The failure is an assertion being hit in libxcb rather than a segfault,
which implies that the library isn't bugged itself but rather it's
catching a fault higher up in the system.
The xcb assertion code is:
if (XLIB_SEQUENCE_COMPARE(event_sequence, >,
dpy->request))
{
throw_thread_fail_assert("Unknown sequence "
"number while "
"processing queue",
xcb_xlib_threads_sequence_lost);
There is a big warning in the comments above this code:
/* Thread-safety rules:
*
* At most one thread can be reading from XCB's event queue at a time.
* If you are not the current event-reading thread and you need to find
* out if an event is available, you must wait.
*
* The same rule applies for reading replies.
*
* A single thread cannot be both the the event-reading and the
* reply-reading thread at the same time.
*
* We always look at both the current event and the first pending reply
* to decide which to process next.
*
* We always process all responses in sequence-number order, which may
* mean waiting for another thread (either the event_waiter or the
* reply_waiter) to handle an earlier response before we can process or
* return a later one. If so, we wait on the corresponding condition
* variable for that thread to process the response and wake us up.
*/
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https://bugs.launchpad.net/bugs/901675
Title:
apport-gtk assert failure: python: ../../src/xcb_io.c:273:
poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.
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