Packer: Allow to reference to user variable in variables block

[nikos]

For example I want to use a variable for reuse inside a variables block:

"variables": {
    "worker_shortname": "Archival",
    "component_name": "{{user `worker_shortname`}}Worker"
}

Unfortunately this seems to be not supported with the current packer (0.5.1) version, but I get the following error message:

Error processing user variable 'component_name': template: tpl0:1:2: executing "tpl0" at <user `worker_shortna...>: error calling user: User variable can't be used within a default value for a user variable: worker_shortname'

[jpluscplusm]

This would be a really handy enhancement; it would allow for significantly more DRY templates here!

[mitchellh]

This is a purposeful limitation. Supporting this would require a dependency graph and a lot more complication, and limiting this is a simplification. Sorry!

[bdurrow]

It seems if there was a requirement that a composite variable was only allowed later in the variable block than the variables it was dependent on it would not require a dependency graph. The way it fails now (silently replacing the variable with nothing) isn't very helpful or obvious.

[dentarg]

Yeah, couldn't packer validate catch this?

[dmitrytokarev]

With Packer v1.0.0 i get more confusing error message:

Error initializing core: error interpolating default value for 'vm_name': template: root:1:9: executing "root" at <user `centos_version...>: error calling user: test

Would be REALLY helpful if this feature was implemented. :pray:

[nitrocode]

Ran into this issue today. You can get around this by using the racker preprocessor.

iso_url = 'http://cdimage.ubuntu.com/releases/18.04/release/ubuntu-18.04.2-server-amd64.iso'

Racker::Processor.register_template do |t|
  t.variables = {
    'iso_url' => iso_url,
    'notes' => "Built on {{ timestamp }} using ${iso_url}",
    ...

Then

racker ubuntu.rb - | packer build -timestamp-ui -

or

racker ubuntu.rb - > ubuntu.json && packer build -timestamp-ui ubuntu.json

[SwampDragons]

@nitrocode FYI this feature is now available within Packer directly :).

[nitrocode]

Hmmm. I tried in 1.3.5 and had issues so I had to go the racker route. I'll retry with the latest 1.4.2.

[lorengordon]

Nice, didn't realize this shipped! Found the mention in the changelog for 1.4.0:

core: Allow user variables to be interpreted within the variables section [GH-7390]

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