Ansible result.failed v.s. failed_when, result.changed v.s. changed_when etc

Question

Ansible in 1.4 and later provides a way to specify this behavior as follows:

- name: Fail task when the command error output prints FAILED
  command: /usr/bin/example-command -x -y -z
  register: command_result
  failed_when: "'FAILED' in command_result.stderr"

but the common Return Values already include .failed and .changed.

It seems to me these features contradict each other. Is there an official resolution to it? Does Ansible define one of failed_when or changed_when as being evaluated first and affecting the latter? Does it define that both are evaluated without seeing the effect of the other?

E.g. what is the defined behaviour of

1. command_result.failed within a failed_when expression
2. command_result.changed within a changed_when expression
3. command_result.failed within a changed_when expression
4. command_result.changed within a changed_when expression

更清楚地说，例如对于第三个，我对以下结果的定义（可靠）程度感兴趣：

- package:
    name: systemd
    state: present
  check_mode: yes
  register: command_result
  failed_when: command_result.changed

假设我对 Ansible 2.0 及更高版本感兴趣。

Answer 1

这看起来不对

您的示例为您的剧本添加了另一条路径。如果未安装 systemd，则输出将与安装时不同。第一次运行后它将被安装。这违背了 ansible 原则：

幂等性

如果执行一次操作的结果与重复执行操作而无需任何干预操作的结果完全相同，则该操作是幂等的。

如果你仍然这样做，请尽可能明确

我建议您运行命令which systemctl并注册输出。检查输出以安装 systemd 并失败并显示失败任务。

这仍然是一个非常有趣的问题

我想没有真正的文件，我们必须调查。

我希望我抓住了所有案例:)但我现在无法填写图表。

剧本.yml：

---

- name: test some stuff
  hosts: all
  tasks:
    - include_tasks: tasks.yml
      with_items:
        - { data: ping, changed: true }
        - { data: ping, changed: false }
        - { data: crash, changed: true }
        - { data: crash, changed: false }

任务.yml

---

- name: Check for command_result is defined and command_result
  ping:
    data: "{{ item.data }}"
  register: command_result
  changed_when: item.changed
  failed_when: command_result is defined and command_result
  ignore_errors: true

- name: Check for command_result is defined and command_result
  file:
    path: ./file
  register: command_result
  changed_when: item.changed
  failed_when: command_result is defined and command_result
  ignore_errors: true

- name: Check for command_result
  ping:
    data: "{{ item.data }}"
  register: command_result
  changed_when: item.changed
  failed_when: command_result
  ignore_errors: true

- name: Check for command_result
  file:
    path: ./file
  register: command_result
  changed_when: item.changed
  failed_when: command_result
  ignore_errors: true

- name: Check for command_result.changed is defined and command_result.changed
  ping:
    data: "{{ item.data }}"
  register: command_result
  changed_when: item.changed
  failed_when: command_result.changed is defined and command_result.changed

- name: Check for command_result.changed is defined and command_result.changed
  ping:
    data: "{{ item.data }}"
  register: command_result
  changed_when: item.changed
  failed_when: command_result.changed is defined and command_result.changed
  ignore_errors: true

- name: Check for command_result.changed
  ping:
    data: "{{ item.data }}"
  register: command_result
  changed_when: item.changed
  failed_when: command_result.changed
  ignore_errors: true

- name: Check for command_result.changed
  file:
    path: ./file
  register: command_result
  changed_when: item.changed
  failed_when: command_result.changed
  ignore_errors: true