Avoid infinite loops in caching layer

Add a global variable, cgit_max_lock_attemps, to avoid the possibility of
infinite loops when failing to acquire a lockfile. This could happen on
broken setups or under crazy server load.

Incidentally, this also fixes a lurking bug in cache_lock() where an
uninitialized returnvalue was used.

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

3 files changed, 31 insertions, 14 deletions

diff --git a/cache.c b/cache.c
index 0ffa502..2e1ef8c 100644
--- a/cache.c
+++ b/cache.c
@@ -58,32 +58,41 @@ int cache_create_dirs()
                 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 ret;
+        int i = 0;
         char *lockfile = fmt("%s.lock", item->name);
 
- top:  
-               item->fd = open(lockfile, O_WRONLY | O_CREAT | O_EXCL, S_IRUSR|S_IWUSR);
+ 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) {
-                struct stat st;
-                time_t t;
-                if (stat(lockfile, &st))
-                        return ret;
-                t = time(NULL);
-                if (t-st.st_mtime > cgit_cache_max_create_time && 
-                    !unlink(lockfile))
+
+        if (item->fd == NOLOCK && errno == EEXIST &&
+            cache_refill_overdue(lockfile) && !unlink(lockfile))
                         goto top;
-                return 0;
-        }
+
         return (item->fd > 0);
 }
 
 int cache_unlock(struct cacheitem *item)
 {
         close(item->fd);
diff --git a/cgit.c b/cgit.c
index 7f14016..dc91125 100644
--- a/cgit.c
+++ b/cgit.c
@@ -28,12 +28,13 @@ char *cgit_css          = "/cgit.css";
 char *cgit_logo         = "/git-logo.png";
 char *cgit_logo_link    = "http://www.kernel.org/pub/software/scm/git/docs/";
 char *cgit_virtual_root = NULL;
 
 char *cgit_cache_root   = "/var/cache/cgit";
 
+int cgit_max_lock_attempts     =  5;
 int cgit_cache_root_ttl        =  5;
 int cgit_cache_repo_ttl        =  5;
 int cgit_cache_dynamic_ttl     =  5;
 int cgit_cache_static_ttl      = -1;
 int cgit_cache_max_create_time =  5;
 
@@ -462,17 +463,23 @@ static void cgit_fill_cache(struct cacheitem *item)
         else
                 cgit_print_repolist();
 }
 
 static void cgit_refresh_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)) {
-                        sched_yield();
+                        sleep(1);
                         goto top;
                 }
                 if (!cache_exist(item))
                         cgit_fill_cache(item);
                 cache_unlock(item);
         } else if (cache_expired(item) && cache_lock(item)) {
diff --git a/cgit.h b/cgit.h
index 3b0994a..7e4bfef 100644
--- a/cgit.h
+++ b/cgit.h
@@ -20,12 +20,13 @@ extern char *cgit_root_title;
 extern char *cgit_css;
 extern char *cgit_logo;
 extern char *cgit_logo_link;
 extern char *cgit_virtual_root;
 extern char *cgit_cache_root;
 
+extern int cgit_max_lock_attempts;
 extern int cgit_cache_root_ttl;
 extern int cgit_cache_repo_ttl;
 extern int cgit_cache_dynamic_ttl;
 extern int cgit_cache_static_ttl;
 extern int cgit_cache_max_create_time;