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, 10 insertions, 4 deletions

diff --git a/cgit.c b/cgit.c
index d1abaa0..0f72f2d 100644
--- a/cgit.c
+++ b/cgit.c
@@ -40,55 +40,61 @@ static void cgit_fill_cache(struct cacheitem *item)
 {
         htmlfd = item->fd;
         item->st.st_mtime = time(NULL);
         if (cgit_query_repo)
                 cgit_print_repo_page(item);
         else
                 cgit_print_repolist(item);
 }
 
 static void cgit_check_cache(struct cacheitem *item)
 {
         int i = 0;
 
         cache_prepare(item);
  top:
         if (++i > cgit_max_lock_attempts) {
                 die("cgit_refresh_cache: unable to lock %s: %s",
                     item->name, strerror(errno));
         }
         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);
+                        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);
+                        cache_unlock(item);
+                } else {
+                        cache_cancel_lock(item);
+                }
         }
 }
 
 static void cgit_print_cache(struct cacheitem *item)
 {
         static char buf[4096];
         ssize_t i;
 
         int fd = open(item->name, O_RDONLY);
         if (fd<0)
                 die("Unable to open cached file %s", item->name);
 
         while((i=read(fd, buf, sizeof(buf))) > 0)
                 write(STDOUT_FILENO, buf, i);
 
         close(fd);
 }
 
 int main(int argc, const char **argv)
 {
         struct cacheitem item;
 
         cgit_read_config("/etc/cgitrc", cgit_global_config_cb);
         cgit_querystring = xstrdup(getenv("QUERY_STRING"));