HCP Packer data sources causes Packer to crash on validate

[nywilken]

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Overview of the Issue

Running packer validate on an otherwise validate build template the contains an hcp-packer-image data source block causes Packer to crash. Running a build on the template does not result in the same error.

This issue does not exist in v1.8.0 - main culprit for this issue is likely #11832

Reproduction Steps

1. Run packer validate on template.

Packer version

v1.8.2

Simplified Packer Template

data "hcp-packer-iteration" "centos7-base" {
  bucket_name = var.hcp_bucket_name
  channel          = var.hcp_channel_name
}
data "hcp-packer-image" "centos7-base" {
  bucket_name    = data.hcp-packer-iteration.centos7-base.bucket_name
  iteration_id   = data.hcp-packer-iteration.centos7-base.id
  cloud_provider = "aws"
  region         = var.region
}
source "amazon-ebs" "centos7-web" {
  ami_name      = "${var.prefix}-centos7-web-${local.timestamp}"
  ami_regions   = ["${var.region}", "eu-west-1", "eu-north-1"]
  instance_type = "t2.micro"
  region        = var.region
  source_ami = data.hcp-packer-image.centos7-base.id
  ssh_username         = var.ssh_username
  ssh_private_key_file = var.ssh_private_key_file
  ssh_keypair_name     = var.ssh_keypair_name
}
build {
  hcp_packer_registry {
    bucket_name = "centos7-web-aws"
    description = "This is a CentOS7-based build that has been prepared as a web server in AWS."
    bucket_labels = {
      "build-version" = "0.0.1",
      "service"       = "aws"
    }
    build_labels = {
      "os"      = "centos7",
      "version" = "web"
    }
  }

  sources = ["source.amazon-ebs.centos7-web"]
}

Operating system and Environment details

All

Log Fragments and crash.log files

 packer validate .
panic: can't use ElementIterator on unknown value
​
goroutine 1 [running]:
github.com/zclconf/go-cty/cty.Value.ElementIterator({{{0x69f8348, 0xc000dd1860}}, {0x535f020, 0xa692de0}})
        /Users/runner/go/pkg/mod/github.com/zclconf/go-cty@v1.10.0/cty/value_ops.go:1118 +0xd8
github.com/zclconf/go-cty/cty.Value.AsValueMap({{{0x69f8348, 0xc000dd1860}}, {0x535f020, 0xa692de0}})
        /Users/runner/go/pkg/mod/github.com/zclconf/go-cty@v1.10.0/cty/value_ops.go:1324 +0xa5
github.com/hashicorp/packer/hcl2template.(*Parser).decodeBuildConfig(0x53bb660, 0xc000de91e0, 0xc00037b040)
        /Users/runner/work/packer/packer/hcl2template/types.build.go:267 +0x1b14
github.com/hashicorp/packer/hcl2template.(*Parser).parseConfig(0xc00037b040, 0xc0006ba160, 0xc00037b040)
        /Users/runner/work/packer/packer/hcl2template/parser.go:373 +0x1eb
github.com/hashicorp/packer/hcl2template.(*PackerConfig).Initialize(0xc00037b040, {0x0})
        /Users/runner/work/packer/packer/hcl2template/parser.go:326 +0x59a
github.com/hashicorp/packer/command.(*ValidateCommand).RunContext(0xc000810840, {0xc0001181a0, 0x1}, 0xc000a04000)
        /Users/runner/work/packer/packer/command/validate.go:59 +0x82
github.com/hashicorp/packer/command.(*ValidateCommand).Run(0xc000810840, {0xc0001181a0, 0x1, 0x1})
        /Users/runner/work/packer/packer/command/validate.go:25 +0xc5
github.com/mitchellh/cli.(*CLI).Run(0xc000152140)
        /Users/runner/go/pkg/mod/github.com/mitchellh/cli@v1.1.2/cli.go:262 +0x5f8
main.wrappedMain()
        /Users/runner/work/packer/packer/main.go:262 +0xaf2
main.realMain()
        /Users/runner/work/packer/packer/main.go:49 +0xf3
main.main()
        /Users/runner/work/packer/packer/main.go:35 +0x19
​
​
​
!!!!!!!!!!!!!!!!!!!!!!!!!!! PACKER CRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!
​
Packer crashed! This is always indicative of a bug within Packer.
A crash log has been placed at "crash.log" relative to your current
working directory. It would be immensely helpful if you could please
report the crash with Packer[1] so that we can fix this.
​
[1]: https://github.com/hashicorp/packer/issues

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