[stream] support piping multiple streams into a single stream

[missinglink]

In the following example sink should wait for all piped streams to unpipe() before calling _flush.

As it currently stands the following code outputs:

$ node test1.js 
a
done

If you remove the // applyFix(); comment you get:

$ node test1.js 
a
b
done

var through = require('through2');

var tap1 = through.obj();
var tap2 = through.obj();
var sink = through.obj(transform, flush);

var pipes = 0;
// applyFix();

tap1.pipe(sink);
tap2.pipe(sink);

tap1.write('a');
tap1.end();

setTimeout(function(){
  tap2.write('b');
  tap2.end();
}, 100);

function applyFix(){
  sink.on( 'pipe', function(){
    pipes++;
  });
  sink.end = function(){
    if( !--pipes ){
      sink._flush();
    }
  };
}

function flush(){
  console.log( 'done' );
}

function transform( item, e, next ){
  console.log( item );
  this.push(item);
  next();
}

ref: #89

[chrisdickinson]

Ah, interesting -- it seems like the pipe mechanism isn't designed to handle multiple sources piping into a single sink (it assumes it the source is the only stream writing into the destination). This should definitely be made explicit in the docs, at the very least.

I'm curious: what is your use case for piping multiple streams into a single stream?

Also, do you mind editing the title to "[stream] support piping multiple streams into a single stream"?

[missinglink]

For me this is a pretty common case when forking the output from a parser to separate pipelines (one per 'type' of record) and then saving to a database using a db client singelton sink.

[phpnode]

Another use case - piping data from multiple clients into one stream and then piping the stream back into each client, e.g. for chat rooms.

[vkurchatkin]

this only works for object mode streams, and only if the order of objects is not significant

[benjamingr]

+1 for making this explicit in the docs. Are we sure this change won't break code existing code though? Also what vkurchatkin said.

[indutny]

Summoning @caineio