[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: .
        stdout2: ..stdout
        stdout2_r: ansible_result.stdout
      changed_when: ansible_result.stdout_lines|first == 'not two'

    - debug:
        var: result2

    - debug:
        var: stdout2

    - command: "echo three"
      register:
        stdout3: ..stdout_lines|first
        stdout3_r: (ansible_result.stdout_lines)[0]
      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 or a leading . in the projection is required, 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.

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)

[briantist]

so cool to see work on this again, it's going to be great 🙌

[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

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