[OE-core] Automatic prebuilt management

[chris.laplante at agilent.com]

> But I would like something smarter, e.g. first trying to check if the SRC_URI is available, if not switching on using the prebuilt package if available (e.g.
> in a DIR_PREBUILT)...
> 
> Before going further is there already an existing solution for that? do you have any recommendation on the easier/best way to
> achieve this?

I don't have a "best practice" recommendation to offer. But coincidentally I suggested something similar for upstreaming a few weeks ago. It was (rightfully) rejected as too "hacky" for inclusion in core code. Regardless, here's a bbclass we currently use:

addhandler skip_eventhandler
python skip_eventhandler() {
    try:
        bb.fetch2.Fetch(d.getVar("SRC_URI").split(), d)
    except bb.fetch2.FetchError:
        raise bb.parse.SkipRecipe("skip-inaccessible: could not access upstream repo; check SRC_URI, access rights, and network availability")
}
skip_eventhandler[eventmask] = "bb.event.RecipePreFinalise"


As was pointed out to me, this is a pretty heavy-handed approach - it can fail for any number of reasons, not just the URI actually being unavailable. So you may consider a more surgical check, e.g. actually trying to fetch the SRC_URI directly using raw GET requests.

To implement the handover, you'd have separate recipes for the build-from-source and the prebuilt case which PROVIDE the same package. You probably also want to set PREFERRED_PROVIDER in your layer conf to prefer the build-from-source recipe. 

Alternatively, you can do it all in a single recipe by rewriting the SRC_URI from within the event handler. But I think separate recipes is nicer.

Chris