separation between PVA and PVXS no longer holds

[gilesknap]

Up until commit 5265fdf it was possible to build ADCore against an epics-base without PVA submodules, simply by specifying USE_PVA=NO.

However this is no longer the case as ntndArrayConverterPvxs.cpp now includes a header file from PVA.

This is critical to our epics-containers infrastructure as we keep the containers small and tidy by only building PVXS in them and not PVA.

@MarkRivers is there some way we can keep that separation and still have a working PVXS plugin?

Thanks!

FYI @coretl @stephen-jd-dann @jsouter

[MarkRivers]

@gilesknap I want to get rid of that include since it is ugly code mixing 2 APIs. The issue is that compressed NTNDArrays need a way of identifying the actual datatype of the array. NDArrays have a dataType property that does not change when the array is compressed. NTNDArrays do not, they only have the actual datatype of the value field, which is byte for compressed data. Years ago we adopted the convention that codec.parameters would encode the actual datatype, using the ScalarType enum which is defined in pv/introspect.h. It is defined identically in the Java PVA library, so ImageJ and Phoebus have access to it as well. The problem is that I don't think PVXS defines an enum with the same values as PVA. We could cheat and just define it independently in ntndArrayConverterPvxs.cpp, but that is also ugly and prone to future issues.

I'm open to suggestions on how best to resolve this issue.

[gilesknap]

Thanks @MarkRivers.

@mdavidsaver is there, or should there be an equivalent enum in PVXS?

[mdavidsaver]

ADCore/ADApp/ntndArrayConverterSrc/ntndArrayConverterPvxs.cpp

Lines 5 to 10 in 72593ed

	#include <pv/pvIntrospect.h>
	using namespace std;
	using namespace epics::pvData;
	// Maps NDDataType to ScalarType
	static const enum ScalarType NDDataTypeToScalar[NDFloat64 + 1] = {

Ah, somehow I looked past this include. And I still had pvAccessCPP in RELEASE.local for ADCore. So my build "just worked".

The problem is that I don't think PVXS defines an enum with the same values as PVA.

Correct. These enum values are not part of the PVA wire protocol.

However, they are now de facto part of the NTNDArray specification. (a bit like how the layout of struct epicsTimeStamp unintentionally became part of the ChannelArchiver file format)

In the short term, I think it would be best if NDDataTypeToScalar in ntndArrayConverterPvxs.cpp became an int[].

At the same time, I think it would make sense to add some kind of "compressed type" mapper to struct NTNDArray in pvxs/nt.h. Preferably in a form which could be straightforwardly reflected into python for P4P.

[mdavidsaver]

simply by specifying USE_PVA=NO.

WITH_PVA? I ask because I tried unsuccessfully to build ADCore (only) with:

$ cat configure/CONFIG_SITE.local 
WITH_PVXS=YES
WITH_PVA=NO

but somehow in ADApp/ntndArrayConverterSrc/Makefile WITH_PVA is back to YES. Makefiles hurt my head...

[mdavidsaver]

Right, never mind. I had an assignment in configure/CONFIG_SITE.linux-x86_64.Common.

[MarkRivers]

Fixed in #569

[gilesknap]

@mdavidsaver. @MarkRivers thanks for a speedy fix. I can confirm this has worked for our container build.

Please can you add a version tag?