Cannot call `prepare` outside of a run

[callumforrester]

Pending #1210 it will be impossible to call prepare outside of a run with our current setup because prepare eventually calls the path provider, which depends on the state that is the current run.

sequenceDiagram
    plan->>+RE: prepare()
    RE->>+detector: prepare()
    detector->>+writer: arm()
    writer->>+path_provier: __call__()
    path_provier->>+writer: path (from numtracker, dependant on current run)
    writer->>+ioc: set_path(path)
    writer->>+ioc: open()
    writer->>-detector:
    detector->>-RE:
    RE->>-plan:

Loading

This issue is to discuss if we actually want that behaviour long-term

Steps To Reproduce

Steps to reproduce the behaviour:

1. Fix StartDocumentPathProvider does not clear its state after a run #1210
2. See that you can now never call prepare outside of a run

Acceptance Criteria

• Either
  • prepare does not depend on the current state that is the run
  • prepare gives a very clear error message when called outside of a run

[callumforrester]

From a discussion with @coretl and @abbiemery:

We should consider just providing more complete scanning plan stubs, since the most common use case of this is plans that call prepare to set the exposure time e.g.

def my_plan() -> MsgGenerator[None]:
    yield from bps.prepare(detector, TriggerInfo(livetime=0.1))  # This should fail because its outside of a run
    yield from bp.count([detector])  # This plan includes a run wrapper, so we can't wrap the whole of `my_plan` in a run

It would be nice to have a more composable solution e.g.

@bpp.plan_decorator()
def my_plan() -> MsgGenerator[None]:
    yield from bps.prepare(detector, TriggerInfo(livetime=0.1))
    yield from bps.count_stub([detector])  

However this still isn't a good pattern for other reasons: prepare is still opening the file writer, which may not be necessary if you just want to set the exposure time. count calls stage, which closes the file writer, so it will need to be reopened later anyway. Further discussion is needed on this point.

[abbiemery]

This is a feature not a bug. Prepare should not be able to be called outside of a run at DLS. Because prepare arms a detector's writers and opens the file. Since we have file naming that depends on the run you are in, this can't happen outside of one.

So without changing the responsibility of prepare (ie should it arm the detector or not) acceptance criteria 2, it should shout.

[coretl]

count calls stage, which closes the file writer, so it will need to be reopened later anyway. Further discussion is needed on this point.

So maybe count stub shouldn't open a run or stage?