[OE-core] how does "PREFERRED_PROVIDER" interact with layer priorities?

[Andre McCurdy]

On Mon, Feb 17, 2020 at 4:02 AM Robert P. J. Day <rpjday at crashcourse.ca> wrote:
>
>   (note: when i finally learn python, i'll be able to RTFS and figure
> out stuff like this without assistance. until then ...)
>
>   was going to enhance the explanation of PREFERRED_PROVIDER in the
> docs, until i realized i don't understand it completely.
>
>   first, if there is a recipe named, say, "foo", then even if there
> are other recipes (with different names) that include:
>
>   PROVIDES = "foo"
>
> by default (and ignoring layer priority complications), if there is a
> dependency on "foo", it is the "foo" recipe that will be selected as
> the provider, correct? that is, if there is a recipe that *exactly*
> matches the name requested, that's the selected provider?

Never seen that stated before. The recipe name (actually ${PN}) is
known to be a provider for the recipe because it's prepended to
PROVIDES from bitbake.conf. There probably isn't anything special (ie
no guaranteed precedence) about PROVIDES being set to the recipe name
from bitbake.conf vs being set to something else from within the
recipe.

>   conversely, if one wants to override that default selection, one
> would need to:
>
>   PREFERRED_PROVIDER_foo = "foo_experimental"
>
> in short, if the above is true, then there would seem to be no value
> in ever stating:
>
>   PREFERRED_PROVIDER_foo = "foo"
>
> but i just noticed that that's done a fair bit in oe-core's
> default-providers.inc, as in (just a few examples):
>
>   PREFERRED_PROVIDER_opkg ?= "opkg"
>   PREFERRED_PROVIDER_pkgconfig ?= "pkgconfig"
>   PREFERRED_PROVIDER_openssl ?= "openssl"
>
> that puzzled me until i saw this snippet from the same file:
>
>   # Alternative is ltp-ddt in meta-oe: meta-oe/recipes-devtools/ltp-ddt/ltp-ddt_0.0.4.bb
>   PREFERRED_PROVIDER_ltp ?= "ltp"
>
> which inspired the conclusion that if there is a higher priority layer
> that provides that recipe name, *that* recipe would be used instead.
> and since prio of oe-core is 5 and prio of meta-oe is 6, if you want
> the *regular* oe-core recipe of "ltp", you need to explicitly ask for
> it. is that what's going on here? because if it isn't, then i'm
> unclear on the purpose of lines like the above.

If there are multiple possible providers then bitbake will generate a
warning if you don't specify which one you prefer.

> rday
>
> p.s. ironically, i was just going to see what meta-oe's recipe of
> "ltp-ddt" was doing, but that recipe was deleted so, at the very
> least, that comment should be removed.
> --
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