util: add util.callbackify()

doc/api/util.md

@@ -10,6 +10,64 @@ module developers as well. It can be accessed using:
 const util = require('util');
 ```
 
+## util.callbackify(original)
+<!-- YAML
+added: REPLACEME
+-->
+
+* `original` {Function} An `async` function
+* Returns: {Function} a callback style function
+
+Takes an `async` function (or a function that returns a Promise) and returns a
+function following the Node.js error first callback style. In the callback, the
+first argument will be the rejection reason (or `null` if the Promise resolved),
+and the second argument will be the resolved value.
+
+For example:
+
+```js
+const util = require('util');
+
+async function fn() {
+  return await Promise.resolve('hello world');
+}
+const callbackFunction = util.callbackify(fn);
+
+callbackFunction((err, ret) => {
+  if (err) throw err;
+  console.log(ret);
+});
+```
+
+Will print:
+
+```txt
+hello world
+```
+
+*Note*:
+
+* The callback is executed asynchronously, and will have a limited stack trace.
+If the callback throws, the process will emit an [`'uncaughtException'`][]
+event, and if not handled will exit.
+
+* Since `null` has a special meaning as the first argument to a callback, if a
+wrapped function rejects a `Promise` with a falsy value as a reason, the value
+is wrapped in an `Error` with the original value stored in a field named
+`reason`.
+  ```js
+  function fn() {
+    return Promise.reject(null);
+  }
+  const callbackFunction = util.callbackify(fn);
+
+  callbackFunction((err, ret) => {
+    // When the Promise was rejected with `null` it is wrapped with an Error and
+    // the original value is stored in `reason`.
+    err && err.hasOwnProperty('reason') && err.reason === null;  // true
+  });
+  ```
+
 ## util.debuglog(section)
 <!-- YAML
 added: v0.11.3
@@ -955,6 +1013,7 @@ Deprecated predecessor of `console.log`.
 [`Object.assign()`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
 [`console.error()`]: console.html#console_console_error_data_args
 [`console.log()`]: console.html#console_console_log_data_args
+[`'uncaughtException'`]: process.html#process_event_uncaughtexception
 [`util.inspect()`]: #util_util_inspect_object_options
 [`util.promisify()`]: #util_util_promisify_original
 [Custom inspection functions on Objects]: #util_custom_inspection_functions_on_objects

lib/internal/errors.js

@@ -164,6 +164,7 @@ E('ERR_SOCKET_BAD_PORT', 'Port should be > 0 and < 65536');
 E('ERR_SOCKET_DGRAM_NOT_RUNNING', 'Not running');
 E('ERR_V8BREAKITERATOR', 'full ICU data not installed. ' +
   'See https://github.com/nodejs/node/wiki/Intl');
+E('FALSY_VALUE_REJECTION', 'Promise was rejected with falsy value');
 // Add new errors from here...
 
 function invalidArgType(name, expected, actual) {

lib/util.js

@@ -1047,3 +1047,53 @@ process.versions[exports.inspect.custom] =
   (depth) => exports.format(JSON.parse(JSON.stringify(process.versions)));
 
 exports.promisify = internalUtil.promisify;
+
+function callbackifyOnRejected(reason, cb) {
+  // `!reason` guard inspired by bluebird (Ref: https://goo.gl/t5IS6M).
+  // Because `null` is a special error value in callbacks which means "no error
+  // occurred", we error-wrap so the callback consumer can distinguish between
+  // "the promise rejected with null" or "the promise fulfilled with undefined".
+  if (!reason) {
+    const newReason = new errors.Error('FALSY_VALUE_REJECTION');
+    newReason.reason = reason;
+    reason = newReason;
+    Error.captureStackTrace(reason, callbackifyOnRejected);
+  }
+  return cb(reason);
+}
+
+
+function callbackify(original) {
+  if (typeof original !== 'function') {
+    throw new errors.TypeError(
+      'ERR_INVALID_ARG_TYPE',
+      'original',
+      'function');
+  }
+
+  // We DO NOT return the promise as it gives the user a false sense that
+  // the promise is actually somehow related to the callback's execution
+  // and that the callback throwing will reject the promise.
+  function callbackified(...args) {
+    const maybeCb = args.pop();
+    if (typeof maybeCb !== 'function') {
+      throw new errors.TypeError(
+        'ERR_INVALID_ARG_TYPE',
+        'last argument',
+        'function');
+    }
+    const cb = (...args) => { Reflect.apply(maybeCb, this, args); };
+    // In true node style we process the callback on `nextTick` with all the
+    // implications (stack, `uncaughtException`, `async_hooks`)
+    Reflect.apply(original, this, args)
+      .then((ret) => process.nextTick(cb, null, ret),
+            (rej) => process.nextTick(callbackifyOnRejected, rej, cb));
+  }
+
+  Object.setPrototypeOf(callbackified, Object.getPrototypeOf(original));
+  Object.defineProperties(callbackified,
+                          Object.getOwnPropertyDescriptors(original));
+  return callbackified;
+}
+
+exports.callbackify = callbackify;

test/fixtures/uncaught-exceptions/callbackify1.js

@@ -0,0 +1,15 @@
+'use strict';
+
+// Used to test that `uncaughtException` is emitted
+
+const { callbackify } = require('util');
+
+{
+  async function fn() { }
+
+  const cbFn = callbackify(fn);
+
+  cbFn((err, ret) => {
+    throw new Error(__filename);
+  });
+}

test/fixtures/uncaught-exceptions/callbackify2.js

@@ -0,0 +1,22 @@
+'use strict';
+
+// Used to test the `uncaughtException` err object
+
+const assert = require('assert');
+const { callbackify } = require('util');
+
+{
+  const sentinel = new Error(__filename);
+  process.once('uncaughtException', (err) => {
+    assert.strictEqual(err, sentinel);
+    // Calling test will use `stdout` to assert value of `err.message`
+    console.log(err.message);
+  });
+
+  async function fn() {
+    return await Promise.reject(sentinel);
+  }
+
+  const cbFn = callbackify(fn);
+  cbFn((err, ret) => assert.ifError(err));
+}