	} catch (error) {
-		return Promise.reject(makeError({error, stdout: '', stderr: '', all: ''}, {
+		const promise = Promise.reject(makeError({error, stdout: '', stderr: '', all: ''}, {
 			joinedCommand,
 			parsed,
 			timedOut: false,
 			isCanceled: false,
 			killed: false
 		}));
+		// We make sure `child_process` properties are present even though no child process was created.
+		// This is to ensure the return value always has the same shape.
+		return mergePrototypes(promise, new childProcess.ChildProcess());
 	}
 
 	const kill = spawned.kill.bind(spawned);

lib/merge.js

@@ -0,0 +1,24 @@
+'use strict';
+
+// Merge two objects, including their prototypes
+function mergePrototypes(to, from) {
+	const prototypes = [...getPrototypes(to), ...getPrototypes(from)];
+	const newPrototype = prototypes.reduce(reducePrototype, {});
+	return Object.assign(Object.setPrototypeOf(to, newPrototype), to, from);
+}
+
+function getPrototypes(object, prototypes = []) {
+	const prototype = Object.getPrototypeOf(object);
+	if (prototype !== null) {
+		return getPrototypes(prototype, [...prototypes, prototype]);
+	}
+
+	return prototypes;
+}
+
+function reducePrototype(prototype, constructor) {
+	return Object.defineProperties(prototype, Object.getOwnPropertyDescriptors(constructor));
+}
+
+module.exports = mergePrototypes;
+

test.js

@@ -288,6 +288,15 @@ test('execa() returns a promise with kill() and pid', t => {
 	t.is(typeof pid, 'number');
 });
 
+test('child_process.spawn() propagated errors have correct shape', t => {
+	const cp = execa('noop', {uid: -1});
+	t.notThrows(() => {
+		cp.catch(() => {});
+		cp.unref();
+		cp.on('error', () => {});
+	});
+});
+
 test('child_process.spawn() errors are propagated', async t => {
 	const {exitCodeName} = await t.throwsAsync(execa('noop', {uid: -1}));
 	t.is(exitCodeName, process.platform === 'win32' ? 'ENOTSUP' : 'EINVAL');