[Openembedded-architecture] Yocto Compatible 2.0 + signature changes

[Phil Blundell]

On Wed, 2017-05-10 at 16:41 -0400, Trevor Woerner wrote:
> If simply adding a Yocto Compatible 2.0 layer to a build doesn't
> cause
> any of its *bbappends to affect the build by default, but simply
> adding any other (presumably non Yocto Compatible 2.0) layer to a
> build has the potential to affect the build simply by virtue of
> having
> added the layer, wouldn't that be horribly confusing for users?

Well, the status quo is that some layers do modify the build and others
don't.  Adding some sort of branding to indicate which ones are known
not to have such effects (given a known set of other layers in use)
seems like a positive step and I don't really understand how this would
be "horribly confusing". 

> Shouldn't this sort of complete reversal of policy regarding layers
> be
> something that's enforced by the tool (bitbake) so that either all
> layers affect the build simply by virtual of being added, or they
> don't?

I don't entirely understand your comment about a complete reversal. 
But that aside, no, I don't think it would be desirable for bitbake to
attempt to police this, nor am I entirely convinced that it is
technically feasible for bitbake to do that in the general case. 
Consider:

1. If layer A and layer B both include different versions of the same
recipe then bitbake has to pick one or other of them.  If a user was
previously including one of these layers and then adds the other one
then there is no way to guarantee that bitbake will make the same
choice it did before.  So in the most general case, the only way to
ensure that a layer cannot conflict with any other layer is for it
either to contain no recipes, or to have an infinitely low preference
such that it is never selected.  And the latter of these two approaches
is not scalable to more than one layer, so the logical conclusion would
be that all layers must contain no recipes.  This does not seem like a
useful conclusion in the real world :-}

2. Some users, including me, have layers for internal use that contain
.bbappends whose whole purpose is to modify the behaviour of recipes in
other layers (generally oe-core).  It would be annoying if such things
were outlawed or required a lot of extra contortion.

p.