Bluebird.coroutine() is sometimes synchronous

[STRML]

This is with latest Bluebird (3.4.1) and Babel (6.11.4).

Many Babel users are using Bluebird.coroutine() in combination with transform-async-to-module-method to have Bluebird shim async/await.

Unfortunately, it's not a perfect shim. The following code:

async function foo() {
  await syncFn();
  console.log(3);
}
async function syncFn() {
  return;
}
console.log(1);
foo();
console.log(2);

prints 1 3 2 with Bluebird.coroutine() and 1 2 3 with the default transform-async-to-generator method.

I've created this Phabricator ticket to track but it appears the issue lies here.

The above code transpiles to this, which I have cleaned up for readability:

var Bluebird = require('bluebird');

function foo() {
  return Bluebird.coroutine(function* () {
    yield syncFn();
    console.log(3);
  })();
}

function syncFn() {
  return Bluebird.coroutine(function* () {
    return;
  })();
}

console.log(1);
foo();
console.log(2);

This is easily runnable in any Node runtime that supports generators:

$ node syncCoroutines.js
1
3
2

Is this intended behavior?

[spion]

Looks like a bug.

[spion]

Minimal test case:

var Bluebird = require('bluebird');
var foo = Bluebird.coroutine(function* () {
    yield Bluebird.resolve();
    console.log(3);
})
console.log(1);
foo();
console.log(2);

Basically, "yield" will synchronously inspect the promise and continue execution immediately if its already resolved. Since no handlers are added to the promise in this case, the async trampoline is completely avoided.

[overlookmotel]

Here's a very hacky workaround for until this gets fixed:

var shimmer = require('shimmer');
var AltPromise = Promise.getNewLibraryCopy();

shimmer.wrap(Promise, 'coroutine', function(original) {
    return function(generatorFunction, options) {
        if (typeof generatorFunction === 'function') {
            var generatorFunctionOriginal = generatorFunction;
            generatorFunction = function() {
                var generator = generatorFunctionOriginal.apply(this, arguments);
                var next = generator.next;
                generator.next = function() {
                    var ret = next.apply(this, arguments);
                    if (ret.value instanceof Promise && !ret.value.isPending()) ret.value = AltPromise.resolve(ret.value);
                    return ret;
                };
                return generator;
            };
        }

        return original.call(this, generatorFunction, options);
    };
});

(the problem only occurs when the yielded promise is an instance of the main bluebird Promise constructor)

[overlookmotel]

Amazing you guys fixed this so fast. Any idea when it's likely to get pushed to npm?