37 alternative concurrency handling

[Monarda]

This is a messy PR because it includes fixes for the problems in the previous PR merged into main. However, once those are excluded there are two places to look. Firstly, the file test/integrations/test_mixed_concurrency.py shows what these changes allow. It's not possible to use threading and asyncio in the same program.

Then the files in p4pillon/thread and p4pillon/asyncio use a version of monkey-patching to slightly alter the base files to use the appropriate concurrency model.

Many of the remaining changes are then switching from the p4pillon.concurrency string to the new method.

[Monarda]

This example may make it more obvious than the new tests what this allows:

from p4p.server import Server

from p4pillon.asyncio.sharednt import SharedNT as AsyncioSharedNT
from p4pillon.nt import NTScalar
from p4pillon.thread.sharednt import SharedNT as ThreadSharedNT

a = AsyncioSharedNT(nt=NTScalar("d"), initial=5.5)
b = ThreadSharedNT(nt=NTScalar("d"), initial=9.9)

Server.forever(
    providers=[
        {
            "demo:a": a,
            "demo:b": b,
        }
    ]
)

You'll need to do your own pvgets

[Monarda]

A little more thought... I think I can see how to do some (all?) of this with straightforward subclassing rather than weird monkey-patching.

[Monarda]

The latest commit shows an example for server of using subclasses in a straightforward way to control the concurrency model.

I've also begun adding ABC to those classes that aren't intended to have concrete instances without specifying the concurrency model.

[Monarda]

These changes weren't as wide-ranging as I expected. And, in fact, even the change to server isn't fundamentally different to the monkey-patching route.

However, this does now include adding ABC (abstract base class) in a number of places. This means that, for example. only p4pillon.thread.server or p4pillon.asyncio.server may be instantiated. p4pillon.server.server can be used to specify an interface but it can't be itself instantiated any longer. I've even added this into p4p.server.raw.SharedPV so you can no longer accidentally use it.