Bluebird: Promise.promisify fails on fs.createReadStream

[chrisprobst]

Hi,

I just wanted to checkout this library and played a little bit with the pomisify stuff but it does not work for all methods:

var Promise = require('bluebird');
var createReadStream = Promise.promisify(require('fs').createReadStream);


var p1 = createReadStream('app.js').then(console.log);

setTimeout(function() {
    console.log(p1);
}, 1000);

The p1 gets never fulfilled, no error, no success. I did test it for createWriteStream and it did not work, either.

[petkaantonov]

createReadStream is not a callback taking function, from the docs:

Returns a function that will wrap the given nodeFunction. Instead of taking a callback, the returned function will return a promise whose fate is decided by the callback behavior of the given node function. The node function should conform to node.js convention of accepting a callback as last argument and calling that callback with error as the first argument and success value on the second argument.

There is no need to promisify createReadStream since it's a synchronous function, you can just call it normally and get the return value immediately back.

[chrisprobst]

Oh gosh, you are right. It happened because I used promisifyAll (createReadStreamAsync), my bad. Sry 8-)

[manast]

Something to note here is that createReadStream can fail in the future, and even if you created the stream inside a promise chain the error will be reported uncatched (and your application die). This is quite unfortunate, for example in the case of trying to open a missing file, you get a stream and in the next tick you get an exception.
You need to create a stream like this to avoid problems, although this is far from ideal:

function createReadStream(filename){
  return new Promise(function(resolve, reject){
    function onError(err){
      reject(err);
    }

    function onReadable(){
      cleanup();
      resolve(stream);
    }

    function cleanup(){
      stream.removeListener('readable', onReadable);
      stream.removeListener('error', onError);
    }

    var stream = fs.createReadStream(filename);
    stream.on('error', onError);
    stream.on('readable', function(){
      resolve(stream);
    });
  });
}

@petkaantonov have you give any thought about a proper way to make streams work well together with promises?

[servatj]

Your comment is really useful, thanks @manast !!

[danjl1100]

Regret digging up an old/closed thread, but I found @manast 's example useful and informative.

Hoping this will help anyone else who stumbles upon it: (tiny correction at the end)

function createReadStream(filename){
  return new Promise(function(resolve, reject){
    function onError(err){
      reject(err);
    }

    function onReadable(){
      cleanup();
      resolve(stream);
    }

    function cleanup(){
      stream.removeListener('error', onError);
      stream.removeListener('readable', onReadable);
    }

    var stream = fs.createReadStream(filename);
    stream.on('error', onError);
    stream.on('readable', onReadable);    // <-- small correction
  });
}

Expanded for the Writable case:

function createWriteStream(filename){
  return new Promise(function(resolve, reject){
    function onError(err){
      reject(err);
    }

    function onOpen(){
      cleanup();
      resolve(stream);
    }

    function cleanup(){
      stream.removeListener('error', onError);
      stream.removeListener('open', onOpen);
    }

    const stream = fs.createWriteStream(filename);
    stream.on('error', onError);
    stream.on('open', onOpen);
  });
}