Redesign the caching layer

The original caching layer in cgit has no upper bound on the number of
concurrent cache entries, so when cgit is traversed by a spider (like the
googlebot), the cache might end up filling your disk. Also, if any error
occurs in the cache layer, no content is returned to the client.

This patch redesigns the caching layer to avoid these flaws by
* giving the cache a bound number of slots
* disabling the cache for the current request when errors occur

The cache size limit is implemented by hashing the querystring (the cache
lookup key) and generating a cache filename based on this hash modulo the
cache size. In order to detect hash collisions, the full lookup key (i.e.
the querystring) is stored in the cache file (separated from its associated
content by ascii 0).

The cache filename is the reversed 8-digit hexadecimal representation of

  hash(key) % cache_size

which should make the filesystem lookup pretty fast (if directory content
is indexed/sorted); reversing the representation avoids the problem where
all keys have equal prefix.

There is a new config option, cache-size, which sets the upper bound for
the cache. Default value for this option is 0, which has the same effect
as setting nocache=1 (hence nocache is now deprecated).

Included in this patch is also a new testfile which verifies that the
new option works as intended.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

1 files changed, 21 insertions, 12 deletions

diff --git a/cache.h b/cache.h
index 4dcbea3..5f2178d 100644
--- a/cache.h
+++ b/cache.h
@@ -6,18 +6,27 @@
 #ifndef CGIT_CACHE_H
 #define CGIT_CACHE_H
 
-struct cacheitem {
-        char *name;
-        struct stat st;
-        int ttl;
-        int fd;
-};
+typedef void (*cache_fill_fn)(void *cbdata);
 
-extern char *cache_safe_filename(const char *unsafe);
-extern int cache_lock(struct cacheitem *item);
-extern int cache_unlock(struct cacheitem *item);
-extern int cache_cancel_lock(struct cacheitem *item);
-extern int cache_exist(struct cacheitem *item);
-extern int cache_expired(struct cacheitem *item);
+
+/* Print cached content to stdout, generate the content if necessary.
+ *
+ * Parameters
+ *   size    max number of cache files
+ *   path    directory used to store cache files
+ *   key     the key used to lookup cache files
+ *   ttl     max cache time in seconds for this key
+ *   fn      content generator function for this key
+ *   cbdata  user-supplied data to the content generator function
+ *
+ * Return value
+ *   0 indicates success, everyting else is an error
+ */
+extern int cache_process(int size, const char *path, const char *key, int ttl,
+                         cache_fill_fn fn, void *cbdata);
+
+
+/* Print a message to stdout */
+extern void cache_log(const char *format, ...);
 
 #endif /* CGIT_CACHE_H */