Unorthodox MkDocs layouts cause docs to become stale

[orlnub123]

My docs haven't been updating for a while now so I've looked into the problem. It started when I changed how they were laid out, or more specifically when I took the MkDocs configuration file out of the root folder, in this commit. The only hint was the difference at the end of the build outputs from the commit before and the commits since that successfully built.

Here's the output from the commit before:

[rtd-command-info] start-time: 2018-12-12T15:24:31.500295Z, end-time: 2018-12-12T15:24:32.154653Z, duration: 0, exit-code: 0
python /home/docs/checkouts/readthedocs.org/user_builds/django-class-settings/envs/latest/bin/mkdocs build --clean --site-dir _build/html --config-file /home/docs/checkouts/readthedocs.org/user_builds/django-class-settings/checkouts/latest/mkdocs.yml
INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: /home/docs/checkouts/readthedocs.org/user_builds/django-class-settings/checkouts/latest/_build/html

And the output from the earliest successfully built commit after:

[rtd-command-info] start-time: 2019-03-06T09:01:08.583601Z, end-time: 2019-03-06T09:01:09.203502Z, duration: 0, exit-code: 0
python /home/docs/checkouts/readthedocs.org/user_builds/django-class-settings/envs/latest/bin/mkdocs build --clean --site-dir _build/html --config-file /home/docs/checkouts/readthedocs.org/user_builds/django-class-settings/checkouts/latest/docs/mkdocs.yml
INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: /home/docs/checkouts/readthedocs.org/user_builds/django-class-settings/checkouts/latest/docs/_build/html

If you look closely you'll see that while the first one builds into _build/html, the one after builds into docs/_build/html. I haven't tested it but I'm guessing this discrepancy is why the docs aren't updating, even after environment resets.

The reason it's getting built into a different directory is because of the build command: https://github.com/rtfd/readthedocs.org/blob/5f78337874cb7dd0f73864d43283fe819d03594d/readthedocs/doc_builder/backends/mkdocs.py#L249-L258

The directory passed to --site-dir, self.build_dir, gets set to _build/html here: https://github.com/rtfd/readthedocs.org/blob/5f78337874cb7dd0f73864d43283fe819d03594d/readthedocs/doc_builder/backends/mkdocs.py#L294-L297

This is a problem because MkDocs assumes local paths to be local to the configuration file, not to the current working directory. The change happens here: https://github.com/mkdocs/mkdocs/blob/eb5f3f84e533cff42e13492af2a5e1ab391865e9/mkdocs/config/config_options.py#L341-L342

A fix would be to change self.build_dir to be an absolute path if the MkDocs configuration file isn't inside the root directory.

[stsewd]

I already have a PR to fix this in #5539

[orlnub123]

That's great! I'll take a look at it.