tmp: avoid dead lock in callback

Author: qzhuyan

c_src/main.c

@@ -81,17 +81,20 @@ erlfdb_future_cb(FDBFuture* fdb_future, void* data)
     if(enif_thread_type() == ERL_NIF_THR_UNDEFINED) {
         caller = NULL;
     } else {
+        enif_mutex_lock(future->lock);
         caller = future->pid_env;
     }
 
-    enif_mutex_lock(future->lock);
+
 
     if(!future->cancelled) {
         msg = T2(future->msg_env, future->msg_ref, ATOM_ready);
         enif_send(caller, &(future->pid), future->msg_env, msg);
     }
 
-    enif_mutex_unlock(future->lock);
+    if (caller) {
+        enif_mutex_unlock(future->lock);
+    }
 
     // We're now done with this future which means we need
     // to release our handle to it. See erlfdb_create_future