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.
Due to how often the state and plan types are referenced throughout
Terraform, there isn't a great way to switch them out gradually. As a
consequence, this huge commit gets us from the old world to a _compilable_
new world, but still has a large number of known test failures due to
key functionality being stubbed out.
The stubs here are for anything that interacts with providers, since we
now need to do the follow-up work to similarly replace the old
terraform.ResourceProvider interface with its replacement in the new
"providers" package. That work, along with work to fix the remaining
failing tests, will follow in subsequent commits.
The aim here was to replace all references to terraform.State and its
downstream types with states.State, terraform.Plan with plans.Plan,
state.State with statemgr.State, and switch to the new implementations of
the state and plan file formats. However, due to the number of times those
types are used, this also ended up affecting numerous other parts of core
such as terraform.Hook, the backend.Backend interface, and most of the CLI
commands.
Just as with 5861dbf3fc49b19587a31816eb06f511ab861bb4 before, I apologize
in advance to the person who inevitably just found this huge commit while
spelunking through the commit history.
Due to how deeply the configuration types go into Terraform Core, there
isn't a great way to switch out to HCL2 gradually. As a consequence, this
huge commit gets us from the old state to a _compilable_ new state, but
does not yet attempt to fix any tests and has a number of known missing
parts and bugs. We will continue to iterate on this in forthcoming
commits, heading back towards passing tests and making Terraform
fully-functional again.
The three main goals here are:
- Use the configuration models from the "configs" package instead of the
older models in the "config" package, which is now deprecated and
preserved only to help us write our migration tool.
- Do expression inspection and evaluation using the functionality of the
new "lang" package, instead of the Interpolator type and related
functionality in the main "terraform" package.
- Represent addresses of various objects using types in the addrs package,
rather than hand-constructed strings. This is not critical to support
the above, but was a big help during the implementation of these other
points since it made it much more explicit what kind of address is
expected in each context.
Since our new packages are built to accommodate some future planned
features that are not yet implemented (e.g. the "for_each" argument on
resources, "count"/"for_each" on modules), and since there's still a fair
amount of functionality still using old-style APIs, there is a moderate
amount of shimming here to connect new assumptions with old, hopefully in
a way that makes it easier to find and eliminate these shims later.
I apologize in advance to the person who inevitably just found this huge
commit while spelunking through the commit history.
