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>

3 files changed, 14 insertions, 2 deletions

diff --git a/cache.c b/cache.c
index 2e1ef8c..b947a34 100644
--- a/cache.c
+++ b/cache.c
@@ -96,12 +96,17 @@ int cache_lock(struct cacheitem *item)
 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);
 }
diff --git a/cgit.c b/cgit.c
index d1abaa0..0f72f2d 100644
--- a/cgit.c
+++ b/cgit.c
@@ -58,19 +58,25 @@ static void cgit_check_cache(struct cacheitem *item)
         }
         if (!cache_exist(item)) {
                 if (!cache_lock(item)) {
                         sleep(1);
                         goto top;
                 }
-                if (!cache_exist(item))
+                if (!cache_exist(item)) {
                         cgit_fill_cache(item);
                 cache_unlock(item);
+                } else {
+                        cache_cancel_lock(item);
+                }
         } else if (cache_expired(item) && cache_lock(item)) {
-                if (cache_expired(item))
+                if (cache_expired(item)) {
                         cgit_fill_cache(item);
                 cache_unlock(item);
+                } else {
+                        cache_cancel_lock(item);
+                }
         }
 }
 
 static void cgit_print_cache(struct cacheitem *item)
 {
         static char buf[4096];
diff --git a/cgit.h b/cgit.h
index c1dcc97..c9554a7 100644
--- a/cgit.h
+++ b/cgit.h
@@ -63,12 +63,13 @@ extern void html_link_close(void);
 extern int cgit_read_config(const char *filename, configfn fn);
 extern int cgit_parse_query(char *txt, configfn fn);
 
 extern void cache_prepare(struct cacheitem *item);
 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);
 
 extern char *cgit_repourl(const char *reponame);
 extern char *cgit_pageurl(const char *reponame, const char *pagename, 
                           const char *query);