terraform: splatting with computed values is computed [GH-2744]

[mitchellh]

Fixes #2744

If any value in a splat (multi-resource variable access) is computed, then the whole value should be computed.

[dalehamel]

❤️ nice!

cc @thegedge

[phinze]

LGTM

[justinclayton]

Hi @mitchellh: Unfortunately this PR seems to have introduced unexpected behavior that is being described in #3885, #3449, and elsewhere. Consider this simple example:

# test.tf
resource "template_file" "foo" {
  template = "I AM ${count.index}"
  count = 2
}

resource "template_file" "no_splat" {
  template = "the first foo says ${template_file.foo.0.rendered}"
}

resource "template_file" "with_splat" {
  template = "the first foo says ${element(template_file.foo.*.rendered, 0)}"
}

With both template_file.no_splat and template_file.with_splat we should expect the same result. And indeed we do, on the first plan and apply:

$ terraform plan
Refreshing Terraform state prior to plan...

<...>

+ template_file.foo.0
    rendered: "" => "<computed>"
    template: "" => "I AM 0"

+ template_file.foo.1
    rendered: "" => "<computed>"
    template: "" => "I AM 1"

+ template_file.no_splat
    rendered: "" => "<computed>"
    template: "" => "the first foo says ${template_file.foo.0.rendered}"

+ template_file.with_splat
    rendered: "" => "<computed>"
    template: "" => "the first foo says ${element(template_file.foo.*.rendered, 0)}"


Plan: 4 to add, 0 to change, 0 to destroy.

$ terraform apply
template_file.foo.1: Creating...
  rendered: "" => "<computed>"
  template: "" => "I AM 1"
template_file.foo.0: Creating...
  rendered: "" => "<computed>"
  template: "" => "I AM 0"
template_file.foo.0: Creation complete
template_file.foo.1: Creation complete
template_file.no_splat: Creating...
  rendered: "" => "<computed>"
  template: "" => "the first foo says I AM 0"
template_file.with_splat: Creating...
  rendered: "" => "<computed>"
  template: "" => "the first foo says I AM 0"
template_file.with_splat: Creation complete
template_file.no_splat: Creation complete

Apply complete! Resources: 4 added, 0 changed, 0 destroyed.

Good so far. Now we increment template_file.foo.count from 2 to 3. Our other resources still only ever reference template_file.foo.0, and not this new resource being created. We run plan again:

$ terraform plan
Refreshing Terraform state prior to plan...

template_file.foo.0: Refreshing state... (ID: aabd55fb4b7b23363c246265d1c4daedc3798149a7230f1c082c8994aa0db39c)
template_file.foo.1: Refreshing state... (ID: 2b9e553f514d0789ecbee4ec00710b11645aebe744bd3de99b1ab6af9130c1e8)
template_file.with_splat: Refreshing state... (ID: 7f2cbd10b2517e514a9a7a6052f08a17e16b829df003f60e612b56f533e982dd)
template_file.no_splat: Refreshing state... (ID: 7f2cbd10b2517e514a9a7a6052f08a17e16b829df003f60e612b56f533e982dd)

<...>

+ template_file.foo.2
    rendered: "" => "<computed>"
    template: "" => "I AM 2"

-/+ template_file.with_splat
    rendered: "the first foo says I AM 0" => "<computed>"
    template: "the first foo says I AM 0" => "the first foo says ${element(template_file.foo.*.rendered, 0)}" (forces new resource)


Plan: 2 to add, 0 to change, 1 to destroy.

Here we can see:

• template_file.no_splat registers no change (because it references template_file.foo.0.rendered directly)
• template_file.with_splat is marked to be destroyed and recreated

This is even though the value of template_file.foo.0.rendered was never altered as part of this change. Harmless in this case, but needlessly destructive in a host of other common real-world scenarios, like when instances are rebuilt because of userdata (#3449), or when LB pool members must be destroyed and recreated before adding additional ones (#4359).

I've tested this same scenario after rolling back the changes made in this PR, and this time I get the expected result from plan:

$ terraform plan
Refreshing Terraform state prior to plan...

template_file.foo.1: Refreshing state... (ID: 2b9e553f514d0789ecbee4ec00710b11645aebe744bd3de99b1ab6af9130c1e8)
template_file.foo.0: Refreshing state... (ID: aabd55fb4b7b23363c246265d1c4daedc3798149a7230f1c082c8994aa0db39c)
template_file.with_splat: Refreshing state... (ID: 7f2cbd10b2517e514a9a7a6052f08a17e16b829df003f60e612b56f533e982dd)
template_file.no_splat: Refreshing state... (ID: 7f2cbd10b2517e514a9a7a6052f08a17e16b829df003f60e612b56f533e982dd)

<...>

+ template_file.foo.2
    rendered: "" => "<computed>"
    template: "" => "I AM 2"


Plan: 1 to add, 0 to change, 0 to destroy.

Of course, this reintroduces the bug from #2744, which you can see with this same example if we destroy and start over with an initial plan:

$ terraform plan
Refreshing Terraform state prior to plan...

<...>

+ template_file.foo.0
    rendered: "" => "<computed>"
    template: "" => "I AM 0"

+ template_file.foo.1
    rendered: "" => "<computed>"
    template: "" => "I AM 1"

+ template_file.no_splat
    rendered: "" => "<computed>"
    template: "" => "the first foo says ${template_file.foo.0.rendered}"

+ template_file.with_splat
    rendered: "" => "<computed>"
    template: "" => "the first foo says 74D93920-ED26-11E3-AC10-0800200C9A66"


Plan: 4 to add, 0 to change, 0 to destroy.

In this particular case the apply still worked as expected, but I understand this bug sometimes causes diffs to fail when they shouldn't, which is not good either.

Not sure where to go from here. In theory, reevaluating dependent values during the apply phase after each resource is created could solve for this, but Terraform seems intently designed to only ever evaluate these values during the plan phase. This is how plans can be generated and executed in a decoupled manner, giving the feel that you're applying a patch to your infrastructure just like you would to some C project. I get that. On the other hand, we're not talking about a Makefile needlessly recompiling a couple of libs, but active running components that are often critical to our production stacks. I feel like more precision is needed with these types of evaluations, but it's not immediately clear to me how to elegantly tackle this myself. Would love to hear others' opinions on the matter.

Oh, what a difference 3 lines of code makes!

[ghost]

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