util: add util.promisify()

Add `util.promisify(function)` for creating promisified functions.
Includes documentation and tests.

Fixes: nodejs/CTC#12
PR-URL: #12442
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: William Kapke <william.kapke@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>

Author: addaleax

doc/api/util.md

@@ -399,6 +399,86 @@ util.inspect.defaultOptions.maxArrayLength = null;
 console.log(arr); // logs the full array
 ```
 
+## util.promisify(original)
+<!-- YAML
+added: REPLACEME
+-->
+
+* `original` {Function}
+
+Takes a function following the common Node.js callback style, i.e. taking a
+`(err, value) => ...` callback as the last argument, and returns a version
+that returns promises.
+
+For example:
+
+```js
+const util = require('util');
+const fs = require('fs');
+
+const stat = util.promisify(fs.stat);
+stat('.').then((stats) => {
+  // Do something with `stats`
+}).catch((error) => {
+  // Handle the error.
+});
+```
+
+Or, equivalently using `async function`s:
+
+```js
+const util = require('util');
+const fs = require('fs');
+
+const stat = util.promisify(fs.stat);
+
+async function callStat() {
+  const stats = await stat('.');
+  console.log(`This directory is owned by ${stats.uid}`);
+}
+```
+
+If there is an `original[util.promisify.custom]` property present, `promisify`
+will return its value, see [Custom promisified functions][].
+
+`promisify()` assumes that `original` is a function taking a callback as its
+final argument in all cases, and the returned function will result in undefined
+behaviour if it does not.
+
+### Custom promisified functions
+
+Using the `util.promisify.custom` symbol one can override the return value of
+[`util.promisify()`][]:
+
+```js
+const util = require('util');
+
+function doSomething(foo, callback) {
+  // ...
+}
+
+doSomething[util.promisify.custom] = function(foo) {
+  return getPromiseSomehow();
+};
+
+const promisified = util.promisify(doSomething);
+console.log(promisified === doSomething[util.promisify.custom]);
+  // prints 'true'
+```
+
+This can be useful for cases where the original function does not follow the
+standard format of taking an error-first callback as the last argument.
+
+### util.promisify.custom
+<!-- YAML
+added: REPLACEME
+-->
+
+* {symbol}
+
+A Symbol that can be used to declare custom promisified variants of functions,
+see [Custom promisified functions][].
+
 ## Deprecated APIs
 
 The following APIs have been deprecated and should no longer be used. Existing
@@ -878,7 +958,9 @@ Deprecated predecessor of `console.log`.
 [`console.error()`]: console.html#console_console_error_data_args
 [`console.log()`]: console.html#console_console_log_data_args
 [`util.inspect()`]: #util_util_inspect_object_options
+[`util.promisify()`]: #util_util_promisify_original
 [Custom inspection functions on Objects]: #util_custom_inspection_functions_on_objects
 [Customizing `util.inspect` colors]: #util_customizing_util_inspect_colors
+[Custom promisified functions]: #util_custom_promisified_functions
 [constructor]: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/constructor
 [semantically incompatible]: https://github.com/nodejs/node/issues/4179

lib/internal/util.js

@@ -4,6 +4,8 @@ const errors = require('internal/errors');
 const binding = process.binding('util');
 const signals = process.binding('constants').os.signals;
 
+const { createPromise, promiseResolve, promiseReject } = binding;
+
 const kArrowMessagePrivateSymbolIndex = binding['arrow_message_private_symbol'];
 const kDecoratedPrivateSymbolIndex = binding['decorated_private_symbol'];
 const noCrypto = !process.versions.openssl;
@@ -217,3 +219,62 @@ module.exports = exports = {
   // default isEncoding implementation, just in case userland overrides it.
   kIsEncodingSymbol: Symbol('node.isEncoding')
 };
+
+const kCustomPromisifiedSymbol = Symbol('util.promisify.custom');
+const kCustomPromisifyArgsSymbol = Symbol('customPromisifyArgs');
+
+function promisify(orig) {
+  if (typeof orig !== 'function') {
+    const errors = require('internal/errors');
+    throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'original', 'function');
+  }
+
+  if (orig[kCustomPromisifiedSymbol]) {
+    const fn = orig[kCustomPromisifiedSymbol];
+    if (typeof fn !== 'function') {
+      throw new TypeError('The [util.promisify.custom] property must be ' +
+                          'a function');
+    }
+    Object.defineProperty(fn, kCustomPromisifiedSymbol, {
+      value: fn, enumerable: false, writable: false, configurable: true
+    });
+    return fn;
+  }
+
+  // Names to create an object from in case the callback receives multiple
+  // arguments, e.g. ['stdout', 'stderr'] for child_process.exec.
+  const argumentNames = orig[kCustomPromisifyArgsSymbol];
+
+  function fn(...args) {
+    const promise = createPromise();
+    try {
+      orig.call(this, ...args, (err, ...values) => {
+        if (err) {
+          promiseReject(promise, err);
+        } else if (argumentNames !== undefined && values.length > 1) {
+          const obj = {};
+          for (var i = 0; i < argumentNames.length; i++)
+            obj[argumentNames[i]] = values[i];
+          promiseResolve(promise, obj);
+        } else {
+          promiseResolve(promise, values[0]);
+        }
+      });
+    } catch (err) {
+      promiseReject(promise, err);
+    }
+    return promise;
+  }
+
+  Object.setPrototypeOf(fn, Object.getPrototypeOf(orig));
+
+  Object.defineProperty(fn, kCustomPromisifiedSymbol, {
+    value: fn, enumerable: false, writable: false, configurable: true
+  });
+  return Object.defineProperties(fn, Object.getOwnPropertyDescriptors(orig));
+}
+
+promisify.custom = kCustomPromisifiedSymbol;
+
+exports.promisify = promisify;
+exports.customPromisifyArgs = kCustomPromisifyArgsSymbol;

lib/util.js

@@ -1057,3 +1057,5 @@ exports._exceptionWithHostPort = function(err,
 // process.versions needs a custom function as some values are lazy-evaluated.
 process.versions[exports.inspect.custom] =
   (depth) => exports.format(JSON.parse(JSON.stringify(process.versions)));
+
+exports.promisify = internalUtil.promisify;

src/node_util.cc

@@ -21,6 +21,7 @@ using v8::Value;
 
 
 #define VALUE_METHOD_MAP(V)                                                   \
+  V(isAsyncFunction, IsAsyncFunction)                                         \
   V(isDataView, IsDataView)                                                   \
   V(isDate, IsDate)                                                           \
   V(isExternal, IsExternal)                                                   \

test/parallel/test-util-promisify.js

@@ -0,0 +1,76 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const vm = require('vm');
+const { promisify } = require('util');
+
+common.crashOnUnhandledRejection();
+
+const stat = promisify(fs.stat);
+
+{
+  const promise = stat(__filename);
+  assert(promise instanceof Promise);
+  promise.then(common.mustCall((value) => {
+    assert.deepStrictEqual(value, fs.statSync(__filename));
+  }));
+}
+
+{
+  const promise = stat('/dontexist');
+  promise.catch(common.mustCall((error) => {
+    assert(error.message.includes('ENOENT: no such file or directory, stat'));
+  }));
+}
+
+{
+  function fn() {}
+  function promisifedFn() {}
+  fn[promisify.custom] = promisifedFn;
+  assert.strictEqual(promisify(fn), promisifedFn);
+  assert.strictEqual(promisify(promisify(fn)), promisifedFn);
+}
+
+{
+  function fn() {}
+  fn[promisify.custom] = 42;
+  assert.throws(
+      () => promisify(fn),
+      (err) => err instanceof TypeError &&
+                err.message === 'The [util.promisify.custom] property must ' +
+                                'be a function');
+}
+
+{
+  const fn = vm.runInNewContext('(function() {})');
+  assert.notStrictEqual(Object.getPrototypeOf(promisify(fn)),
+                        Function.prototype);
+}
+
+{
+  function fn(callback) {
+    callback(null, 'foo', 'bar');
+  }
+  promisify(fn)().then(common.mustCall((value) => {
+    assert.deepStrictEqual(value, 'foo');
+  }));
+}
+
+{
+  function fn(callback) {
+    callback(null);
+  }
+  promisify(fn)().then(common.mustCall((value) => {
+    assert.strictEqual(value, undefined);
+  }));
+}
+
+{
+  function fn(callback) {
+    callback();
+  }
+  promisify(fn)().then(common.mustCall((value) => {
+    assert.strictEqual(value, undefined);
+  }));
+}