WIP: compilation-queue BLACS implementation

[chrisjbillington]

Inform runmanager how many shots are remaining in BLACS' queue.

Required for the "delayed compilation" feature in labscript-suite/runmanager#104 to work.

This is a bit of a hack presently, it happens in the queue's thread and so blocks for up to 5 seconds if runmanager is not running.

This would be better done as an extension that the queue pipes events to, and ideally the information should go out to whoever is listening over PUB/SUB rather than a method call to a specific runmanager instance.

It is possible to have multiple runmanager instances sending shots to BLACS, only the one specified in the labconfig file will receive the information, eg

[servers]
runmanager = localhost

I'll think about how to switch this to PUB/SUB. If designed to work with multiple runmanagers:

• Should BLACS tell each runmanager instance how many of its shots are remaining in the queue? Or how many shots total? Maybe a dict containing all the info, and runmanager instances can decide what to do with it.
• If the former, shots will need to contain information from what runmanager instance they came from. This can be gleaned either at shot submission time (zmq can give you the IP address of the client that sent the shots), or it could be in the shot file itself.

When I first wrote this code (this is ~2 years old), I remember thinking that PUB/SUB was going to be a problem, whereby e.g. closing and reopening runmanager or BLACS could result in deadlocks where they're both waiting on each other. But thinking about it now I don't see the problem, so will think more about it and switch this and the corresponding runmanager PR to use PUB/SUB for this if it seems to make sense. In the meantime, this WIP PR can be used to get the feedback feature working.