Middleware does not resolve when parallel is followed by serial and both are async

[JedWatson]

I have isolated a bug in pre('save', ...) middleware resolution, based on this issue: keystonejs/keystone#759

This works:

schema.pre('save', true, function(next, done) {
    console.log('locking 1');
    process.nextTick(function() {
        console.log('releasing lock 1');
        done();
    });
    next();
});

schema.pre('save', true, function(next, done) {
    console.log('locking 2');
    process.nextTick(function() {
        console.log('releasing lock 2');
        done();
    });
    next();
});

This also works:

schema.pre('save', true, function(next, done) {
    console.log('locking 1');
    process.nextTick(function() {
        console.log('releasing lock 1');
        done();
    });
    next();
});

schema.pre('save', function(done) {
    console.log('locking 2');
    console.log('releasing lock 2');
    done();
});

This does not work:

schema.pre('save', true, function(next, done) {
    console.log('locking 1');
    process.nextTick(function() {
        console.log('releasing lock 1');
        done();
    });
    next();
});

schema.pre('save', function(done) {
    console.log('locking 2');
    process.nextTick(function() {
        console.log('releasing lock 2');
        done();
    });
});

The order is important; if the serial middleware is included first in the bottom example, it works as expected.

[vkarpov15]

The last example is the same as the first one. Do you mean:

schema.pre('save', true, function(next, done) {
    console.log('locking 1');
    console.log('releasing lock 1');
    done();
});

schema.pre('save', true, function(next, done) {
    console.log('locking 2');
    process.nextTick(function() {
        console.log('releasing lock 2');
        done();
    });
    next();
});

[JedWatson]

@vkarpov15 The middleware functions are not identical; the second function is serial middleware in the third example, not parallel.

The issue I'm reporting is that if you add a serial middleware after a parallel one, and both functions are async (caused in this case by use of process.nextTick but the issue was originally found by having middleware that would execute queries) the middleware chain never resolves, and everything hangs.

The output of example 1 is:

locking 1
locking 2
releasing lock 1
releasing lock 2
(save continues)

The output of example 2 is:

locking 1
locking 2
releasing lock 2
releasing lock 1
(save continues)

The output of example 3 is:

locking 1
locking 2
releasing lock 1
releasing lock 2
(process stops)

After both middleware functions in example 3 have resolved, the save does not complete.

[JedWatson]

This is a problem because KeystoneJS sometimes adds middleware to schemas based on configuration, and that middleware is sometimes async depending on other configuration.

If a developer using KeystoneJS adds their own middleware that is safe to run in parallel, before Keystone's middleware that runs in series, mongoose hangs, and there's nothing I can do to work around that.

[vkarpov15]

Got it, I'll investigate this today. Thanks for opening up this issue.

[vkarpov15]

See bnoguchi/hooks-js#20 . Bad bug in underlying hooks library, thanks for catching it.

[JedWatson]

Thanks for tackling it!

[vkarpov15]

So looks like hooks is a kinda dead module, and I unfortunately don't have push access to it. I'll probably just fix this by switching over to kareem entirely - it powers the query middleware right now, might as well use it for all the middleware.