[Xcb] libxcb deadlocking on select() call in _xcb_in_read_block

[Bryce Harrington]

Hi,

Xubuntu users are seeing a fairly serious intermittent-but-frequent
issue where dbus-launch hangs on the select() call in
_xcb_in_read_block().  Both xfce4-session and dbus-launch get stuck;
killing dbus-launch releases the deadlock - so it sounds like possibly a
race condition?

I've also posted this bug to bugzilla the other day (fdo #16420), but
wanted to raise it here since it seems to be an extremely critical issue
for Xubuntu Hardy.

  https://bugs.freedesktop.org/show_bug.cgi?id=16420

We're not absolutely certain that it's a bug in libxcb or an underlying
issue that libxcb is just exposing when in the Xubuntu environment, but
if so would appreciate pointers to the real culprit.  (There's also a
small number of reports it affects Gnome too, but these are pretty
anecdotal at this point.)

#0 0xb8002424 in __kernel_vsyscall ()
#1 0xb7e8484d in select () from /lib/tls/i686/cmov/libc.so.6
#2 0xb7da309a in _xcb_in_read_block (c=0x80579a8, buf=0x8057040, len=8)
    at xcb_in.c:248
#3 0xb7da2343 in xcb_connect_to_fd (fd=13, auth_info=0xbff1cdf0)
    at xcb_conn.c:133
#4 0xb7da4a51 in xcb_connect (displayname=0x0, screenp=0x0) at
xcb_util.c:279
#5 0xb7f43717 in _XConnectXCB () from /usr/lib/libX11.so.6
#6 0xb7f2c029 in XOpenDisplay () from /usr/lib/libX11.so.6
#7 0x0804b3de in x11_init () at dbus-launch-x11.c:218
#8 0x0804abb2 in main (argc=5, argv=0xbff1d5a4) at dbus-launch.c:432

Advice would be most appreciated, thanks ahead of time.

Bryce