gitweb/lib - Serve stale data when waiting for filling cache

gitweb/lib - Serve stale data when waiting for filling cache

When process fails to acquire exclusive (writers) lock, then instead
of waiting for the other process to (re)generate and fill cache, serve
stale (expired) data from cache.  This is of course possible only if
there is some stale data in cache for given key.

This feature of GitwebCache::FileCacheWithLocking is used only for
->update($key, $code) and ->update_fh($key, $code_fh) methods.  It is
controlled by 'max_lifetime' cache parameter; you can set it to -1 to
always serve stale data if it exists, and you can set it to 0 (or any
value smaller than 'expires_min') to turn this feature off.

This feature, as it is implemented currently, makes ->update() method a
bit asymmetric with respect to process that acquired writers lock and
those processes that didn't, which can be seen in the new test in t9503.
The process that is to regenerate (refresh) data in cache must wait for
the data to be generated in full before showing anything to client, while
the other processes can show stale (expired) data immediately.  In order
to remove or reduce this asymmetry gitweb would need to employ one of
two alternate solutions.  Either data should be (re)generated in background,
so that process that acquired writers lock would generate data in
background while serving stale data, or alternatively the process that
generates data should pass output to original STDOUT while capturing it
("tee" output).

Note that process that got stale data serves it immediately, therefore
it wouldn't be available to regenerate data if process regenerating
data died; see commented-out TODO test in t9503.  Otherwise it would
have to wait to check if data got regenerated, which would negate the
idea of serving stale data for a fast return.

When developing this feature, ->is_valid() method in base class
GitwebCache::SimpleFileCache acquired additional extra optional
parameter, where one can pass expire time instead of using whole-cache
global (adaptive) expire time.

Inspired-by-code-by: John 'Warthog9' Hawley <warthog9@kernel.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>