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>

1 files changed, 5 insertions, 0 deletions

diff --git a/cache.c b/cache.c
index 2e1ef8c..b947a34 100644
--- a/cache.c
+++ b/cache.c
@@ -54,54 +54,59 @@ int cache_create_dirs()
 
 	if (cgit_query_page) {
 		path = fmt("%s/%s/%s", cgit_cache_root, cgit_query_repo, 
 			   cgit_query_page);
 		if (mkdir(path, S_IRWXU) && errno!=EEXIST)
 			return 0;
 	}
 	return 1;
 }
 
 int cache_refill_overdue(const char *lockfile)
 {
 	struct stat st;
 
 	if (stat(lockfile, &st))
 		return 0;
 	else
 		return (time(NULL) - st.st_mtime > cgit_cache_max_create_time);
 }
 
 int cache_lock(struct cacheitem *item)
 {
 	int i = 0;
 	char *lockfile = fmt("%s.lock", item->name);
 
  top:
 	if (++i > cgit_max_lock_attempts)
 		die("cache_lock: unable to lock %s: %s",
 		    item->name, strerror(errno));
 
        	item->fd = open(lockfile, O_WRONLY|O_CREAT|O_EXCL, S_IRUSR|S_IWUSR);
 
 	if (item->fd == NOLOCK && errno == ENOENT && cache_create_dirs())
 		goto top;
 
 	if (item->fd == NOLOCK && errno == EEXIST &&
 	    cache_refill_overdue(lockfile) && !unlink(lockfile))
 			goto top;
 
 	return (item->fd > 0);
 }
 
 int cache_unlock(struct cacheitem *item)
 {
 	close(item->fd);
 	return (rename(fmt("%s.lock", item->name), item->name) == 0);
 }
 
+int cache_cancel_lock(struct cacheitem *item)
+{
+	return (unlink(fmt("%s.lock", item->name)) == 0);
+}
+
 int cache_expired(struct cacheitem *item)
 {
 	if (item->ttl < 0)
 		return 0;
 	return item->st.st_mtime + item->ttl * 60 < time(NULL);
 }