The fix that landed in #6557 was unfortunately the wrong subset of the
work I had been doing locally, and users of the attached bugs are still
reporting problems with Terraform v0.6.16.
At the very last step, I attempted to scope down both the failing test
and the implementation to their bare essentials, but ended up with a
test that did not exercise the root of the problem and a subset of the
implementation that was insufficient for a full bugfix.
The key thing I removed from the test was a _referencing output_ for the
module, which is what breaks down the #6557 solution.
I've re-tested the examples in #5440 and #3268 to verify this solution
does indeed solve the problem.
Wow this one was tricky!
This bug presents itself only when using planfiles, because when doing a
straight `terraform apply` the interpolations are left in place from the
Plan graph walk and paper over the issue. (This detail is what made it
so hard to reproduce initially.)
Basically, graph nodes for module variables are visited during the apply
walk and attempt to interpolate. During a destroy walk, no attributes
are interpolated from resource nodes, so these interpolations fail.
This scenario is supposed to be handled by the `PruneNoopTransformer` -
in fact it's described as the example use case in the comment above it!
So the bug had to do with the actual behavor of the Noop transformer.
The resource nodes were not properly reporting themselves as Noops
during a destroy, so they were being left in the graph.
This in turn triggered the module variable nodes to see that they had
another node depending on them, so they also reported that they could
not be pruned.
Therefore we had two nodes in the graph that were effectively noops but
were being visited anyways. The module variable nodes were already graph
leaves, which is why this error presented itself as just stray messages
instead of actual failure to destroy.
Fixes#5440Fixes#5708Fixes#4988Fixes#3268
