Stream not emitting data event after pipe+unpipe

[dmail]

The following should log 'ok' to the console but nothing is logged.

var stream = require('stream');
var pass = new stream.PassThrough();
var writable = new stream.Writable();

pass.pipe(writable); pass.unpipe(writable);
pass.on('data', function(chunk){ console.log(chunk.toString()); });
pass.write('ok');

Of course, commenting the line pass.pipe(writable); pass.unpipe(writable); fix the problem.

Can you explain me why?

[micnic]

you should get the error "chunk is not defined" on line 6, when fixed it should print "ok" on the stdout.

[dmail]

I'm sorry @micnic I forgot the chunk argument when I wrote this snipet I've modified the line.
When I exec this script nothing is logged to the console, but when I comment the pipe/unpipe line, ok is logged.

var stream = require('stream');
var pass = new stream.PassThrough();
var writable = new stream.Writable();

// When the line below is commented, ok is logged
//pass.pipe(writable); pass.unpipe(writable); 
pass.on('data', function(chunk){ console.log(chunk.toString()); });
pass.write('ok');

[micnic]

pass gets paused and it needs to be resumed with pass.resume() before writing. I'm not sure, but this could be a bug, in node 0.10 it works as you expect, in 0.12 and io.js it needs to be resumed.

[dmail]

You're right !

With 0.10 no need to call resume()
With 0.12 I have to call resume() to get the 'ok' log

var stream = require('stream');
var pass = new stream.PassThrough();
var writable = new stream.Writable();

pass.pipe(writable); pass.unpipe(writable); 
pass.resume();
pass.on('data', function(chunk){ console.log(chunk.toString()); });
pass.write('ok');

Thank you very much :)

[chrisdickinson]

.unpipe sets state.flowing = false, which is the same as explicitly .pause()'ing the stream. Since it's in an explicitly paused state, .on('data') does not implicitly resume the stream. This seems like a regression. cc @iojs/streams

[micnic]

ping @iojs/streams

[sonewman]

@chrisdickinson This "feature" seems to have been introduced here 444bbd4

streams: Support objects other than Buffers

...
Fixed a bug with unpipe where the pipe would break because
the flowing state was not reset to false.

I believe this was during the streams2 era. So in that case I think we weren't expecting to get any data events.

We'll have to see what the implications are to streams with only a readable listener if we remove this.

[Trott]

@sonewman @chrisdickinson @nodejs/streams I don't suppose there's anything new to add to this issue at this time, is there?

[iliakan]

That's a bug indeed, tracked down in the following:

var fs = require('fs');

var src = fs.createReadStream(__filename);
var dst = fs.createWriteStream('/tmp/test');

src.pipe(dst);
src.unpipe(dst);
src.on('data', function() {
    console.log("Never happens")
  });

As @chrisdickinson wrote above, unpipe sets the stream to "explicitly paused" state, then on('data') does not resume it.

P.S. And yes, extra resume call at the end makes it flow again.