* callback which XCB can call when it wants the write side of the
* socket back to make a request. This callback synchronizes with the
* external socket owner and flushes any output queues if appropriate.
+ * The callback might be called from different threads at the same time.
* If you are sending requests which won't cause a reply, please note the
* comment for xcb_writev which explains some sequence number wrap issues.
* */