Hello,

This patch applies cache locking behaviour to stale cache entries, so
in the case where the *_cache_lock directives are used, the same
locking behaviour is used for stale content as it is for new entries.

Previously, this was only done for new cache entries.
(see: http://mailman.nginx.org/pipermail/nginx/2016-January/049734.html)

This is useful when serving stale content is not permissable but sending
many requests upstream is undesriable.


This patch exposes the ngx_http_file_cache_lock function; this function
is then called in ngx_http_upstream if a cache entry has expired.

I have attached two versions of this patch, a "minimal" one that avoids
changes where not strictly necessary, and a "cleaner" version which is
more invasive but (in my opinion) cleaner.

Both patches cause no (additional) failures in the nginx test suite.
I tested with as many modules as I could reasonably enable.

Additionally, a colleague will add tests for this new behaviour in a
follow-up patch.


Regards, Elliot.



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