How to write Meteor.wrapAsync fs.readFile? - asynchronous

Question

I need a function that emits individual lines from a file with newlines. Nothing hard.
But with node, it is hard, and with Meteor, there's an additional complication: you must use Meteor.wrapAsync. Surprisingly, there isn't an example of how to use wrapAsync in the docs, and I could only find a couple of examples online, none of which helped.
I have something like:
var readFileAsync = function (file, cb) {
// From here to below comment works synchronously
var instream = fs.createReadStream(file, function () {
var outstream = new stream;
outstream.readable = true;
outstream.writable = true;
var rl = readline.createInterface({
input: instream,
output: outstream,
terminal: false
});
rl.on('line', function(line) {
console.log(line);
return line;
});
});
// Reference to aforementioned comment
};
var readWatFile = Meteor.wrapAsync(readFileAsync);
var line = readWatFile('/path/to/my/file');
console.log(line);
I know this is wrong because it doesn't work, so how do I write this?

Answer 1

There are two ways to go around it.
Load the whole file into memory and do whatever you want. To do that you can use the Private Assets API
Use node.js streams and stream the file line by line. You would have something like this.
Example code that you would need to tweak to your favorite streaming methods:
var Future = Npm.require('fibers/future');
var byline = Npm.require('byline');
var f = new Future;
// create stream in whatever way you like
var instream = fs.createReadStream(...);
var stream = byline.createStream(instream);
// run stream handling line-by-line events asynchronously
stream.on('data', Meteor.bindEnvironment(function (line) {
if (line) console.log(line)
else future.return();
}));
// await on the future yielding to the other fibers and the line-by-line handling
future.wait();