Comment 69 for bug 1421009

Some weekend numbers of how many reboots needed to reproduce the problem on mako: 30, 12, 15, 59, 15, 9, 26, 10, 6. There's no clear upper limit.

In other testing, reverting the libusermetrics landing from February does not seem to cure the problem - I was able to reproduce the problem also with the revert I've pushed to https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/landing-007/+packages

I've improved the test case in the description several times to get fuller backtraces. With the latest version I was able to see that both in case of the current libusermetrics and the reverted one the backtrace leads back to usermetrics's DBus usage. Which is not to say there's anything wrong with libusermetrics, as the bug is in multi-threaded handling of DBus inside Qt. The DBus call being called is:
#17 0xffffffff in dbus_bus_add_match (connection=0xe9d140, rule=0x11ba818 "type='signal',sender='com.canonical.UserMetrics',path='/com/canonical/UserMetrics/DataSource/2',interface='com.canonical.usermetrics.DataSource',member='emptyDataStringChanged'", error=0x0) at ../../dbus/dbus-bus.c:1553
        msg = 0xf76058
        __FUNCTION__ = "dbus_bus_add_match"