[HPR-838] cmd/hcl2_upgrade: Generate variable block for all referenced user input variables

[nywilken]

Currently the HCL2 upgrade command generates a valid variable block for all variables within the JSON variables property.

However JSON templates in Packer support variable interpolation for input variables defined within a variable definition file without it being declared within the variables property. When upgrading a JSON template to HCL2 the user variable reference gets converted to var.<var_name>. The upgraded template ultimately fails on validation or build execution with undefined variable errors. In order to improve the user experience when upgrading a JSON template to HCL2 the upgrade command will now create a variable block for all input variables referenced within the build template to ensure all required variables been defined.

Todo

• Update HCL2 upgrade command docs with an example of this behavior. (In a separate PR)

Example Runs

Current HCL2 upgrade command experience

~>  packer hcl2_upgrade command/test-fixtures/hcl2_upgrade/undeclared-variables/input.json
Successfully created command/test-fixtures/hcl2_upgrade/undeclared-variables/input.json.pkr.hcl. Exit 0

~>  packer validate -var-file=/tmp/var-file.json command/test-fixtures/hcl2_upgrade/undeclared-variables/input.json.pkr.hcl
Warning: Undefined variable

A "communicator_type" variable was set but was not declared as an input
variable. To declare variable "communicator_type", place this block in one of
your .pkr.hcl files, such as variables.pkr.hcl

variable "communicator_type" {
  type = string
}


Error: Unsupported attribute

  on command/test-fixtures/hcl2_upgrade/undeclared-variables/input.json.pkr.hcl line 8:
  (source code not available)

This object does not have an attribute named "communicator_type".

Updated HCL2 upgrade command experience

~>  packer hcl2_upgrade command/test-fixtures/hcl2_upgrade/undeclared-variables/input.json
Successfully created command/test-fixtures/hcl2_upgrade/undeclared-variables/input.json.pkr.hcl. Exit 0

~>  packer validate -var-file=/tmp/var-file.json command/test-fixtures/hcl2_upgrade/undeclared-variables/input.json.pkr.hcl
The configuration is valid.

[lbajolet-hashicorp]

LGTM!

[nywilken]

I'm going to merge this change in and work on the documentation in a separate PR.

[github-actions]

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