We previously had mechanisms to clean up only individual instance states,
leaving behind empty resource husks in the state after they were all
destroyed.
This takes care of it in the "orphan" case. It does not yet do it in the
"terraform destroy" or "terraform plan -destroy" cases because we don't
have anywhere to record in the plan that we're actually destroying and so
the resource configurations should be ignored and _everything_ should be
cleaned. We'll let the state be not-quite-empty in that case for now,
since it doesn't really hurt; cleaning up orphans is the main case because
the state will live on afterwards and so leftover cruft will accumulate
over the course of many changes.
If we don't set it, we end up creating an invalid plan where the destroy
changes don't have a provider address set, which then later fails
decoding when round-tripped through a planfile.
This also includes some extra safety checks in EvalDiff and
EvalDiffDestroy so that we can catch this bug sooner in future.
This change is verified by
TestContext2Apply_plannedDestroyInterpolatedCount, which is now passing.
Previously we used a single plan action "Replace" to represent both the
destroy-before-create and the create-before-destroy variants of replacing.
However, this forces the apply graph builder to jump through a lot of
hoops to figure out which nodes need it forced on and rebuild parts of
the graph to represent that.
If we instead decide between these two cases at plan time, the actual
determination of it is more straightforward because each resource is
represented by only one node in the plan graph, and then we can ensure
we put the right nodes in the graph during DiffTransformer and thus avoid
the logic for dealing with deposed instances being spread across various
different transformers and node types.
As a nice side-effect, this also allows us to show the difference between
destroy-then-create and create-then-destroy in the rendered diff in the
CLI, although this change doesn't fully implement that yet.
Previously our handling of create_before_destroy -- and of deposed objects
in particular -- was rather "implicit" and spread over various different
subsystems. We'd quietly just destroy every deposed object during a
destroy operation, without any user-visible plan to do so.
Here we make things more explicit by tracking each deposed object
individually by its pseudorandomly-allocated key. There are two different
mechanisms at play here, building on the same concepts:
- During a replace operation with create_before_destroy, we *pre-allocate*
a DeposedKey to use for the prior object in the "apply" node and then
pass that exact id to the destroy node, ensuring that we only destroy
the single object we planned to destroy. In the happy path here the
user never actually sees the allocated deposed key because we use it and
then immediately destroy it within the same operation. However, that
destroy may fail, which brings us to the second mechanism:
- If any deposed objects are already present in state during _plan_, we
insert a destroy change for them into the plan so that it's explicit to
the user that we are going to destroy these additional objects, and then
create an individual graph node for each one in DiffTransformer.
The main motivation here is to be more careful in how we handle these
destroys so that from a user's standpoint we never destroy something
without the user knowing about it ahead of time.
However, this new organization also hopefully makes the code itself a
little easier to follow because the connection between the create and
destroy steps of a Replace is reprseented in a single place (in
DiffTransformer) and deposed instances each have their own explicit graph
node rather than being secretly handled as part of the main instance-level
graph node.
Martin Atkins