Now that outputs changes are tracked in full, we can remove the
comparisons with the prior state and use the planned changes directly.

This change refreshes the instance state during plan, so a complete
Refresh no longer needs to happen before Plan.

* unlock the state if Context() has an error, exactly as backend/remote does today
* terraform console and terraform import will exit before unlocking state in case of error in Context()
* responsibility for unlocking state in the local backend is pushed down the stack, out of backend.go and into each individual state operation
* add tests confirming that state is not locked after apply and plan

* backend/local: add checks that the state is unlocked after operations

This adds tests to plan, apply and refresh which validate that the state
is unlocked after all operations, regardless of exit status. I've also
added specific tests that force Context() to fail during each operation
to verify that locking behavior specifically.

This is a baby-step towards an intended future where all Terraform actions
which have side-effects in either remote objects or the Terraform state
can go through the plan+apply workflow.

This initial change is focused only on allowing plan+apply for changes to
root module output values, so that these can be written into a new state
snapshot (for consumption by terraform_remote_state elsewhere) without
having to go outside of the primary workflow by running
"terraform refresh".

This is also better than "terraform refresh" because it gives an
opportunity to review the proposed changes before applying them, as we're
accustomed to with resource changes.

The downside here is that Terraform Core was not designed to produce
accurate changesets for root module outputs. Although we added a place for
it in the plan model in Terraform 0.12, Terraform Core currently produces
inaccurate changesets there which don't properly track the prior values.

We're planning to rework Terraform Core's evaluation approach in a
forthcoming release so it would itself be able to distinguish between the
prior state and the planned new state to produce an accurate changeset,
but this commit introduces a temporary stop-gap solution of implementing
the logic up in the local backend code, where we can freeze a snapshot of
the prior state before we take any other actions and then use that to
produce an accurate output changeset to decide whether the plan has
externally-visible side-effects and render any changes to output values.

This temporary approach should be replaced by a more appropriately-placed
solution in Terraform Core in a release, which should then allow further
behaviors in similar vein, such as user-visible drift detection for
resource instances.

Located all non-test paths where a Diagnostic type was assigned to an
error variable.

The documentation for the -target option warns that it's intended for
exceptional circumstances only and not for routine use, but that's not a
very prominent location for that warning and so some users miss it.

Here we make the warning more prominent by including it directly in the
Terraform output when -target is in use. We first warn during planning
that the plan might be incomplete, and then warn again after apply
concludes and direct the user to run "terraform plan" to make sure that
there are no further changes outstanding. The latter message is intended
to reinforce that -target should only be a one-off operation and that you
should always run without it soon after to ensure that the workspace is
left in a consistent, converged state.

When failing to write the state, the local backend writes the state to a local file called `errrored.tfstate`. Previously it would do so by creating a new state file which would use a new serial and lineage. By exorting the existing state file and directly assigning the new state, the serial and lineage are preserved.

replaced because the instance was tainted.

This is a refactored version of the `remote` backend that was initially added to Terraform v0.11.8 which should now be compatible with v0.12.0.

There are several steps here and a number of them can include reaching out
to remote servers or executing local processes, so it's helpful to have
some trace logs to better narrow down causes of errors and hangs during
this step.

This is a light adaptation of our earlier prototype of structural diff
rendering, as a starting point for what we'll actually ship. This is not
consistent with the latest mocks, so will need some additional work before
it is ready, but integrating this allows us to at least see the plan
contents while fixing up remaining issues elsewhere.

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 5861dbf3 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-...

The new config loader requires some steps to happen in a different
order, particularly in regard to knowing the schema in order to
decode the configuration.

Here we lean directly on the configschema package, rather than
on helper/schema.Backend as before, because it's generally
sufficient for our needs here and this prepares us for the
helper/schema package later moving out into its own repository
to seed a "plugin SDK".

* cli: show workspace name in destroy confirmation

If the workspace name is not "default", include it in the confirmation
message for `terraform destroy`.

Fixes #15480

Simplify the use of clistate.Lock by creating a clistate.Locker
instance, which stores the context of locking a state, to allow unlock
to be called without knowledge of how the state was locked.

This alows the backend code to bring the needed UI methods to the point
where the state is locked, and still unlock the state from an outer
scope.

Provide a NoopLocker as well, so that callers can always call Unlock
without verifying the status of the lock.

Add the StateLocker field to the backend.Operation, so that the state
lock can be carried between the different function scopes of the backend
code. This will allow the backend context to lock the state before it's
read, while allowing the different operations to unlock the state when
they complete.

This reverts commit e88bd74b.

This reverts commit 0b804a96.

Use the new StateLocker field to provide a wrapper for locking the state
during terraform.Context creation. We can then remove all the state
locking code from individual operations, and unlock them in one place
inside the main Operation method.

Simplify the use of clistate.Lock by creating a clistate.Locker
instance, which stores the context of locking a state, to allow unlock
to be called without knowledge of how the state was locked.

This alows the backend code to bring the needed UI methods to the point
where the state is locked, and still unlock the state from an outer
scope.

Moves the nested select statements for backend operations into a single
function. The only difference in this part was that apply called
PersistState, which should be harmless regardless of the type of
operation being run.

If the user wishes to interrupt the running operation, only the first
interrupt was communicated to the operation by canceling the provided
context. A second interrupt would start the shutdown process, but not
communicate this to the running operation. This order of event could
cause partial writes of state.

What would happen is that once the command returns, the plugin system
would stop the provider processes. Once the provider processes dies, all
pending Eval operations would return return with an error, and quickly
cause the operation to complete. Since the backend code didn't know that
the process was shutting down imminently, it would continue by
attempting to write out the last known state. Under the right
conditions, the process would exit part way through the writing of the
state file.

Add Stop and Cancel CancelFuncs to the RunningOperation, to allow it to
easily differentiate between the two signals. The backend will then be
able to detect a shutdown and abort more gracefully.

In order to ensure that the backend is not in the process of writing the
state out, the command will always attempt to wait for the process to
complete after cancellation.

Since an early version of Terraform, the `destroy` command has always
had the `-force` flag to allow an auto approval of the interactive
prompt. 0.11 introduced `-auto-approve` as default to `false` when using
the `apply` command.

The `-auto-approve` flag was introduced to reduce ambiguity of it's
function, but the `-force` flag was never updated for a destroy.

People often use wrappers when automating commands in Terraform, and the
inconsistency between `apply` and `destroy` means that additional logic
must be added to the wrappers to do similar functions. Both commands are
more or less able to run with similar syntax, and also heavily share
their code.

This commit updates the command in `destroy` to use the `-auto-approve` flag
making working with the Terraform CLI a more consistent experience.

We leave in `-force` in `destroy` for the time-being and flag it as
deprecated to ensure a safe switchover period.

Cancellation in the local backend was only implemented for apply.

The previous diff presentation was rather "wordy", and not very friendly
to those who can't see color either because they have color-blindness or
because they don't have a color-supporting terminal.

This new presentation uses the actual symbols used in the plan output
and tries to be more concise. It also uses some framing characters to
try to separate the different stages of "terraform plan" to make it
easier to visually navigate.

The apply command also adopts this new plan presentation, in preparation
for "terraform apply" (with interactive plan confirmation) becoming the
primary, safe workflow in the next major release.

Finally, we standardize on the terminology "perform" and "actions" rather
than "execute" and "changes" to reflect the fact that reading is now an
action and that isn't actually a _change_.

Previously the rendered plan output was constructed directly from the
core plan and then annotated with counts derived from the count hook.
At various places we applied little adjustments to deal with the fact that
the user-facing diff model is not identical to the internal diff model,
including the special handling of data source reads and destroys. Since
this logic was just muddled into the rendering code, it behaved
inconsistently with the tally of adds, updates and deletes.

This change reworks the plan formatter so that it happens in two stages:
- First, we produce a specialized Plan object that is tailored for use
  in the UI. This applies all the relevant logic to transform the
  physical model into the user model.
- Second, we do a straightforward visual rendering of the display-oriented
  plan object.

For the moment this is slightly overkill since there's only one rendering
path, but it does give us the benefit of letting the counts be derived
from the same data as the full detailed diff, ensuring that they'll stay
consistent.

Later we may choose to have other UIs for plans, such as a
machine-readable output intended to drive a web UI. In that case, we'd
want the web UI to consume a serialization of the _display-oriented_ plan
so that it doesn't need to re-implement all of these UI special cases.

This introduces to core a new diff action type for "refresh". Currently
this is used _only_ in the UI layer, to represent data source reads.
Later it would be good to use this type for the core diff as well, to
improve consistency, but that is left for another day to keep this change
focused on the UI.

Some tweaks to the messaging and presentation.

A common reason to want to use `terraform plan` is to have a chance to
review and confirm a plan before running it.  If in fact that is the
only reason you are running plan, this new `terraform apply -auto-approve=false`
flag provides an easier alternative to

    P=$(mktemp -t plan)
    terraform refresh
    terraform plan -refresh=false -out=$P
    terraform apply $P
    rm $P

The flag defaults to true for now, but in a future version of Terraform it will
default to false.

When the backend operation is cancelled, immediately call PersistState.
The is a high likelihood that the user is going to terminate the process
early if the provider doesn't return in a timely manner, so persist as
much state as possible.

In the old remote state system we had the idea of a local backup, which
is actually still present for the legacy backends but no longer applies
for the new-style backends like the s3 backend.

It's problematic when an apply runs for long enough that someone's
time-limited AWS STS credentials expire and then Terraform fails and can't
persist state to S3.

To reduce the risk of lost state, here we add some extra fallback code
for the local apply operation in particular. If either state writing
or state persisting fail then we attempt to write the state to a special
backup file errored.tfstate, and produce an error message that guides the
user on how to retry uploading this state.

In the unlikely event that we can't write to local disk either (e.g.
permissions problems) we take a last-ditch attempt to dump the JSON onto
stdout and advise the user to manually copy it into a file for import.
If even that doesn't work for some reason, we assume a critical Terraform
bug (JSON-serialization problem with states?) and bail out with an
apologetic error message.

This is implemented for the apply command in particular because this is
the one command where new objects are created in real APIs that we don't
want to lose track of. For other operations it's less bad to just generate
a simple error message and have the user retry.

This fixes #14298.

The backend apply operation doesn't need to output the same text as the
cli itself. Instead notify the user that we are in the process of
stopping the operation.

The documentation for Refresh indicates that it will always return a
valid state, but that wasn't true in the case of a graph builder error.
While this same concept wasn't documented for Apply, it was still
assumed in the terraform apply code.

Since the helper testing framework relies on the absence of a state to
determine if it can call Destroy, the Context can't can't start
returning a state in all cases. Document this, and use the State method
to fetch the correct state value after Apply.

Add a nil check to the WriteState function, so that writing a nil state
is a noop.

Make sure to init before sorting the state, to make sure we're not
attempting to sort nil values. This isn't technically needed with the
current code, but it's just safer in general.

Add fields required to create an appropriate context for all calls to
clistate.Lock.

Add missing checks for Meta.stateLock, where we would attempt to lock,
even if locking should be skipped.