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.
Due to these tests happening in the wrong order, removing an object from
the end of a sequence of objects would previously cause a bounds-check
panic.
Rather than a more severe rework of the logic here, for now we'll just
introduce an extra precondition to prevent the panic. The code that
follows already handles the case where there _is_ no new object (i.e. the
"old" object is being deleted) as long as we're able to pass through this
type-checking logic.
The new "JSON list of objects - removing item" test covers this problem
by rendering a diff for an object being removed from the end of a list
of objects within a JSON value.
Our initial prototype of new-style diff rendering excluded this because
the old SDK has no support for this construct. However, we want to be able
to introduce this construct in the new SDK without breaking compatibility
with existing versions of Terraform Core, so we need to implement it now
so it's ready to be used once the SDK implements it.
The key associated with each block allows us to properly correlate the
items to recognize the difference between an in-place update of an
existing block and the addition/deletion of a block.
Our null-to-empty normalization was previously assuming these would always
be collection types, but that isn't true when a block contains something
dynamic since we must then use tuple or object types instead to properly
represent all of the individual element types.
We are now allowing the legacy SDK to opt out of the safety checks we try
to do after plan and apply, and so in such cases the before/after values
in planned changes may be inconsistent with our usual rules.
To avoid adding lots of extra complexity to the diff renderer to deal with
these situations, instead we'll normalize the handling of nested blocks
prior to using these values.
In the long run it'd be better to do this normalization at the source,
immediately after we receive an object from a provider using the opt-out,
but we're doing this at the outermost layer for now to avoid risking
unintended impacts on other Terraform Core components when we're just
about to enter the beta phase of the v0.12.0 release cycle.
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.
We'll now show an "update" symbol prior to the argument to this synthetic
jsonencode(...) call, for consistency with how we show nested values in
other cases and to attach a verb to any "# forces replacement".
We'll also show a special form in the case where the value seems to differ
only in whitespace, so users can understand what's going on in that
hopefully-rare situation, particularly if those whitespace-only changes
end up forcing us to replace a remote object.
Since our own syntax for primitive values is similar to that of JSON, and
since we permit automatic conversions from number and bool to string, we
must do this special JSON value diff formatting only if the value is a
JSON array or object to avoid confusing results.
Because so far we've not supported dynamically-typed complex data
structures, several providers have used strings containing JSON to stand
in for these.
In order to get a readable diff in those cases, we'll recognize situations
where old and new are both JSON and present a diff of the effective value
of the JSON, using a faux call to the jsonencode(...) function to indicate
when we've done so.
This is a bit of a "cute" heuristic, but is important at least for now
until we can migrate away from that practice of passing large JSON strings
to providers and use dynamically-typed attributes instead.
This extra comment line gives us a place to show the full resource address
(since the block header line only includes type and name) and also allows
us to explain in long form the meaning of the change icon on the following
line.
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.
Martin Atkins