Can't interrupt Status._set_thread waiting for value

[thopkins32]

Problem Description

I am building a Caproto IOC which has an acquire PV like so:

@acquire.putter
async def acquire(self, instance, value):
    ...
    # Start acquisition
    if value > 0:
        if self.unsafe_to_start:
            raise RuntimeError("Acquisition cannot start because it is unsafe to do so")
    ...

Now when I connect to this PV via Ophyd and try the following, it restricts my ability to try again once it is safe to do so:

acquire = EpicsSignal("ACQUIRE", timeout=None)

acquire.set(1)  # Signal.set completes but spins up a thread that waits forever

acquire.stop_set_in_progress()  # new method to interrupt?

# Assume it is now safe to start
acquire.set(1)  # Should succeed

Without the proposed stop_set_in_progress method, the failure I get is:

RuntimeError: Another set() call is still in progress for acquire

From what I can tell, there is no way to safely stop the Signal._set_thread once it is waiting. The value will never match 1 because of the RuntimeError on the server-side. Since the timeout=None, the Signal._set_thread will be waiting forever and I have to restart Python for the next acquire.set(1) call to work.

Alternatives Considered

You could configure a shorter timeout on this EpicsSignal as a workaround, but I still think there should be a way to stop the Signal._set_thread from waiting.

You could do a EpicsSignal.put instead but then I have to monitor the readback value in my own code to ensure that the value matches.