Add internal caching

[jesse-gallagher]

It'd probably be best to add some filesystem caches like Tycho, but that always has some expiration problems.

[mickaelistria]

What about directly reusing some Tycho clases as API?
Or what about brining the p2-layout-provider extension directly as a feature into Tycho?

[jesse-gallagher]

I think the former is how I'd go about doing it - I'd want to make sure that the cache use is compatible with how Tycho does it, and so reusing the same code directly makes the most sense.

As for integrating this directly in Tycho, part of it is the daunting prospect of properly contributing to the Tycho codebase, but a more-technical problem is that Tycho already has its own concept of a "p2" repository type that isn't quite the same. However, the actual Maven repository handlers for it in Tycho are, I believe, mostly stubbed out, so it'd likely be possible to add "real" resolution capabilities to them without disrupting their existing use as hooks for the Equinox resolver Tycho uses.

[mickaelistria]

the daunting prospect of properly contributing to the Tycho codebase

the codebase has some complex part, but some parts are, I hope, easy enough to contribute to.

Tycho already has its own concept of a "p2" repository type that isn't quite the same.

I don't think there is a difference in the definition of a p2 repository. How it can be used or not can make a difference but...

the actual Maven repository handlers for it in Tycho are, I believe, mostly stubbed out, so it'd likely be possible to add "real" resolution capabilities to them without disrupting their existing use as hooks for the Equinox resolver Tycho uses.

This resolver are only used for the eclipse-plugin, eclipse-feature, eclipse-repository and other Tycho-specific packaging types; they wouldn't interfer for a plain "jar" project. And even if there are some conflicts at execution, I think it would still be valuable for Tycho, and for this code, to have it as a separate org.eclipse.tycho:p2-resolution extension, but inside Tycho.
Tycho doesn't strongly maintain APIs, so if you start using APIs, they may change at some point and break this extension; while with being directly inside Tycho, this extension will be built together, and there would be no such risk of unnoticed breaking change before release.