The graph walking mechanism is specified as requiring a graph with a single
root, which in practice means there's exactly one node in the graph
which doesn't have any dependencies.
However, we previously weren't verifying that invariant is true for
subgraphs returned from DynamicExpand. It was working anyway, but it's not
ideal to be relying on a behavior that isn't guaranteed by our underlying
infrastructure.
We also previously had the RootTransformer being a bit clever and trying
to avoid adding a new node if there is already only a single graph with
no dependencies. That special case isn't particularly valuable since
there's no harm in turning a one-node graph into a two-node graph with
an explicit separate root node, and doing that allows us to assume that
the root node is always present and is always exactly terraform.rootNode.
Many existing DynamicExpand implementations were not producing valid
graphs and were previously getting away with it. All of them now produce
properly-rooted graphs that should pass validation, and we will guarantee
that with an explicit check of the DynamicExpand return value before we
try to walk that subgraph. For good measure we also verify that the root
node is exactly terraform.rootNode, even though that isn't strictly
required by our graph walker, just to help us catch potential future bugs
where a DynamicExpand implementation neglects to add our singleton root
node.
In earlier incarnations of these transformers we used the set of all
available providers for tasks such as generating implied provider
configuration nodes.
However, in modern Terraform we can extract all of the information we need
from the configuration itself, and so these transformers weren't actually
using this set of provider addresses.
These also ended up getting left behind as sets of string rather than sets
of addrs.Provider in our earlier refactoring work, which didn't really
matter because the result wasn't used anywhere anyway.
Rather than updating these to use addrs.Provider instead, I've just
removed the unused arguments entirely in the hope of making it easier to
see what inputs these transformers use to make their decisions.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
