I am not sure of what information to provide since I am not sure how the internals of kwin_wayland works.
But trying to run kwin_wayland with master xserver, and it hangs, running it with GDB, and IIRC it hangs trying to connect to the display. (and it deadlocks pretty hard, where it ignores CTRL+C)
I narrowed it down to commit 08843efc5940563a2275c654804c999cfc772987
the commit before, and kwin_wayland runs fine
https://gitlab.freedesktop.org/xorg/xserver/merge_requests/99 claims to fix this.
P.S. Xwayland bugs should be filed at https://gitlab.freedesktop.org/xorg/xserver/issues/new now.
Thanks! That works! I'll make a note of that, sorry for reporting it in the wrong place
Thanks for the report, fixed in Git master:
Author: Roman Gilg <email@example.com>
Date: Wed Jan 2 21:55:23 2019 +0100
xwayland: Fix wm_fd readin with no listen_fds