reimplement errname lookup.

Fixes sindresorhus#31

index.js

@@ -8,6 +8,7 @@ var npmRunPath = require('npm-run-path');
 var isStream = require('is-stream');
 var _getStream = require('get-stream');
 var pathKey = require('path-key')();
+var errname = require('./lib/errname');
 var TEN_MEBIBYTE = 1024 * 1024 * 10;
 
 function handleArgs(cmd, args, opts) {
@@ -149,10 +150,7 @@ module.exports = function (cmd, args, opts) {
 				// err.killed = spawned.killed || killed;
 				err.killed = spawned.killed;
 
-				// TODO: child_process applies the following logic for resolving the code:
-				// var uv = process.bind('uv');
-				// ex.code = code < 0 ? uv.errname(code) : code;
-				err.code = code;
+				err.code = code < 0 ? errname(code) : code;
 			}
 
 			err.stdout = stdout;

lib/errname.js

@@ -0,0 +1,38 @@
+'use strict';
+// The Node team wants to deprecate `process.bind(...)`.
+//   https://github.com/nodejs/node/pull/2768
+//
+// However, we need the 'uv' binding for errname support.
+// This is a defensive wrapper around it so `execa` will not fail entirely if it stops working someday.
+//
+// If this ever stops working. See: https://github.com/sindresorhus/execa/issues/31#issuecomment-215939939 for another possible solution.
+var uv;
+
+try {
+	uv = process.binding('uv');
+	if (typeof uv.errname !== 'function') {
+		throw new Error('uv.errname is not a function');
+	}
+} catch (e) {
+	console.error('execa/lib/errname: unable to establish process.binding("uv")', e);
+	uv = null;
+}
+
+function errname(uv, code) {
+	if (uv) {
+		return uv.errname(code);
+	}
+
+	if (!(code < 0)) {
+		throw new Error('err >= 0');
+	}
+
+	return 'UNKNOWN CODE: ' + code;
+}
+
+module.exports = function getErrname(code) {
+	return errname(uv, code);
+};
+
+// used for testing the fallback behavior.
+module.exports._test = errname;

test/errname.js

@@ -0,0 +1,23 @@
+import test from 'ava';
+import errname from '../lib/errname';
+
+// simulates failure to capture process.binding('uv');
+function fallback(code) {
+	return errname._test(null, code);
+}
+
+function makeTests(name, m, expected) {
+	test(name, t => {
+		// throws >= 0
+		t.throws(() => m(0), /err >= 0/);
+		t.throws(() => m(1), /err >= 0/);
+		t.throws(() => m('2'), /err >= 0/);
+		t.throws(() => m('foo'), /err >= 0/);
+
+		t.is(m(-2), expected);
+		t.is(m('-2'), expected);
+	});
+}
+
+makeTests('native', errname, 'ENOENT');
+makeTests('fallback', fallback, 'UNKNOWN CODE: -2');