Query GetEngine Prior to Open

[ax3l]

In the call to adios2::IO::Open( file, mode ), engines according to file, mode and options are picked.
For some situations when we use ADIOS2, i.e. #2487, but also future transitions to new transports and formats, we would like to know before the Open() call which engine will be picked by it.

Why is this feature important?

This allows us to quickly address some high-level selections and work-arounds, i.e. if an engine is not 100% compatible with older functionality and/or if we do breaking changes in the way we represented data in ADIOS1, ADIOS2 or future versions of ADIOS2.

What is the potential impact of this feature in the community?

It would allow us to implement robust "workflow"/"backend"/"engine" selectors in openPMD-api without having to test-open potentially large files.

Is your feature request related to a problem? Please describe.

It's related to confusion for users in issues such as:

• BP3 and BP4 (and BP5) File Magic #2487
• openPMD adios1 output defective ComputationalRadiationPhysics/picongpu#3378
• BP3 in ADIOS2: Reads Fail on Some Attributes openPMD/openPMD-api#726

Describe the solution you'd like and potential required effort

A potential solution could implement an GetEngine( file, mode ) function that takes the same parameters as Open() but does only perform the quick logic checks outlined in #2487 and returns an enum or string that uniquely describes the engine it would pick.

I am not sure if GetEngine() would need to be a member of the adios2::IO object, since in some cases we might decide to not use the adios::IO object to read the file at all based on the result of it. So potentially, a standalone function will do.

Describe alternatives you've considered and potential required effort

Alternatives would need to probe-open files (can be slow).
We can also duplicate the logic inside ADIOS2's IO::Open() in openPMD-api, but this would not be very robust since it might change over time (e.g. with new engines being added).

Additional context

Discussed in the WarpX-ADIOS2 meeting on Oct 21st, 2020.

cc @pnorbert @guj @franzpoeschel @dmitry-ganyushin

[williamfgc]

@ax3l @pnorbert just an idea as I ran into a similar situation in another project. This could be another Mode to Open the file to just verify if it exist, only Engine::bool(), Engine::Name() and Engine::Type() would work. That way the API is not altered.

//Ping is an option, I don't have a good name
adios2::Engine engine = io.Open("file.bp", adios2::Mode::Ping);  
if( engine && engine.Type() == "BP3" ){ //do something }