ansible_exec_ok

Purpose

This is the exec_ok ansible test plugin.

With it you can test whether a task has been successfully executed:

- name: test something
  action: some action
  when: registered_variable is exec_ok

exec_ok exists to remedy ansible's built-in success plugin, which will evaluate to true if a task was not executed.

The problem: why exec_ok exists

This playbook:

- hosts: machine
  tasks:
  # in our setup /foo/bar does __not__ exist!!!
  - name: check if /foo/bar exists
    stat: path=/foo/bar
    #                   # this task:
    failed_when:  False # * does not fail
    changed_when: False # * never changes anything
    register: foo_bar_status

  - name: register semantically expressive variable for foo_bar_status
    local_action: command "true"
    changed_when: False
    when: foo_bar_status.stat.exists
    register: baz_needs_to_be_run
  
  # this should not run, but it __does__!
  - name: run baz if /foo/bar exists
    command: run_baz
    when: baz_needs_to_be_run is success

... will probably not do what you expect!

In the first task the existence of /foo/bar will be checked. It does not exist. So foo_bar_status.stat.exists will be false.

Subsequently we could use the condition when: foo_bar_status.stat.exists, however that condition is a technical one and does not express the reason why we are checking ("the semantics").

So in order for our playbook to be semantically more expressive we register the variable baz_needs_to_be_run.

Naively we could expect that if foo_bar_status.stat.exists then baz_needs_to_be_run is success be true. And if foo_bar_status does not exist, then baz_needs_to_be_run is success would not be true. However that is not the case.

The problem is that when the condition when: foo_bar_status.stat.exists is not true, then the second task will be skipped.

The third task will now check whether baz_needs_to_be_run is success where ansible defines success as not failed.

The problem with this is that the second task will register the baz_needs_to_be_run variable in order to be able to mark the variable, which for ansible semantically refers to the task being executed. But ansible will not set the variable to failed (since it was skipped).

And thus, since the third task checks for baz_needs_to_be_run being success and success is defined as not failed and the failed property has not been set in the second task, then not failed evaluates to true and thus the condition when: baz_needs_to_be_run is success evaluates to true.

So even though the condition that determines whether baz should be run (when: foo_bar_status.stat.exists) is false, ansible will run baz nonetheless.

This behavior has been reported as a bug against ansible

Installation

I suggest to add to check out the plugin into your roles/ directory as a submodule (provided that you keep the directory with your ansible playbooks in a git repo):

$ cd roles
$ git submodule add https://github.com/sourcepole/ansible_exec_ok.git exec_ok

Then you'll need to make the test plugin available to ansible as a test plugin. I suggest you symlink it into your library:

$ cd library/test_plugins # create that directory in case it doesn't exist yet
$ ln -s ../../roles/exec_ok/library/exec_ok.py

and tell ansible where to find the test plugin:

$ cat ~/.ansible.cfg
[...]
test_plugins = /usr/share/ansible/plugins/test:/full/path/to/library/test_plugins