When support for osbuild result was added into osbuild-composer it was in
a bit hacky way - localtarget's location was reused as a path for the
result. This didn't make much sense because we want to store the result
even when image build has no localtarget.
Several past commits made store less dependant on the localtarget. The
responsibility for "holding the paths" to build artifacts was gradually
switched from the localtarget to the store while still maintaining
backwards compatibility - localtarget.Location still pointed at the
correct location.
This commit finishes the switch: local target now has no Location field.
The store is now fully responsible for managing the artifacts and paths
to them. LocalTarget is now just a simple "switch" - if image build has it,
then worker uploads an image into the store and it's then available for
download using the weldr API.
Go doesn't really do variants, so we must somehow emulate it. The
json objects we use are essentially tagged unions, with a `name`
field in reverse domain name notation identifying the type and a
type specific 'options' object.
In Go we represent this by having an BarOptions interface, which
implements a private method `isBarOptions()`, making sure that only
types in the same package are able to implement it. Each type FooBar
that should belong to the variant implements the interface, and a
constructor `NewFooBar(options *FooBarOptions) *Bar` that makes sure
the `name` field is set correctly.
This would be enough to represent our types and marshal them into
JSON, but unmarshalling would not work (json does not know about
our tags, so would not know what concrete types to demarshal to).
We therefore must also implement the Unmarshall interface for Bar,
to select the right types for the Options field.
We implement his logic for Target, Stage and Assembler. A handful
of concrete types are also implemented, matching what osbuild
supports.
Signed-off-by: Tom Gundersen <teg@jklm.no>
