Why do I require a `wl_event_loop_add_idle` in creating a fork-exec wayland client

[Sichem Zhou]

Hi All,

I have a question related to the wl_client creation in the compositor. I
followed these step like weston did:

1. create socketpair.
2. fork a process and close `socket[0]` in child, close `socket[1]` in in
parent process.
3. set the `WAYLAND_SOCKET` to `socket[1]`.
4. exec the program.

I found out if I do this directly both compositor and client will stuck in
the eventloop. It seems the compositor is stuck in the
`wl_display_flush_clients` and client is stuck in `wl_event_queue_dispatch`
and internally stuck in the epoll.  So I think both compositor and client
were waiting each other for some update.

Then I found out that  I need to pack the code in a
`wl_event_loop_idle_func_t` and add it to the event_loop. But I am couldn't
figure out why did the compositor stuck in the first place and why did the
wl_event_loop_add_idle helped.

Thanks very much.
SZ
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