[Users] Code failure apparently due to changes in Reduction.c

[Roland Haas]

Hello Lorenzo,

> So is the call to CCTK_ReduceLocArrayToArray1D valid or not in Carpet? 
The function itself is valid (that function, ignoring its arguments, can
be used with Carpet) but the operator handle that was passed to it was
not. Hence the call (use of the function) was wrong. You must pass an
operator handle obtained from the Handle function matching the reduction
function you will call (and for the correct "old" / "new" interface
style). From the original email I concluded that you had used to obtain
a handle from "CCTK_LocalArrayReductionHandle" and that is wrong with
Carpet (though it has been long enough that I do not remember my actual
reasoning anymore). Thus any use of that handle with Carpet is wrong.

This is b/c Carpet only supports the "old" reduction interface. That
interface is (only) described in the "Old Reduction Specification API
Documentation" section fond just below here:

https://www.einsteintoolkit.org/usersguide/UsersGuide.html#x1-110000C1.7.5

> I'm
> asking because in Spritz we use Carpet and we have a call to
> CCTK_ReduceLocArrayToArray1D with the handle obtained from
> CCTK_ReductionArrayHandle (which should work according to the second thing
> you wrote) and this gives an error in reduction.
In my email that you quote
(http://lists.einsteintoolkit.org/pipermail/users/2021-August/008154.html)
I tried to give a list of the "correct" combinations and also the
combination one has to use with Carpet:

--8<--
"CCTK_ReduceLocArrayToArray1D" however is "old" interface, so is valid
with Carpet, but you must use "CCTK_ReductionArrayHandle" to get the
reduction handle for it. You must *not* use "CCCTK_ReductionHandle" or
"CCTK_LocalArrayReductionHandle" to get a handle to use with
"CCTK_ReduceLocArrayToArray1D".
--8<--

So if in Spritz you are using CCTK_ReductionArrayHandle and
CCTK_ReduceLocArrayToArray1D then you are doing things correctly and
all is fine.

Yours,
Roland

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