startpar-bridge cycles when logging upstart events (trusty)

Trusty 14.04. When installing a custom init script to log upstart events (see http://upstart.ubuntu.com/cookbook/#record-all-jobs-and-events-which-emit-an-event), PID 1 init consumes significant CPU and events are logged by the custom script in a repeating stream, like this:

upstart_events/ (/proc/self/fd/9):14382:Thu May 29 17:06:03 UTC 2014:Job startpar-bridge/upstart_events--started stopping.
upstart_events/ (/proc/self/fd/9):14386:Thu May 29 17:06:03 UTC 2014:Job startpar-bridge/upstart_events--stopped stopping.
upstart_events/ (/proc/self/fd/9):14390:Thu May 29 17:06:03 UTC 2014:Job startpar-bridge/upstart_events--started stopping.
upstart_events/ (/proc/self/fd/9):14394:Thu May 29 17:06:03 UTC 2014:Job startpar-bridge/upstart_events--stopped stopping.

It's possible that these reports are also reflect a variant of this issue:
* http://askubuntu.com/questions/367079/strange-boot-log-multiple-startpar-bridge-starting-and-stopping-repeatedly
* http://askubuntu.com/questions/419431/ubuntu-13-10-not-booting-startpar-bridge
* https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1244989/comments/3

To reproduce on Vagrant:
1. vagrant box add trusty64 https://cloud-images.ubuntu.com/vagrant/trusty/current/trusty-server-cloudimg-amd64-vagrant-disk1.box
2. vagrant init trusty64
3. vagrant up
4. vagrant ssh
5. Copy this script into /etc/init/upstart_events.conf:
description "Log upstart events"

start on ( starting JOB!=upstart_events \
        or started JOB!=upstart_events \
        or stopping JOB!=upstart_events \
        or stopped JOB!=upstart_events )
script
  exec 1>>/tmp/upstart_events.log
  echo -n "$UPSTART_JOB/$UPSTART_INSTANCE ($0):$$:`date`:"
  echo "Job $JOB/$INSTANCE $UPSTART_EVENTS."
end script
6. sudo reboot
7. vagrant ssh
8. tail -f /tmp/upstart_events.log