multipipe(readable, multipipe(transform, writable, cb1), cb2) fails

[kc97ble]

This code works:

multipipe(readable, transform, writable, cb)

This code does not work:

multipipe(readable, multipipe(transform, writable, cb1), cb2)

Reason: multipipe(transform, writable, cb1) returns a transform stream, not a writable stream

[juliangruber]

Do you have a self contained code example (eg gist) for me to check out?

[kc97ble]

Sorry for the late reply.

Here is an working example that read from a.txt, pass the data through a bunch of stream.PassThrough(), then write to b.txt.

const stream = require("stream");
const multipipe = require("multipipe");
const fs = require("fs");

function identity() {
  return new stream.PassThrough();
}

multipipe(
  fs.createReadStream("a.txt"), identity(), identity(),
  identity(), identity(), fs.createWriteStream("b.txt")
).then(() => console.error("Done."));

One can test by running the following commands:

dd if=/dev/urandom of=a.txt bs=1M count=1024
node script.js
diff a.txt b.txt

However, the following code DOES NOT WORK:

// NOT WORKING
const stream = require("stream");
const multipipe = require("multipipe");
const fs = require("fs");

function identity() {
  return new stream.PassThrough();
}

multipipe(
  multipipe(fs.createReadStream("a.txt"), identity(), identity()),
  multipipe(identity(), identity(), fs.createWriteStream("b.txt"))
).then(() => console.error("Done."));

It does not write any bytes to b.txt, and also does not output Done..

[kc97ble]

PS: the example above is quite a common pattern.

function createMyReadStream() { return multipipe(fs.createReadStream(...), transform1(), transform2()); }
function createMyWriteStream() { return multipipe(transform3(), transform4(), fs.createWriteStream(...)); }

multipipe(createMyReadStream(), createMyWriteStream());