How to make sure a task is executed before starting different services

We have the following scenario in upstart:

We have some task called T, and some services, A and B with the following requirements:

T must run completely isolated from the services A and B
Both A and B can only run if task T has completed
A and B can be started independently

In simple words, T is a requirement for both A and B, but running T doesn't necessarily mean that either A or B should be started.

How can we enforce these requirements in upstart? Adding other "helper" jobs is fine, of course.

We tried the following, that doesn't work:

# T.conf
task
start on (starting A or starting B)

The problem is that if T is already running when starting B, e.g. because A is already about to start, then B will just start without waiting for T to finish. This violates the first two requirements above.

Another option is to explicitly start T from the pre-start sections of the services. However, that causes a service to fail to start, instead of waiting, if T is already being executed.

Solution

There is a workaround using this extra helper task (better suggestions are still welcome):

start on (starting A or starting B)
task
instance $JOB
script
    until start T; do sleep 1; done
end script

This helper job is started just about when either A or B is about to start, blocking those services. There will be one instance of this task for each service. It will block until T is successfully completed.