Terraform uses "implied" move statements to represent the situation where
it automatically handles a switch from count to no-count on a resource.
Because that situation requires targeting only a specific resource
instance inside a specific module instance, implied move statements are
always presented as if they had been declared in the root module and then
traversed through the exact module instance path to reach the target
resource.
However, that means they can potentially cross a module package boundary,
if the changed resource belongs to an external module. Normally we
prohibit that to avoid the root module depending on implementation details
of the called module, but Terraform generates these implied statements
based only on information in the called module and so there's no need to
apply that same restriction to implied move statements, which will always
have source and destination addresses belonging to the same module
instance.
This change therefore fixes a misbehavior where Terraform would reject
an attempt to switch from no-count to count in a called module, where
previously the author of the calling configuration had no recourse to fix
it because the change has actually happened upstream.
Create a separate `validateMoveStatementGraph` function so that
`ValidateMoves` and `ApplyMoves` both check the same conditions. Since
we're not using the builtin `graph.Validate` method, because we may have
multiple roots and want better cycle diagnostics, we need to add checks
for self references too. While multiple roots are an error enforced by
`Validate` for the concurrent walk, they are OK when using
`TransitiveReduction` and `ReverseDepthFirstWalk`, so we can skip that
check.
Apply moves must first use `TransitiveReduction` to reduce the graph,
otherwise nodes may be skipped if they are passed over by a transitive
edge.
This is a first pass at implementing refactoring.ValidateMoves, covering
the main validation rules.
This is not yet complete. A couple situations not yet covered are
represented by commented test cases in TestValidateMoves, although that
isn't necessarily comprehensive. We'll do a further pass of filling this
out with any other subtleties before we ship this feature.
As of this commit, refactoring.ValidateMoves doesn't actually do anything
yet (always returns nil) but the goal here is to wire in the set of all
declared instances so that refactoring.ValidateMoves will then have all
of the information it needs to encapsulate our validation rules.
The actual implementation of refactoring.ValidateMoves will follow in
subsequent commits.
This is a whole lot of nothing right now, just stubbing out some control
flow that ultimately just leads to TODOs that cause it to do nothing at
all.
My intent here is to get this cross-cutting skeleton in place and thus
make it easier for us to collaborate on adding the meat to it, so that
it's more likely we can work on different parts separately and still get
a result that tessellates.
Martin Atkins