Fix up reference counting for cloud log items.

[bluesky.git] / bluesky / log.c

diff --git a/bluesky/log.c b/bluesky/log.c
index 9b4b62b..55daf7a 100644 (file)
--- a/bluesky/log.c
+++ b/bluesky/log.c
@@ -282,10 +282,12 @@ BlueSkyRCStr *bluesky_log_map_object(BlueSkyLog *log,
         map = g_new0(BlueSkyMmap, 1);
 
         off_t length = lseek(fd, 0, SEEK_END);
+        map->log_seq = log_seq;
         map->addr = (const char *)mmap(NULL, length, PROT_READ, MAP_SHARED,
                                        fd, 0);
         map->len = length;
-        g_atomic_int_set(&map->refcount, 1);
+        map->log = log;
+        g_atomic_int_set(&map->refcount, 0);
 
         g_hash_table_insert(log->mmap_cache, GINT_TO_POINTER(log_seq), map);
 
@@ -296,3 +298,31 @@ BlueSkyRCStr *bluesky_log_map_object(BlueSkyLog *log,
 
     return bluesky_string_new_from_mmap(map, log_offset, log_size);
 }
+
+void bluesky_mmap_unref(BlueSkyMmap *mmap)
+{
+    if (mmap == NULL)
+        return;
+
+    if (g_atomic_int_dec_and_test(&mmap->refcount)) {
+        /* There is a potential race condition here: the BlueSkyLog contains a
+         * hash table of currently-existing BlueSkyMmap objects, which does not
+         * hold a reference.  Some other thread might grab a new reference to
+         * this object after reading it from the hash table.  So, before
+         * destruction we need to grab the lock for the hash table, then check
+         * the reference count again.  If it is still zero, we can proceed with
+         * object destruction. */
+        BlueSkyLog *log = mmap->log;
+        g_mutex_lock(log->mmap_lock);
+        if (g_atomic_int_get(&mmap->refcount) > 0) {
+            g_mutex_unlock(log->mmap_lock);
+            return;
+        }
+
+        g_hash_table_remove(log->mmap_cache, GINT_TO_POINTER(mmap->log_seq));
+        munmap((void *)mmap->addr, mmap->len);
+        g_free(mmap);
+        g_mutex_unlock(log->mmap_lock);
+    }
+}
+