I believe commit https://github.com/undertow-io/undertow/commit/595322a869494ba03610773b2e8084da8d4a8372 should be reverted.

The issue is that sendRequest() calls clientCallback.completed() which is on the caller side and can block for a long time, thus blocking other users/threads from calling sendRequest().

So, I think that any synchronization must be done by the callers. Or it must be implemented different than just making this whole method synchronized.