PHP File Serving Script: Unreliable Downloads?

问题

This post started as a question on ServerFault ( https://serverfault.com/questions/131156/user-receiving-partial-downloads ) but I determined that our php script was the culprit. So I'm issuing an updated question here about what I believe is the actual issue.

I am using a php script to verify permissions and then serve up a file for users of my website to download. Most of the time, this works, but recently one user has been seeing problems with larger downloads. He is only getting ~80% of downloads for files that are > 100MB in size. Also, all downloads from this script fail to report a filesize. Further, tests revealed that the same user COULD reliably download each of the failed files if given a direct link (at which point the filesize is reported).

Here's the relevant snippet of code that we are using to serve the file:

header("Content-type:$contenttype");
$len = filesize($filename);
header("Content-Length: $len");
header("Content-Disposition: attachment; filename=".$title.".".$ext);
readfile($filename);

Note that $contenttype, $filename, $title, and $ext are all set correctly before we get here. These have been triple-checked. None of them are the problem. Also, $len does provide the correct filesize.

While researching this issue, I came across this post: Content-Length header always zero

It seems that I am encountering the same issue. When I use the script, I get chunked encoding on the file and no size is set for content-length. I'm hypothesizing that something is going wrong on the large downloads, leading him to get a zero-length chunk before the end of the file.

Here's what the headers look like for a direct request:

http://www.grinderschool.com/videos/zfff5061b65ae00e8b21/KillsAids021.wmv

GET /videos/zfff5061b65ae00e8b21/KillsAids021.wmv HTTP/1.1
Host: www.grinderschool.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Referer: http://www.grinderschool.com/phpBB3/viewtopic.php?f=14&p=29468
Cookie: style_cookie=printonly; phpbb3_7c544_u=2; phpbb3_7c544_k=44b832912e5f887d; phpbb3_7c544_sid=e8852df42e08cc1b2250300c2897f78f; __utma=174624884.2719561324781918700.1251850714.1270986325.1270989003.575; __utmz=174624884.1264524375.411.12.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=low%20stakes%20poker%20videos; phpbb3_cmviy_k=; phpbb3_cmviy_u=2; phpbb3_cmviy_sid=d8df5c0943863004ca40ef9c392d371d; __utmb=174624884.4.10.1270989003; __utmc=174624884
Pragma: no-cache
Cache-Control: no-cache

HTTP/1.1 200 OK
Date: Sun, 11 Apr 2010 12:57:41 GMT
Server: Apache/2.2.14 (Unix) mod_ssl/2.2.14 OpenSSL/0.9.8l DAV/2 mod_auth_passthrough/2.1 FrontPage/5.0.2.2635
Last-Modified: Sun, 04 Apr 2010 12:51:06 GMT
Etag: "eb42d6-7d9b843-48368aa6dc280"
Accept-Ranges: bytes
Content-Length: 131708995
Keep-Alive: timeout=10, max=30
Connection: Keep-Alive
Content-Type: video/x-ms-wmv

And here's what they look like for the request answered by my script:

http://www.grinderschool.com/download_video_test.php?t=KillsAids021&format=wmv

GET /download_video_test.php?t=KillsAids021&format=wmv HTTP/1.1
Host: www.grinderschool.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Cookie: style_cookie=printonly; phpbb3_7c544_u=2; phpbb3_7c544_k=44b832912e5f887d; phpbb3_7c544_sid=e8852df42e08cc1b2250300c2897f78f; __utma=174624884.2719561324781918700.1251850714.1270986325.1270989003.575; __utmz=174624884.1264524375.411.12.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=low%20stakes%20poker%20videos; phpbb3_cmviy_k=; phpbb3_cmviy_u=2; phpbb3_cmviy_sid=d8df5c0943863004ca40ef9c392d371d; __utmb=174624884.4.10.1270989003; __utmc=174624884

HTTP/1.1 200 OK
Date: Sun, 11 Apr 2010 12:58:02 GMT
Server: Apache/2.2.14 (Unix) mod_ssl/2.2.14 OpenSSL/0.9.8l DAV/2 mod_auth_passthrough/2.1 FrontPage/5.0.2.2635
X-Powered-By: PHP/5.2.11
Content-Disposition: attachment; filename=KillsAids021.wmv
Vary: Accept-Encoding
Content-Encoding: gzip
Keep-Alive: timeout=10, max=30
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: video/x-ms-wmv

So the question is...what can I do to make downloads from the script work properly? Again, for 99% of users, it works as is (though I find it annoying now that no filesize is reported and thus that no time estimate can be computed about the download).

回答1:

It's your GZIP compression. When you specify a content length but turn compression on, it gums everything up. It's happened to me a few times: try turning it off in your script.

Generally you'd turn it on with:

ob_start("ob_gzhandler");

...so just comment that line out. If that's not in your code, chances are there's a setting in either your php.ini file somewhere or your apache.conf/conf.d.

Hope this helps!

回答2:

Content-Encoding: gzip

Hmm. Presumably PHP's zlib.output_compression is doing that. (Doesn't look like Apache's mod_deflate.)

Try turning it off and seeing if it's that that's forcing chunked encoding. You don't want to compress the download of a filetype like WMV which is already highly compressed.

However, chunked encoding would only explain the lack of size report. The download should still work. Is it possible you're being hit by a timeout (eg. PHP's set_time_limit, or Apache Timeout)?

回答3:

If it is the script then have you tried using a substitute function for the readfile() function that reads and outputs a bit at a time? The reasoning behind this could be that a memory limit is reached somewhere and it fails.

From http://php.net/manual/en/function.readfile.php :

function readfile_chunked ($filename) {
  $chunksize = 1*(1024*1024); // how many bytes per chunk
  $buffer = '';
  $handle = fopen($filename, 'rb');
  if ($handle === false) {
    return false;
  }
  while (!feof($handle)) {
    $buffer = fread($handle, $chunksize);
    print $buffer;
  }
  return fclose($handle);
} 

Also, try to flush the output as often as you can.

来源：https://stackoverflow.com/questions/2617190/php-file-serving-script-unreliable-downloads