In our recent refactoring of the state manager interfaces we made serial
and lineage management the responsibility of the state managers
themselves, not exposing them at all to most callers, and allowing for
simple state managers that don't implement them at all.
However, we do have some specific cases where we need to preserve these
properly when available, such as migration between backends, and the
"terraform state push" and "terraform state pull" commands.
These new functions and their associated optional interface allow the
logic here to be captured in one place and access via some simple
calls. Separating this from the main interface leaves things simple for
the normal uses of state managers.
Since these functions are mostly just thin wrappers around other
functionality, they are not yet well-tested directly, but will be
indirectly tested through the tests of their callers. A subsequent commit
will add more unit tests here.
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