streaming / iterative fs.readdir

[jonathanong]

since we're in ES6 territory now, i'm thinking the sync version should be an iterable

var dirs = fs.readdirIter(__dirname);
for (dir of dirs) {

}

and have the async version be an object stream:

var stream = fs.readdirStream(__dirname);
stream.on('data', dir => )

See: nodejs/node-v0.x-archive#388

[novacrazy]

Using generators/iterators in io.js where they make sense would be a good addition.

[cjb]

Does this depend on the work in joyent/libuv#1574 to be merged first? I don't think it's in libuv yet.

[vkurchatkin]

👍

in the future asynchronous version could return an observable:

for (dir on fs.readdir(__dirname)) {

}

[hemanth]

👍

[Qard]

I'm all for the new named versions. We don't want to break compatibility with existing code by changing behaviour of existing functions, but new stuff could work. I'm not sure what the policy is yet on new interfaces that deviate from node.js though.

@iojs/tc Thoughts?

[rvagg]

Where else does an interface like this make sense? Would this be a one-off of would there be a flood of requests to add the same style interface to other core APIs?

[jonathanong]

@rvagg I looked for any other APIs that would look like this but I couldn't find any. I think this may be a one-off.

[rvagg]

I think the way forward on this is for someone here to propose a change in a PR and then we'll escalate that to a TC discussion because it would be good to cover the question of whether adopting a new style of API is desirable. If it's too much work to come up with an initial implementation then we could just elevate this issue, it'd just be more of a hypothetical discussion then.

[Qard]

I think the streams version makes sense. I would test the iterator design in userland though. It can apply to any sort of stream, really. And it's easy enough to abstract a stream as an iterator in userland.

[Fishrock123]

The eventual observables thing sounds pretty rad.

[jonathanong]

@Qard how would you create an iterator from a stream? streams are async... iterators are not...

@cjb i don't think it's merged. i'm going to open an issue in libuv now

[Qard]

Just use co? (or something similar)

[domenic]

OK, a few things:

• Iterables make no sense here; they are synchronous. (Generators are just iterables with .throw() and .return() methods; they don't make any sense either.)
• There are vague designs by one TC39 committee member for adding RxJS-style observables to the language, along with for-on syntax. I doubt that will make the ES2016 train since no implementers have yet expressed interest, but I could be wrong. Regardless, it's very tentative. (There's another competing proposal, async iterables, that might be a better fit for this use case in particular.)
• In the meantime, Node.js has an awkward version of observables/async-iterables/etc. already: object-mode streams. That's clearly the right choice for this situation, even if in the long run there becomes a language-level primitive for plural asynchronous values (similar to promises being the primitive for singular asynchronous values, or iterables being the primitive for plural synchronous values).

So I don't think this should be a new style of API. Just an object mode stream is fine.

[timoxley]

Whoa, yeah this is a totally bogus suggestion as iterators and generators can't be used for iterating over async. This tripped me up at first as well.

But note that though it's not exactly what you had in mind fs.readFileSync(file, 'utf8') is already iterable by way of a String being iterable, and soon fs.readFileSync(file) will too be iterable as Buffers implement the iterable interface: #525

[domenic]

So re-reading the OP: readdirSync already returns an array which is iterable

[bnoordhuis]

It does, but readdirSync() is (too) eager: it slurps the directory in one pass. If you apply it to a directory with 1M+ entries, it will likely bring down the process.

An iterator that walks the entries piecemeal would be great but it requires a number of changes to libuv. @misterdjules already did some of the prep work but I recall that there were still some loose ends. I'm not even sure if we reached consensus that his approach was really the best way forward.

[vkurchatkin]

as iterators and generators can't be used for iterating over async

they can!

for (let promise of iterator) {
  let val = await promise;
}

[domenic]

@bnoordhuis I see, so you'd block on each call to .next(), instead of all at once.

@vkurchatkin yes, you can invent contracts on top of iterators. Especially ones that only work in ES2016. But they won't necessarily be respected. Someone could "forget" an await, or a .then, or just call .next() twice in a row. Now what? How do you know if it's end-of-iterable or I/O error or ...?

[domenic]

@vkurchatkin essentially you are proposing async iterable as next() : -> Iteration<Promise<T>>, whereas a more coherent design is next(): -> Promise<Iteration<T>>.

For example in your example the producer can't decide when to return or throw from the generator side, because the producer has to wait for the async process to complete before doing so, but the only way to signal completion or errors is synchronous return/throw.

[bnoordhuis]

you'd block on each call to .next(), instead of all at once.

Yes, but I'm mostly concerned with memory usage; an iterator brings it down from O(n) to O(1). Currently, trying to scan a large directory exhausts the heap and terminates the process.

[vkurchatkin]

@domenic agreed, ugly and far from ideal.

a more coherent design is next(): -> Promise<Iteration>

I can't see how this can be achieved using existing ES2015/2016 primitives. Thanks for the link, probably it has explanation.

[jonathanong]

Yeah the iterative version in my head was a synchronous version that doesn't load the entire array in memory at once. But i care way more about the async version.

[Qard]

@domenic While generators on their own are sync, they can be used in an async way via co. Maybe you don't like the weird promise trampoline hack it uses, but it works and many people are already using it in that way. I suggested using co because I had already suggested doing the iterator thing in userland, which means those sort of tools are available. If you prefer some different async iterator module, you can write a streams abstraction on that.

I wouldn't mind this being in core, I just suggested a userland approach because I expected some members of the core team might disagree.

[jonathanong]

btw let's not do the iterator design. i realized that if something like this happens:

var i = 0;
for (name of fs.readdirIter(__dirname)) {
  if (i++ > 5) throw new Error('boom');
}

assuming fs.readdirIter() creates a file descriptor, the above may cause a file descriptor leak as the iterator may remain in a paused state. this is assuming fs.readdirIter() opens a file descriptor until it's complete. having users manually close the file descriptor may be too much to ask (not worth people complaining about leaks when they don't know about this).

so... let's just do the stream version for now :D

[domenic]

@jonathanong as long as the iterator implements iterator.return() that closes the FD, there will be no leak.

[vkurchatkin]

@domenic why isn't return here: https://people.mozilla.org/~jorendorff/es6-draft.html#sec-iterator-interface

[domenic]

@vkurchatkin same reason throw isn't.

[vkurchatkin]

not working:

var arr = [1,2,3,4,5];
var it = arr[Symbol.iterator]();

it.return = function() {
  cobsole.log('return');
}


for (var v of it) {
  if (v > 3) break;
  console.log(v)
}

[Fishrock123]

@rijnhard That's gona add a whole bunch of overhead...

But to be clear, you want to: have an object stream of directory entries, transform those into file reads, in the same stream?

That doesn't really make sense to me. You end up muliplexing/splitting the stream anyways and that leads to a very similar can of worms as just "iterating" and "awaiting" until a stream is finished to pull the next entry.

Maybe you could elaborate more? I am presently avoiding making this a stream due to complexity and lack of solid use cases.

[rijnhard]

@Fishrock123 I think I misunderstood, I just need to iterate through directories and not their contents.

[paragi]

While you are working with it, would it be a lot of trouble to add a function with glob or just '?' and '*' wildcard functionality?
I presume performance and memory usages would vastly improve, working on very large filesystems, if you do the scanning in libuv.
It would be much appriciatet 🥇

[Fishrock123]

@paragi This API will do no filtering and will not return entries from subdirectories. That will be up to a module to do.

[Fishrock123]

Ok folks, the pull request is up: #29349

const fs = require('fs');

async function print(path) {
  const dir = await fs.promises.opendir(path);
  for await (const dirent of dir) {
    console.log(dirent.name);
  }
}
print('./').catch(console.error);

[Fishrock123]

Landed in cbd8d71 as fs{Promises}.opendir(), which returns an fs.Dir, which exposes an async iterator. 🎉

[Fishrock123]

This has been released in 12.12.0

[ma11hew28]

What about also making directories sync iterable (as initially suggested)?

I think this could be done by using dir.readSync().

The commit message could be "fs: make directories sync iterable".

[frank-dspeed]

@ma11hew28 sorry to tell you that Sync can't be iterable as its Sync :) a iterable is a async type

[Qard]

There are sync iterators too. It's entirely possible to make a sync version.

[frank-dspeed]

@Qard why should i use a iterator for a Sync call that will return after all is in mermory already but ok your right it could exist it can be done. i for my self would suggest for...of as iterate method but ok

[ma11hew28]

Thank you, @frank-dspeed and @Qard, for responding. I'm sorry for not responding promptly. @frank-dspeed, I'm sorry, but I think you misunderstood me. What you suggested is what I meant, as it is the first part of what @jonathanong initially suggested. Ie, if we make directories sync iterable, then you could do something like this:

const fs = require('fs')

const dir = fs.opendirSync('.')
for (const dirent of dir) {
  console.log(dirent.name)
}

instead of something like this:

const fs = require('fs')

const dir = fs.opendirSync('.')
let dirent
while ((dirent = dir.readSync()) !== null) {
  console.log(dirent.name)
}
dir.closeSync()

As for the second sentence of my initial comment on this issue, by "this", I meant "making directories sync iterable". Ie, I think a directory's default sync iterator's next() method could call dir.readSync() to get the directory's next entry.

[Qard]

@frank-dspeed It doesn't have to all be loaded into memory with a sync iterator. If you have a directory with millions of entries in it, a sync iterator could read and return entries one at a time, or in batches, but synchronously.