Using depends_on in a module definition fails terraform plan when using data source aws_partition in the module

[watsonjm]

When using a depends_on argument in a module definition for a module that uses the data source aws_partition, terraform plan will fail with an error due to not knowing the value until apply, like this:

The "for_each" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created. To work around this, use the -target argument to first apply only the resources that the for_each depends on.

Terraform Version

Terraform v1.1.3
on linux_amd64
+ provider registry.terraform.io/hashicorp/aws v3.71.0
+ provider registry.terraform.io/hashicorp/cloudinit v2.2.0
+ provider registry.terraform.io/hashicorp/random v3.1.0
+ provider registry.terraform.io/hashicorp/tls v3.1.0

Terraform Configuration Files

https://github.com/watsonjm/depends_on-bug

Debug Output

https://gist.github.com/watsonjm/31bdebd61e02aa05f4d4ae2374ceac9e

Expected Behavior

terraform plan should show a successful plan

Actual Behavior

terraform plan fails with 1 or more errors depending on how many times this data source is referenced.

Steps to Reproduce

1. terraform init -var-file=test.tfvars
2. terraform plan -var-file=test.tfvars -out test.tfplan
   This will give you a successful plan, to see the error in action, uncomment the depends_on argument on line 30 of eks.tf and re-run terraform plan.

Additional Context

I have ran into this before with my own custom modules with the same requirements (depends_on argument in module definition, and the module using aws_partition data source), so this issue is not just with the eks module in use here.

References

terraform-aws-modules/terraform-aws-eks#1753

[jbardin]

Hi @watsonjm,

The behavior you have described here is expected form the given configuration. Data sources using depends_on are always deferred until apply when there is a change in any of their dependencies. Because depends_on can't describe which part of the change is relevant, the entire change must be taken into account.

When you add depends_on to a module, you are effectively adding that to every object within the module. This can be a convenient shortcut in some cases, but we generally discourage using it on modules for this very reason, especially when that module contains data sources.

We use GitHub issues for tracking bugs and enhancements, rather than for questions. While we can sometimes help with certain simple problems here, it's better to use the community forum where there are more people ready to help.

Thanks!

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