Limiting Parallel/Simultaneous Downloads - How to know if download was cancelled?

Question

I have a simple file upload service, written out in PHP, which also includes a script that controls download speeds by sending limited-sized packets when a user requests a d

Answer 1

<?php

class DownloadObserver
{
  protected $file;
  public function __construct($file) {
    $this->file = $file;
  }

  public function send() {
    // -> note in DB you've started
    readfile($this->file);
  }

  public function __destruct() {
    // download is done, either completed or aborted
    $aborted = connection_aborted();
    // -> note in DB
  }
}

$dl = new DownloadObserver("/tmp/whatever");
$dl->send();

should work just fine. No need for a shutdown_function or any funky self-built connection observation.

Answer 2

There is no HTTP "cancel" signal that is sent by default. So, it looks like you will need to decide on a timeout, the length of time a connection can sit without sending/receiving another packet. If you are sending rather small packets (as I presume you are) keep the timeout short for best effect.

In your while condition you will need to check the age of the last timestamp update, if its too old, stop sending the file.

Answer 3

You will want to check out the following functions: connection_status(), connection_aborted() and ignore_user_abort() (see the connection handling section of the PHP manual for more info).

Although I can't guarantee the reliability (it's been a while since I've played around with it), with the right combination you should be able to accomplish what you want. There are a few caveats when working with these though, the big one being that if something goes wrong you could end up with stranded PHP scripts running on the server requiring you to kill Apache to stop them.

The following should give you a good idea of how to do it (adapted from the PHP code examples and a couple of the comments):

<?php
//Set PHP not to cancel execution if the connection is aborted
//and drop the time limit to allow for big file downloads
ignore_user_abort(true);
set_time_limit(0);

while(true){
    //See the ignore_user_abort() docs re having to send data
    echo chr(0);

    //Make sure the data gets flushed properly or the connection check won't work
    flush();
    ob_flush();

    //Check then connection status and exit loop if aborted
    if(connection_status() != CONNECTION_NORMAL || connection_aborted()) break;

    //Just to provide some spacing in this example
    sleep(1);
}

file_put_contents("abort.txt", "aborted\n", FILE_APPEND);

//Never hurts to ensure that the script halts execution
die();

Obviously for how you would be using it the data being sent would simply be the download data chunk (just make sure you flush the buffer properly to ensure the data is actually sent). As far as I'm aware, there is no way of making a distinction between pausing and aborting/stopping. Pause/resume functionality (and multi-part downloading - i.e. how download managers accelerate downloads) relies on the "Range" header, basically requesting byte x to byte y of the file. So if you want to allow resumable downloads you'll have to deal with that too.