using $upstream* variables inside map directive

[Kirill K.]

Probably that's the case, and I'm not sure if there's a way to use map
inside upstream {...} or other context apart from http {...}, which makes
your theory sound correct.
What confuses me most: I googled a bit, and using map w/
$upstream_response_length is the most common way offered to avoid caching of
small (or zero-sized) responses, yet it just does not work in a real life
scenario...

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,249880,249892#msg-249892