Waveforms with empty array initial values do not update as expected

[AlexanderWells-diamond]

When using a WaveformOut record and doing .set() and .get(), the behaviour is not as expected when the initial_value is an empty numpy array:

# Import the basic framework components.
from softioc import softioc, builder, asyncio_dispatcher
import numpy
# Create an asyncio dispatcher, the event loop is now running
dispatcher = asyncio_dispatcher.AsyncioDispatcher()

# Set the record prefix
builder.SetDeviceName("PREFIX")

# Create some records
wo = builder.WaveformOut("TEST", initial_value=numpy.array([]))

# Boilerplate get the IOC started
builder.LoadDatabase()
softioc.iocInit(dispatcher)

print(wo.get())
wo.set(numpy.array([10]))
print(wo.get())

# Finally leave the IOC running with an interactive shell.
softioc.interactive_ioc(globals())

This program prints:

...
iocRun: All initialization complete
[]
[]

If you change the initial_value to numpy.array([1]) then the program prints out the expected results:

...
iocRun: All initialization complete
[1]
[10]

Apologies if this is a duplicate of an existing issue, but it certainly is a novel symptom.

[AlexanderWells-diamond]

This is likely to be an issue in ProcessDeviceSupportOut._process. The check against empty arrays is not working as intended - passing a numpy.array(10.) and a numpy.array([]) passes the check and returns EPICS_OK.

The probable fix for this is to extend the if check to do something like this:

if value.shape == self._value.shape and (value == self._value).all() and not self.__always_update:

This needs testing and tweaking for all Out record types, as this'll obviously fail for any non-array values.

[Araneidae]

Ah. The problem is the maximum array length is now zero!

There are two places where an error could be reported here:

1. A zero length waveform is invalid
2. We should probably raise an error when truncating writes to arrays