util: add callbackify

Add `util.callbackify(function)` for creating callback style functions
from functions returning a `Thenable`

PR-URL: https://github.com/nodejs/node/pull/12712
Fixes: https://github.com/nodejs/CTC/issues/109
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

2 files changed, 51 insertions, 0 deletions

diff --git a/lib/internal/errors.js b/lib/internal/errors.js
index 57e73ac56a..1f1dd82c37 100644
--- a/lib/internal/errors.js
+++ b/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) {
diff --git a/lib/util.js b/lib/util.js
index 3c31e0932a..11dd521668 100644
--- a/lib/util.js
+++ b/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;