Allow marking variables as required, and fail task if they're absent

[deepbrook]

I ran into behaviour which causes a lot of annoying trash on my gitlab instance.

Given the following task:

  init:purpose:module:
    dir: deploy/{{.module}}
    vars:
      purpose: "{{.purpose}}"
      module: "{{.module}}"
    cmds:
      - >
        terraform init -reconfigure
        -backend-config="address=https://gitlab.netrtl.com/api/v4/projects/{{.GL_PROJECT_ID}}/terraform/state/{{.purpose}}-{{.module}}-{{.env}}"
        -backend-config="lock_address=https://gitlab.netrtl.com/api/v4/projects/{{.GL_PROJECT_ID}}/terraform/state/{{.purpose}}-{{.module}}-{{.env}}/lock"
        -backend-config="unlock_address=https://gitlab.netrtl.com/api/v4/projects/{{.GL_PROJECT_ID}}/terraform/state/{{.purpose}}-{{.module}}-{{.env}}/lock"
        -backend-config="lock_method=POST"
        -backend-config="unlock_method=DELETE"
        -backend-config="retry_wait_min=5"

I get built-in failure if GL_PROJECT_ID is missing, because the gitlab API is going to throw a 404.
However, If misspell purpose on the cli when running this task like this:

task   init:purpose:module prupose=pass-the-butter module=butter

The task runs and the state file is simply created on GitLab when it doesn't exist:

task: [init:purpose:module] terraform init -reconfigure -backend-config="address=https://gitlab.netrtl.com/api/v4/projects/5729/terraform/state/-butter-prod" -backend-config="lock_address=https://gitlab.netrtl.com/api/v4/projects/5729/terraform/state/-butter-prod/lock" -backend-config="unlock_address=https://gitlab.netrtl.com/api/v4/projects/5729/terraform/state/-butter-prod/lock" -backend-config="lock_method=POST" -backend-config="unlock_method=DELETE" -backend-config="retry_wait_min=5"
[...]

I'd like to be able to declare certain variables as required/existing via the yaml schema, like this for example:

  init:purpose:module:
    dir: deploy/{{.module}}
    vars:
      purpose:
        value: "{{.purpose}}"
        required: true
      module: "{{.module}}"

[treybrisbane]

@deepbrook You can effectively get required variables by leveraging a combination of Go's template syntax and Slim-Sprig's fail function:

tasks:
  example-task:
    vars:
      example_variable: '{{or .example_variable (fail "Variable `example_variable` is required")}}'

Hope that helps! 🙂

[sdemura]

I use preconditions for this, but I will say using sprig is more clever

 example:
    vars:
      AWS_ACCESS_KEY_ID: "{{.AWS_ACCESS_KEY_ID | default nil}}"
      AWS_SECRET_ACCESS_KEY: "{{.AWS_SECRET_ACCESS_KEY | default nil}}"
    cmds:
      - cmd: aws ...
         silent: true
    preconditions:
      - sh: "[ {{.AWS_ACCESS_KEY_ID}} ]"
        msg: AWS_ACCESS_KEY_ID envvar not set.
      - sh: "[ {{.AWS_SECRET_ACCESS_KEY}} ]"
        msg: AWS_SECRET_ACCESS_KEY envvar not set.

[max-sixty]

FYI this is similar to #495

[andreynering]

Closing as duplicate of #1203 and #1204.