validate_html_static_path raises exception when static path and outdir are in different drives

[bizzfitch]

Describe the bug
Hey sphinx team. Made a github account just for this, pretty excited to be potentially contributing to a project I use all the time

I think the bug is caused by the implementation for #1464

Bug is pretty much outlined by the title. In the code for sphinx.builders.html, the validate_html_static_path title will raise an error if the static path is in a different drive than the outdir. I assume this will also happen in validate_html_extra_path. Specifically, the outdir is using a UNC directory (like \network\some\path), but this happens with any two drives.

This occurs because these functions make a call to os.path.commonpath, which raises an error if the arguments are in different drives. Previously, sphinx had no problem with writing the output to a different drive.

To Reproduce
Steps to reproduce the behavior:
Easy enough to reproduce, no need for any real tinkering, since the error occurs before any real run happens.

$ git clone https://github.com/.../some_project
$ cd some_project
$ pip install -r requirements.txt
$ cd docs
$ make html BUILDDIR="D:\any\other\drive"
$ # we don't get past the init stage of the application

Expected behavior
In 2.1.2, neither of these functions exist, so the documents are successfully written in a different
drive than the source files. This is important for my project in particular, but seems like it's probably a common enough use case.

Your project
Can't submit the particular project. Hope this is okay, it's a project for work. I don't believe there's anything specific to the project that affects the bug, I reproduced it easily at home.

Environment info

• OS: Windows 10
• Python version: 3.7.4
• Sphinx version: 2.2.0
• Sphinx extensions: sphinx.ext.autodoc, sphinx.ext.doctest, sphinx.ext.mathjax, sphinx.ext.napoleon, sphxarg.ext
• Extra tools: sphinx_rtd_theme

Additional context
I've solved it temporarily by simply putting the calls to commonpath in try-except blocks. I think this is fine unless there is a specific reason this cross-drive writing shouldn't be allowed.

[bizzfitch]

I don't know if it's kosher to suggest a fix myself, but here's what I did to get around this. Not elegant, obviously.

Original:

def validate_html_static_path(app: Sphinx, config: Config) -> None:
    """Check html_static_paths setting."""
    for entry in config.html_static_path[:]:
        static_path = path.normpath(path.join(app.confdir, entry))
        if not path.exists(static_path):
            logger.warning(__('html_static_path entry %r does not exist'), entry)
            config.html_static_path.remove(entry)
        elif path.commonpath([app.outdir, static_path]) == app.outdir:
            logger.warning(__('html_static_path entry %r is placed inside outdir'), entry)
            config.html_static_path.remove(entry)

Proposed fix:

def validate_html_static_path(app: Sphinx, config: Config) -> None:
    """Check html_static_paths setting."""
    for entry in config.html_static_path[:]:
        static_path = path.normpath(path.join(app.confdir, entry))
        if not path.exists(static_path):
            logger.warning(__('html_static_path entry %r does not exist'), entry)
            config.html_static_path.remove(entry)
        else:
            try:
                commonpath = path.commonpath([app.outdir, static_path])
            except ValueError:
                continue
            if commonpath  == app.outdir:
                logger.warning(__('html_static_path entry %r is placed inside outdir'), entry)
                config.html_static_path.remove(entry)

[tk0miya]

Now your fix is released Sphinx-2.2.1. Thank you for your work!