Fix YAML error message when error is at the end of the file

[samdoran]

SUMMARY

If a YAML file fails to load due to a syntax error in a file, or there is an error in the last line of a file, PyYAML reports the last line number of the file as the index where the error occurred.

When reading the file lines, we use that index to the get the relevant line. If the index value is out of range, the relevant line is lost for error reporting.

Subtract one from the index value to avoid the IndexError in this specific scenario. It is possible to still get an IndexError, which will be handled as it is currently.

Fixes #16456

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

lib/ansible/errors/__init__.py

ADDITIONAL INFORMATION

• Update tests

The additional code that tries to avoid empty lines in the error message is not necessary for this fix. I can omit that if others see problems with it.

[Shrews]

Hrm, looks like the message does change, but it still references the wrong line number when I use the example from the original reported issue (with an invalid, two-line role meta/main.yml).

Without your change:

% ansible-playbook site.yml 
[DEPRECATION WARNING]: Ansible will require Python 3.8 or newer on the controller starting with Ansible 2.12. Current version: 3.7.3 (default, Apr 24 2020, 18:51:23) 
[Clang 11.0.3 (clang-1103.0.32.62)]. This feature will be removed from ansible-core in version 2.12. Deprecation warnings can be disabled by setting 
deprecation_warnings=False in ansible.cfg.
[WARNING]: You are running the development version of Ansible. You should only run Ansible from "devel" if you are modifying the Ansible engine, or trying out features
under development. This is a rapidly changing source of code and can become unstable at any point.
ERROR! We were unable to read either as JSON nor YAML, these are the errors we got from each:
JSON: Expecting value: line 1 column 1 (char 0)

Syntax Error while loading YAML.
  found unexpected end of stream

The error appears to be in '/Users/shrews/Devel/playground/t/roles/a/meta/main.yml': line 3, column 1, but may
be elsewhere in the file depending on the exact syntax problem.

(specified line no longer in file, maybe it changed?)

With your change:

% ansible-playbook site.yml
[DEPRECATION WARNING]: Ansible will require Python 3.8 or newer on the controller starting with Ansible 2.12. Current version: 3.7.3 (default, Apr 24 2020, 18:51:23) 
[Clang 11.0.3 (clang-1103.0.32.62)]. This feature will be removed from ansible-core in version 2.12. Deprecation warnings can be disabled by setting 
deprecation_warnings=False in ansible.cfg.
[WARNING]: You are running the development version of Ansible. You should only run Ansible from "devel" if you are modifying the Ansible engine, or trying out features
under development. This is a rapidly changing source of code and can become unstable at any point.
ERROR! We were unable to read either as JSON nor YAML, these are the errors we got from each:
JSON: Expecting value: line 1 column 1 (char 0)

Syntax Error while loading YAML.
  found unexpected end of stream

The error appears to be in '/Users/shrews/Devel/playground/t/roles/a/meta/main.yml': line 3, column 1, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

dependencies:
 - { role: b, a_value="{{ "foo" }}" }
^ here
We could be wrong, but this one looks like it might be an issue with
missing quotes. Always quote template expression brackets when they
start a value. For instance:

    with_items:
      - {{ foo }}

Should be written as:

    with_items:
      - "{{ foo }}"

[samdoran]

@Shrews Unfortunately, that's expected. That's the position we get from YAML. Since it's an "end of stream" error, the error address is effectively the last line and column of the file. I'm not sure how we would go about getting the "correct" position in this situtaion.

ansible/lib/ansible/parsing/utils/yaml.py

Line 37 in 30d9399

err_obj.ansible_pos = (file_name, yaml_exc.problem_mark.line + 1, yaml_exc.problem_mark.column + 1)