Xmir hangs in mir_buffer_stream_get_graphics_region() from xmir_input_set_cursor()

Xmir freezes in mir_buffer_stream_get_graphics_region() from xmir_input_set_cursor()

Seems to happen a lot when testing Firefox and Chrome when entering/leaving the address bar...

(gdb) bt
#0 pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
#1 0x00007f5ea91e3c5c in std::condition_variable::wait(std::unique_lock<std::mutex>&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2 0x00007f5eac25b6b2 in std::condition_variable::wait<mir::client::PromiseStateBase<std::shared_ptr<mir::client::MirBuffer> >::ReadLock::ReadLock(mir::client::PromiseStateBase<std::shared_ptr<mir::client::MirBuffer> >&)::{lambda()#1}>(std::unique_lock<std::mutex>&, mir::client::PromiseStateBase<std::shared_ptr<mir::client::MirBuffer> >::ReadLock::ReadLock(mir::client::PromiseStateBase<std::shared_ptr<mir::client::MirBuffer> >&)::{lambda()#1}) ()
    at /usr/include/c++/6/condition_variable:99
#3 __base_ctor (parent=..., this=0x7fff74877420)
    at ./src/client/no_tls_future-inl.h:136
#4 mir::client::PromiseStateBase<std::shared_ptr<mir::client::MirBuffer> >::ensure_read_context() () at ./src/client/no_tls_future-inl.h:152
#5 get_value (this=0x55de6688f830) at ./src/client/no_tls_future-inl.h:223
#6 get (this=0x7f5e9c008160) at ./src/client/no_tls_future-inl.h:354
#7 mir::client::BufferDepository::advance_current_buffer(std::unique_lock<std::mutex>&) (this=this@entry=0x7f5e9c008160, lk=...)
    at ./src/client/buffer_stream.cpp:166
#8 0x00007f5eac25f9ed in current_buffer_id (this=<optimised out>)
    at ./src/client/buffer_stream.cpp:183
#9 get_current_buffer (this=0x7f5e9c007f40)
    at ./src/client/buffer_stream.cpp:400
#10 0x00007f5eac25f517 in secure_for_cpu_write (this=0x7f5e9c007f40)
    at ./src/client/buffer_stream.cpp:419
#11 0x00007f5eac263f28 in mir_buffer_stream_get_graphics_region (
    buffer_stream=buffer_stream@entry=0x7f5e9c007f48,
    region_out=region_out@entry=0x7fff748775d0)
    at ./src/client/mir_buffer_stream_api.cpp:149
#12 0x000055de647da90e in xmir_input_set_cursor (cursor=0x55de6687ff60,
    xmir_input=<optimised out>, xmir_input=<optimised out>)
    at xmir-cursor.c:108
#13 0x000055de64829eab in miPointerUpdateSprite (pDev=0x55de66621c10)
    at mipointer.c:468
#14 0x000055de6482a0fa in miPointerDisplayCursor (pDev=0x55de66621c10,
    pScreen=0x55de65c775b0, pCursor=0x55de6687ff60) at mipointer.c:206
#15 0x000055de64816bf1 in CursorDisplayCursor (pDev=0x55de66621c10,
    pScreen=0x55de65c775b0, pCursor=0x55de6687ff60) at cursor.c:150
#16 0x000055de6489f4e0 in AnimCurDisplayCursor (pDev=0x55de66621c10,
    pScreen=0x55de65c775b0, pCursor=0x55de6687ff60) at animcur.c:220
#17 0x000055de64910cc8 in ChangeToCursor (pDev=0x55de66621c10,
    cursor=0x55de6687ff60) at events.c:937
#18 0x000055de64912107 in WindowHasNewCursor (pWin=pWin@entry=0x55de66840be0)
    at events.c:3369
#19 0x000055de64939740 in ChangeWindowAttributes (pWin=0x55de66840be0,
    vmask=<optimised out>, vlist=vlist@entry=0x55de6690e17c,
    client=client@entry=0x55de667e37e0) at window.c:1561
#20 0x000055de64901bed in ProcChangeWindowAttributes (client=0x55de667e37e0)
    at dispatch.c:726
#21 0x000055de64907ed5 in Dispatch () at dispatch.c:479
#22 0x000055de6490be58 in dix_main (argc=6, argv=0x7fff74877a58,
    envp=<optimised out>) at main.c:287
#23 0x00007f5eaa4bf3f1 in __libc_start_main (main=0x55de647d5f70 <main>,
    argc=6, argv=0x7fff74877a58, init=<optimised out>, fini=<optimised out>,
    rtld_fini=<optimised out>, stack_end=0x7fff74877a48)
    at ../csu/libc-start.c:291
#24 0x000055de647d5faa in _start ()