Resetting a compress record to empty it

[Érico Nogueira Rolim via Tech-talk <tech-talk at aps.anl.gov>]

Hi folks!

We are using compress records as circular buffers, and would like to be
able to empty them between events. However, our current implementation
feels hackish, so I'd like to see if we are missing something.

The circular buffer is implemented similarly to below:


record(calc, "scalar"){
     field(CALC, "VAL+1")
}

record(compress, "array"){
     field(ALG, "Circular Buffer")
     field(NSAM, "100")
     field(INP, "scalar PP")
}


Emptying them is done manually, with someone writing to a PV which
writes into the records' RES fields. There are two naive ways of
implementing the reset PV:


record(longout, "reset-array"){
     field(OUT, "array.RES")
}

record(longout, "reset-array-pp"){
     field(OUT, "array.RES PP")
}


"reset-array" doesn't process the record, so monitor clients don't
received its changed value; only new clients (or explicit Get
operations) see the empty vector. It does leave "array" empty, though,
so subsequent processing of the record will leave it with the correct
amount of elements.


"reset-array-pp" processes the record 'too much'. It will be emptied,
then processed once, so "scalar" is processed as well and "array"
fetches that value, ending up with one element, not empty.


The only way we have managed to make it work, so that monitor clients
(GUI and CLI) observe an empty vector, feels somewhat hackish. It's done
with seq:


record(seq, "reset-array-full"){
     field(DOL1, 1)
     field(DOL2, 1)
     field(LNK1, "array.RES PP") # empty record, process it, ends up
with one element
     field(LNK2, "array.RES") # somehow empties record before monitor is
actually posted
}

Is there some better way of implementing this?

If not, is the behavior of "reset-array-full" something we can rely on?
I'd be okay with relying on it, especially if I understood it better.

And could something be done to fix it in epics-base, perhaps always
posting monitors on writes to "special(SPC_RESET)" fields? Would that
make sense?


Cheers,

Érico


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