add a "fail" filter

[tux21b]

SUMMARY

Add a "fail" filter that always returns a FilterError with a specific message.

ISSUE TYPE

• Feature Idea

COMPONENT NAME

core filters

ADDITIONAL INFORMATION

I would like to list and document all variables. Role specific variables could be listed and documented in "roles/*/defaults/*" and global variables in "group_vars/all/global.yml". Unfortunately there are some variables which do not have a good default value. For example, setting a variable like "mysql_password" to an empty string might be fatal. Sure, I could assert at the start of every role that the variable is not empty, but the chances are high that I might forget to add the assert on one place or another. Therefore, something like the following would be nice:

group_vars/all/global.yml:

# the password for the mysql root user.
# this password is used in several roles (like monitoring, application setup, etc.)
global_mysql_root_password: "{{ 'required, generate with pwgen -s 20' | fail }}"

If I then happen to forget to override the variable somewhere, an error like "failed to render global_mysql_root_password: required, generate with pwgen -s 20" is thrown.

The filter is easy to write (I can provide a PR if you want), is less specific than several other filters already present (e.g. "debug_type") and might be useful in a lot of places.

[ansibot]

Files identified in the description:

• lib/ansible/plugins/filter/core.py

If these files are incorrect, please update the component name section of the description or use the !component bot command.

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[sivel]

Is there a reason you couldn't just use the mandatory filter?

Don't define the var at all, but you can put a comment like:

# the password for the mysql root user.
# this password is used in several roles (like monitoring, application setup, etc.)
# required, generate with pwgen -s 20
# global_mysql_root_password: 

And then in your task use:

password: "{{ global_mysql_root_password | mandatory('required, generate with pwgen -s 20') }}"

[tux21b]

Thanks for your reply. That's basically how we are doing it now, but I think this approach has some drawbacks:

1. commenting out the variable makes the file harder to parse visually (no separation, no different syntax color)
2. the variable is used at multiple places in multiple roles. I would have to repeat the mandatory filter multiple times.
3. when reading the variable file it is not clear that this variable is required and how it can be generated.

[MarkusTeufelberger]

This would be the filter equivalent of the fail module: https://docs.ansible.com/ansible/latest/modules/fail_module.html

[Jmennius]

What about extending the mandatory filter to check not only for Undefined but also for the null/None value?
That would allow the following:

# the password for the mysql root user.
# this password is used in several roles (like monitoring, application setup, etc.)
# required, generate with pwgen -s 20
global_mysql_root_password:

or any of the null YAML specifiers like ~, null, Null, NULL like:

global_mysql_root_password: ~
global_mysql_root_password: null

[bcoca]

closing as per #75407