Why crypto.createHash() is not writable until setEncoding() is called?

[iximiuz]

• Version: v6.9.1
• Platform: Darwin 15.2.0 Darwin Kernel Version 15.2.0: Fri Nov 13 19:56:56 PST 2015; root:xnu-3248.20.55~2/RELEASE_X86_64 x86_64

Please look at the code below:

var hasher = crypto.createHash('sha256'); 
// hasher.setEncoding('hex');
console.log(hasher.writable); 

So, if one use hasher with the legacy stream.pipe() implementation there will be an error, because the hash will be always calculated from the empty value.

It's easy to reproduce:

const crypto = require('crypto'); 
const request = require('request');  // this package uses oldschool streams

var hasher = crypto.createHash('sha256'); 
// Uncomment the line below to fix!
// hasher.setEncoding('hex');
console.log(hasher.writable); 
request('http://ya.ru').pipe(hasher).on('finish', function() {
    console.log('Hash is', hasher.read()); 
});

[bnoordhuis]

cc @nodejs/streams - this doesn't come out of lib/crypto.js as far as I can tell.

[mcollina]

@bnoordhuis I will have a look. I think it's LazyTransform.

[calvinmetcalf]

it's coming out of lazyTransform, does removing the console log change anything ?

[mcollina]

I think the problem is that https://github.com/nodejs/node/blob/master/lib/internal/streams/legacy.js#L11-L91 does not cause the lazyness of LazyTransform triggered.

[calvinmetcalf]

@mcollina but in the example, the console.log will actually trigger LazyTransform

[mcollina]

@calvinmetcalf why? the lazyness is on _writableState.

[calvinmetcalf]

ignore me, I thought he was calling console.log on _writableState

[iximiuz]

I found out the root of the problem. Actually this code can reproduce it:

var hasher = crypto.createHash('sha256'); 
const _ = hasher._writableState;  // we don't even have to call .setEnconding() here
console.log(hasher.writable);

So, the main reason is the LazyTransform class and its delayed parent's constructor calls. LazyTransform calls the ctr of the Transform (which extends Duplex which extends Writable) only on the first access to _readableState, or _writableState, or _transformState. However, this.writable = true is set only in the Writable's constructor.

A fix could be another backward compatibility hack - setting this.writable = true in the LazyTransorm thin constructor. Another one could be triggering access to this._writableState in the legacy stream.Stream.pipe().