The diff is a tad misleading give that it looks like close() is being added when it in fact is untouched.

You can already do this by using an iterator as input, see
http://symfony.com/doc/current/components/process.html#streaming-to-the-standard-input-of-a-process

@nicolas-grekas Looks promising. I actually developed this on 3.0.x and rebased. Finally got around to cleaning up and trying to upstream. I'll take a look and close if nothing further.

It seems to work in principal, but interestingly it does not write every time ->isRunning() is called as my approach does.

My use-case is basically (paraphrased):

$process = new Process('cat');
$input = new InputStream();
$process->setInput($input);

$process->start(function ($type, $buffer) use($input) {
  echo "+ $buffer";

  if (/*some buffer output */) {
    $input->write(PHP_EOL); // continue
  }
}

while (true) {
    echo "main loop\n";
    $process->isRunning(); // triggers reading of pipes
    $process->checkTimeout(); // can trigger exception

    sleep(1);
}

main loop is output multiple times before the process responds with output buffer. With my implementation there is output between each loop iteration.

What I end up doing is manually writing to pipe via getPipes() (#19559) in order to have the response be written immediately. Instead of waiting for after sleep.

So to facilitate my usecase, using an InputStream and never writing to it should work fine to keep the input pipe open, but the current implementation seems to be a bit cumbersome to use given it is unclear when it chooses to write (it in fact differs how many loop iteration in output).

What about foreach ($process ....)?

That appears to flush the write buffer, but never times out since timeout is never checked (this feels like a bug). Additionally, for my usecase I need to do other things besides only wait on output from process (ie the stream_select() case describe in other issue).

To sum it up, the iterator input would work if there was a way to call readPipes() / $this->processPipes->readAndWrite() after calling write() on the InputStream.

Not sure stream_select fits into the component's scope. You may need to use proc_open directly, or use an abstraction provided by an async loop (e.g. ReactPHP)

proc_open() works, but then I found myself re-implementing the timeouts and many other features of process. Given all I really need is exactly what process is already doing except merging multiple process pipes together and a custom timeout.