sending 404 responses for epty objects.

[Cord Beermann]

Hello,

Due to issues with a backend beyond my influence i need to fix this with
Nginx.

Root-Cause: A CMS generates empty files on a filesystem which will be later
filled with content. However: those files are there for some time with 0
bytes
and will be served with 200 through a chain of a caching Nginx and a caching
CDN.

User --> CDN --> Caching Nginx (SlowFS) --> Serving Nginx --> Filesystem.

The Backend-Filesystem is served by a Nginx. Solution could be to serve a
404
for all empty files. I tried with $sent_http_content_length, but it seems
to
be empty in the location where i would need it. Is it possible to use some
kind of 'test -s' on the file to decide when to send a 404?

I also tried to tackle it in the caching SlowFS-Nginx, by using
$upstream_http_content_length in if or map-statements [1]. I can see
$upstream_http_content_length set in an X-Debug-Header i added, but can't
get
it to work to use it to act on it.

	if ($upstream_http_content_length = 0) {
	  return 404;
	}

I found the discussion about 'using $upstream* variables inside map
directive'[2]
but even if i would get this to work it wouldn't be enough as i need to
signal the CDN in front (with some headers) that it also should not cache
the
empty response. Best would be to generate a 404 if the upstream-object is
empty.

So maybe someone here has an idea how to tackle this.

Cord

[1] like in
http://syshero.org/post/49594172838/avoid-caching-0-byte-files-on-nginx
[2] http://forum.nginx.org/read.php?2,249880,249899#msg-249899

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,255421,255421#msg-255421