[POC] Add projection feature for registering vars - Option 2

[sivel]

SUMMARY

This PoC PR updates register to allow authors to pre-process module return results, before registering the variables.

In extension, this allows registering multiple variables

See:

• handle register expressions for dict specs #67804
• Introduce "this" for default task results proposals#74
• Ansible 2.2 inconsistent include_vars behaviour (with/without register) #21088

ISSUE TYPE

• Feature Pull Request

COMPONENT NAME

lib/ansible/plugins/strategy/__init__.py

ADDITIONAL INFORMATION

    - command: "echo one"
      register: result

    - debug:
        var: result

    - command: "echo two"
      register:
        result2: ansible_result
        stdout2: ansible_result.stdout
      changed_when: ansible_result.stdout_lines|first == 'not two'

    - debug:
        var: result2

    - debug:
        var: stdout2

    - command: "echo three"
      register:
        stdout3: ansible_result.stdout_lines|first
      changed_when: stdout3 == 'three'

    - command: "echo {{ item }}"
      loop:
        - one
        - two
        - three
      changed_when: ansible_loop_result.stdout_lines|first == 'three'

Either ansible_result must be used in the projection, and represents the task result, all normal jinja follows.

This PR also adds implicit registers named ansible_loop_result and ansible_result. ansible_loop_result will only exist during a looped task, and ansible_result will only exist when not on a looped task. These variables do not persist outside of the TaskExecutor so are only useful for when, changed_when, failed_when, until.

DOCS:

Since docs are no longer maintained in this repo, the docs for this can be located at https://gist.github.com/sivel/376abb3ae5476cfe168361d5ae556f61

TODO:

• Order of projections can affect later projections
• Evaluate moving _project off of TE for access in the Strategy also, to deduplicate code
• Evaluate means of not requiring . as the first character for a projection
• Evaluate the need for setting register 4 times, which includes running the projections 4 times (3 in TE, 1 in Strategy)

[sivel]

Added support for using ansible_result in place of .. . still works as a shorthand, but has a limitation of needing to be the first character.

[ansibot]

The test ansible-test sanity --test docs-build [explain] failed with 1 error:

docs/docsite/rst/playbook_guide/playbooks_variables.rst:242:0: warning: Pygments lexer name 'yaml+jinja2' is not known

click here for bot help

[lachlanmulcahy]

Is there a chance this will get added to the project? Seems like this has been floating around for a couple of years now.

[sivel]

Yes, there is a chance.

This feature has been included in the experimental fallible (https://pypi.org/project/fallible/) build, and we are waiting on feedback about the feature, to determine if it should be included within ansible-core. See this presentation for more information: https://youtu.be/DCf_e6XRsCw?t=7924

So far, feedback has been limited, and I don't have any specific indication whether anyone has actually tried the feature within the above fallible build or not.