Packer optional behavior is different than Terraform

[zeidoo]

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Overview of the Issue

Using optional in variables of type object isn't working in Packer, but working in Terraform.

Reproduction Steps

# foo.pkr.hcl
variable "context" {
  type = object({
    bar_pro = optional(string, "bar")
  })
}

$> packer validate foo.pkr.hcl 
Error: Invalid type specification

  on foo.pkr.hcl line 3, in variable "context":
   3:     bar_pro = optional(string, "bar")

Optional attribute modifier is only for type constraints, not for exact types.

# main.tf has exactly same content as foo.pkr.hcl
# tf init etc ommited
$> terraform validate
Success! The configuration is valid.

Packer version

Packer v1.8.5
Terraform v1.3.6

Operating system and Environment details

Linux fedora 6.0.15-300.fc37.x86_64

[sestegra]

This feature needs newer version of github.com/hashicorp/hcl/v2 and github.com/zclconf/go-cty.

But, the update is blocked by the following issues:

• mapstructure-to-hcl2: . . . Unexpected package creation during export data loading packer-plugin-sdk#128
• hashicorp/packer-plugin-sdk incompatible with zclconf/go-cty v1.11.0 packer-plugin-sdk#131
• Upgrade github.com/zclconf/go-cty to v1.11.1 packer-plugin-sdk#135
• hashicorp/packer-plugin-sdk incompatible with zclconf/go-cty v1.11.0 zclconf/go-cty#134

The root cause is the removal of the support of enconding/gob in cty module (zclconf/go-cty@97bafac).

[github-actions]

This issue has been synced to JIRA for planning.

JIRA ID: HPR-982

[nywilken]

@sestegra thanks for the added context. I'm actually starting to look into hashicorp/packer-plugin-sdk#135. I'll update the thread once I have some insight on how to best proceed.

[sestegra]

@nywilken
This issue is on a closed Milestone.
Since plugin-packer-sdk bumps to 0.5.2 with go-cty update, I guess this issue can be fixed in the next milestone.