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Lars Hjemli fbaf1171b4 Don't truncate valid cachefiles
An embarrassing thinko in cgit_check_cache() would truncate valid cachefiles
in the following situation:
  1) process A notices a missing/expired cachefile
  2) process B gets scheduled, locks, fills and unlocks the cachefile
  3) process A gets scheduled, locks the cachefile, notices that the cachefile
     now exist/is not expired anymore, and continues to overwrite it with an
     empty lockfile.

Thanks to Linus for noticing (again).

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
2006-12-11 22:53:50 +01:00
.gitignore Add caching infrastructure 2006-12-10 22:31:36 +01:00
cache.c Don't truncate valid cachefiles 2006-12-11 22:53:50 +01:00
cgit.c Don't truncate valid cachefiles 2006-12-11 22:53:50 +01:00
cgit.css Import cgit prototype from git tree 2006-12-09 15:18:17 +01:00
cgit.h Don't truncate valid cachefiles 2006-12-11 22:53:50 +01:00
COPYING Add license file and copyright notices 2006-12-10 22:41:14 +01:00
git.h Add caching infrastructure 2006-12-10 22:31:36 +01:00
html.c Add license file and copyright notices 2006-12-10 22:41:14 +01:00
Makefile Move global variables + callback functions into shared.c 2006-12-11 17:25:51 +01:00
parsing.c Rename config.c to parsing.c + move cgit_parse_query from cgit.c to parsing.c 2006-12-11 16:11:40 +01:00
README Add caching infrastructure 2006-12-10 22:31:36 +01:00
shared.c Move global variables + callback functions into shared.c 2006-12-11 17:25:51 +01:00
ui-log.c Move log-functions into ui-log.c 2006-12-11 17:04:19 +01:00
ui-repolist.c Move functions for repolist output into ui-repolist.c 2006-12-11 16:49:18 +01:00
ui-shared.c Move functions for repolist output into ui-repolist.c 2006-12-11 16:49:18 +01:00
ui-summary.c Move log-functions into ui-log.c 2006-12-11 17:04:19 +01:00
ui-view.c Move functions for generic object output into ui-view.c 2006-12-11 17:12:26 +01:00

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.