Future.wait() can't wait without a fiber (while waiting on another future in Meteor.method)

Question

In Meteor, I'm writing a method that will have to check a certain path's subdirectories for new files. I first would like to just list the subdirectories within Meteor after which I child_process.exec a simple bash script that lists files added since the last time it executed.

I'm having some issues getting the directory discovery to be async (Error: Can't wait without a fiber). I've written a synchronous version but having both fs.readdir and fs.stat in stead of their synchronous alternatives allows me to catch errors.

Here's the code:

function listDirs(dir, isDir){

    var future1 = new Future();fs.readdir(dir, function(err, files){

            if (err)
                throw new Meteor.error(500, "Error listing files", err);

            var dirs = _.map(files, function(file){

                var future2 = new Future();
                var resolve2 = future2.resolver();

                fs.stat(dir+file, function(err, stats){

                    if (err)
                        throw new Meteor.error(500, "Error statting files", err);

                    if (stats.isDirectory() == isDir && file.charAt(0) !== '.')
                        resolve2(err, file);

                });

                return future2;

            });

            Future.wait(dirs);

            //var result = _.invoke(dirs, 'get');

            future1['return'](_.compact(dirs));
        });

        return future1.wait();
    }

The error Error: Can't wait without a fiber has to do with future2. When I comment out Future.wait(dirs) the server doesn't crash anymore, but that's about as far as I got in trying to solve this. :/

Another _.map function I use in another part of the method works fine with futures. (see also https://gist.github.com/possibilities/3443021 where I found my inspiration)

Answer 1

Wrap your callback into Meteor.bindEnvironment, example:

fs.readdir(dir, Meteor.bindEnvironment(function (err, res) {
    if (err) future.throw(err);
    future.return(res);
}, function (err) { console.log("couldn't wrap the callback"); });

Meteor.bindEnvironment does a lot of things and one of them is to make sure callback is running in a Fiber.

Another thing that could be helpful is var funcSync = Meteor._wrapAsync(func) which utilizes futures and allows use to call a function in synchronous style (but it is still async).

Watch these videos on evented mind if you want to know more: https://www.eventedmind.com/posts/meteor-dynamic-scoping-with-environment-variables https://www.eventedmind.com/posts/meteor-what-is-meteor-bindenvironment