It looks like the librarian or librarian teardown hung itself with a socket timeout in build 436. The full log is attached.
Here is an excerpt of the log with the relevant information:
{{{
Traceback (most recent call last):
...
File "/srv/buildbot/slaves/launchpad/lucid-db-devel/build/lib/canonical/testing/layers.py", line 861, in _check_and_reset
"shutdown: " + str(e)
canonical.testing.layers.LayerIsolationError: Librarian has been killed or has hung. ... : [Errno socket error] timed out
...
WARNING: A test appears to be hung. There has been no output for 600 seconds.
Forcibly shutting down the test suite
...
The dying processes left behind the following output:
--------------- BEGIN OUTPUT ---------------
**********************************************************************
Could not communicate with subprocess
**********************************************************************
---------------- END OUTPUT ----------------
Tests failed (exit code -15)
}}}
The LOSAs have confirmed that the librarian pidfile was cleaned up properly after the run, so it wasn't a complete process layer crash.