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.
If a planned NestingList block value looks like it may represent a
dynamic block, we don't check the length since it may be unknown. This
check was missing in the NestingSet case, but it applies for the same
reason.
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If a dynamic block (in the HCL dynamic block extension sense) has an
unknown value for its for_each argument, it gets expanded to a single
placeholder block with all of its attributes set to a unknown values.
We can use this as part of a heuristic to relax our object compatibility
checks for situations where the plan included an object that appears to
be (but isn't necessarily) such a placeholder, allowing for the fact that
the one placeholder block could be replaced with zero or more real blocks
once the for_each value is known.
Previously our heuristic was too strict: it would match only if the only
block present was a dynamic placeholder. In practice, users may mix
dynamic blocks with static blocks of the same type, so we need to be more
liberal to avoid generating incorrect incompatibility errors in such
cases.
In study of existing providers we've found a pattern we werent previously
accounting for of using a nested block type to represent a group of
arguments that relate to a particular feature that is always enabled but
where it improves configuration readability to group all of its settings
together in a nested block.
The existing NestingSingle was not a good fit for this because it is
designed under the assumption that the presence or absence of the block
has some significance in enabling or disabling the relevant feature, and
so for these always-active cases we'd generate a misleading plan where
the settings for the feature appear totally absent, rather than showing
the default values that will be selected.
NestingGroup is, therefore, a slight variation of NestingSingle where
presence vs. absence of the block is not distinguishable (it's never null)
and instead its contents are treated as unset when the block is absent.
This then in turn causes any default values associated with the nested
arguments to be honored and displayed in the plan whenever the block is
not explicitly configured.
The current SDK cannot activate this mode, but that's okay because its
"legacy type system" opt-out flag allows it to force a block to be
processed in this way anyway. We're adding this now so that we can
introduce the feature in a future SDK without causing a breaking change
to the protocol, since the set of possible block nesting modes is not
extensible.
We were using the wrong cty operation to access map members, causing a
panic whenever a prior value was present for a resource type with a nested
block backed by a map value.
When dynamically-typed attributes are in the schema, we use different
conventions for representing nested blocks containing them (using tuples
and objects instead of lists and maps).
The normalization code here doesn't deal with those because the legacy
SDK never generates them, but we must still pass them through properly or
else other SDKs will be blocked from using dynamic attributes.
Previously this function would panic in that situation. Now it will just
pass through nested blocks containing dynamic attribute values entirely
as-is, with no normalization whatsoever. That's okay, because the scope
of this function is only to normalize inconsistencies that the legacy
SDK is known to produce, and the legacy SDK never produces dynamic-typed
attributes.
Now that we have an opt-out to let the legacy SDK return values that are
inconsistent with the new conventions for representing configuration,
various parts of Terraform must now be prepared to deal with
inconsistencies.
This function normalizes the most egregious inconsistencies relating to
the representation of nested blocks, freeing any recipient of its result
from worrying about these inconsistencies itself.
Our usual "ground rules" for mapping configschema to cty call for the
collection values representing nested block types to always be known and
non-null, using an empty collection to represent the absense of any blocks
of that type so that users can always safely use length(...) etc on them
without worrying about them sometimes being null.
However, due to some different behaviors in the legacy SDK we've allowed
it an exception to this rule which means that we can see unknown and null
collections in these positions in object values returned from provider
operations like PlanResourceChange and ApplyResourceChange when the legacy
SDK opt-out is activated.
As a consequence of this, we need to be mindful in our safety check
functions, like AssertObjectCompatible here, of tolerating these non-ideal
situations to allow the safety checks to complete. We run these checks
even when the provider requests an opt-out, because we want to note any
inconsistencies as WARNING level log lines to aid in debugging.
Since these error messages get printed in Terraform's output and we
encourage users to share them as part of bug reports, we should avoid
including sensitive information in them to reduce the risk of accidental
exposure.
Completing our set of provider result safety-check functions,
AssertPlanValid checks a result from a provider's PlanResourceChange to
make sure it doesn't propose a change that is not valid within the user
model of Terraform.
Specifically, it forbids the provider from planning a value that
contradicts what the user gave in configuration, which is important to
ensure that making a reference to an attribute elsewhere in the
configuration will produce exactly the given result, as users reasonably
expect.
Providers _are_ allowed (and, in fact, required) to make changes to
any Computed attribute values declared in the schema in order to fill in
the default values that the provider has generated. Later checks during
the apply phase will ensure that the provider remains true to these
planned values, to ensure that Terraform can keep its promise of doing
exactly what was planned or presenting an error explaining why not.
In an earlier commit we changed objchange.ProposedNewObject so that the
task of populating unknown values for attributes not known during apply
is the responsibility of the provider's PlanResourceChange method, rather
than being handled automatically.
However, we were also using objchange.ProposedNewObject to construct the
placeholder new object for a deferred data resource read, and so we
inadvertently broke that deferral behavior. Here we restore the old
behavior by introducing a new function objchange.PlannedDataResourceObject
which is a specialized version of objchange.ProposedNewObject that
includes the forced behavior of populating unknown values, because the
provider gets no opportunity to customize a deferred read.
TestContext2Plan_createBeforeDestroy_depends_datasource required some
updates here because its implementation of PlanResourceChange was not
handling the insertion of the unknown value for attribute "computed".
The other changes here are just in an attempt to make the flow of this
test more obvious, by clarifying that it is simulating a -refresh=false
run, which effectively forces a deferred read since we skip the eager
read that would normally happen in the refresh step.
Previously we would construct a proposed new state with unknown values in
place of any not-set-in-config computed attributes, trying to save the
provider a little work in specifying that itself.
Unfortunately that turns out to be problematic because it conflates two
concerns: attributes can be explicitly set in configuration to an unknown
value, in which case the final result of that unknown overrides any
default value the provider might normally populate.
In other words, this allows the provider to recognize in the proposed new
state the difference between an Optional+Computed attribute being set to
unknown in the config vs not being set in the config at all.
The provider now has the responsibility to replace these proposed null
values with unknown values during PlanResourceChange if it expects to
select a value during the apply step. It may also populate a known value
if the final result can be predicted at plan time, as is the case for
constant defaults specified in the provider code.
This change comes from a realization that from core's perspective the
helper/schema ideas of zero values, explicit default values, and
customizediff tweaks are all just examples of "defaults", and by allowing
the provider to see during plan whether these attributes are being
explicitly set in configuration and thus decide whether the default will
be provided immediately during plan or deferred until apply.
Previously we were just asserting that the number of elements didn't grow
between planned and actual. We still can't precisely correlate elements in
sets with unknown values, but here we adapt some logic we added earlier
to config/hcl2shim to ensure that we can find a plausible correlation for
each element in each set to at least one element in the other set, and
thus catch more cases where set elements might vanish or appear between
plan and apply, for improved safety.
This will still generate false negatives in some cases where unknown
values are present due to having to assume correlation is intended
wherever it is possible, but we'll catch situations where the actual value
is obviously contrary to what was planned.
ProposedNewObject intentionally replaces a null prior with an unknown
prior in order to easily fill in unknown values where they "show through"
under values not set explicitly in config, but it was failing to handle
that situation when dealing with nested blocks that are backed by sets.
incoming values
Addresses an odd state where the priorV of an object to be changed is
known but null.
While this situation should not happen, it seemed prudent to ensure that
core is resilient to providers sending incorrect values (which might
also occur with manually edited state).
We need to check for the known-ness of the prior value before we check for
the null-ness of actual, because it's valid for an unknown value to become
a null.
This algorithm is the usual first step when generating diffs. This package
is a bit of a strange home for it, but since it works with changes to
cty.Value this feels more natural than any other place it could be.
We need to make the collection itself be a tuple or object rather than
list or map in this case, since otherwise all of the elements of the
collection are constrained to be of the same type and that isn't the
intent of a provider indicating that it accepts any type.
This function's goal is to ensure that the "final" plan value produced
by a provider during the apply step is always consistent with the known
parts of the planned value produced during the plan step.
Any error produced here indicates a bug in the provider.
This produces a "proposed new state", which already has prior computed
values propagated into it (since that behavior is standard for all
resource types) but could be customized further by the provider to make
the "_planned_ new state".
In the process of implementing this it became clear that our configschema
DecoderSpec behavior is incorrect, since it's producing list values for
NestingList and map values for NestingMap. While that seems like it should
be right, we should actually be using tuple and object types respectively
to allow each block to have a different runtime type in situations where
an attribute is given the type cty.DynamicPseudoType. That's not fixed
here, and so without a further fix list and map blocks will panic here.
The DecoderSpec implementation will be fixed in a subsequent commit.
Martin Atkins