I hit this while debugging issues with spice-gtk (using spicy) and multi monitor
guests. I hit an assert in spicy while it was running under the debugger and left
it sitting in the debugger (and thus stuck).

This caused spice server to first say:
red_wait_outgoing_item: blocked
red_wait_outgoing_item: timeout

Followed by:
red_clear_surface_drawables_from_pipes: ASSERT red_channel_client_no_item_being_sent(&dcc->common.base) failed

I'm not sure if the ASSERT only happened after I really quit the asserted spicy or before.

Here is a bt:
#0  0x0000003000836435 in raise () from /lib64/libc.so.6
#1  0x0000003000837d4b in abort () from /lib64/libc.so.6
#2  0x00007ffff7cf36e7 in red_clear_surface_drawables_from_pipes (
    worker=<optimized out>, surface_id=<optimized out>, force=<optimized out>, 
    wait_for_outgoing_item=<optimized out>) at red_worker.c:2009
#3  red_clear_surface_drawables_from_pipes (worker=0x7ffff47325e0, surface_id=
    0, force=1, wait_for_outgoing_item=1) at red_worker.c:1996
#4  0x00007ffff7cf78a4 in handle_dev_destroy_primary_surface (worker=
    0x7ffff47325e0) at red_worker.c:10322
#5  handle_dev_input (listener=0x7ffff47325e0, events=<optimized out>)
    at red_worker.c:10476
#6  0x00007ffff7cf95b2 in red_worker_main (arg=<optimized out>)
    at red_worker.c:10811
#7  0x0000003001407d31 in start_thread () from /lib64/libpthread.so.0
#8  0x00000030008efdfd in clone () from /lib64/libc.so.6