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Cache algorithm
===============

Cgit normally returns cached pages when invoked. If there is no cache file, or
the cache file has expired, it is regenerated. Finally, the cache file is 
printed on stdout.

When it is decided that a cache file needs to be regenerated, an attempt is 
made to create a corresponding lockfile. If this fails, the process gives up
and uses the expired cache file instead.

When there is no cache file for a request, an attempt is made to create a 
corresponding lockfile. If this fails, the process calls sched_yield(2) before
restarting the request handling.

In pseudocode:

	name = generate_cache_name(request);
top:
	if (!exists(name)) {
		if (lock_cache(name)) {
			generate_cache(request, name);
			unlock_cache(name);
		} else {
			sched_yield();
			goto top;
		}
	} else if (expired(name)) {
		if (lock_cache(name)) {
			generate_cache(request, name);
			unlock_cache(name);
		}
	}
	print_file(name);


The following options can be set in /etc/cgitrc to control cache behaviour:
  cache-root:        root directory for cache files
  cache-root-ttl:    TTL for the repo listing page
  cache-repo-ttl:    TTL for any repos summary page
  cache-dynamic-ttl: TTL for pages with symbolic references (not SHA1)
  cache-static-ttl:  TTL for pages with sha1 references

TTL is specified in minutes, -1 meaning "infinite caching". 


Naming of cache files
---------------------
Repository listing:  <cachedir>/index.html
Repository summary:  <cachedir>/<repo>/index.html
Repository subpage:  <cachedir>/<repo>/<page>/<querystring>.html

The corresponding lock files have a ".lock" suffix.