Imagine we have "platform:f29.0.0" and "platform:f29.1.0" base modules.
We also have "DBI" module we want to build agaisnt "platform:f29.1.0".
This "DBI" module depends on "perl" module which is only build against
"platform:f29.0.0".
Currently, DBI build would fail to resolve the dependencies, because
it wouldn't find "perl" module, because it is built against different
platform stream.
This PR changes the MSE code to include buildrequired module builds built
against all the compatible platform streams.
It does so by introducing following changes:
- MSE code uses new get_base_module_mmds() method to find out all the
compatible platform modules. This needed new methods in DBResolver
and MBSResolver.
- For each buildrequired module defined by name:stream, the MSE code then
finds particular NSVC built against each compatible platform module.
Side effect of these code changes is that every module now must buildrequire
some base module.
1. Changed ModuleBuild's context property to db column, it's
non-nullable and default value is '00000000' to keep it consistent
with previous behaviour.
2. Changed ModuleBuild.contexts_from_mmd to return a tuple of
(ref_build_context, build_context, runtime_context, context).
3. Updated tests affected by this change.
