Fix Windows symlinks

[kurtmckee]

On Windows, os.readlink(path) returns UNC paths that start with \\?\.

The UNC prefix can simply be removed when running on Windows.

Fixes #307

[anishathalye]

Thank you for the pull request! By the way, did you get the unit tests running on Windows? What did you need to do to make that happen?

[kurtmckee]

My pleasure!

I was able to get the unit tests running with Windows as the host -- it still ultimately runs in a VirtualBox VM that Vagrant set up. (And THANK YOU for excellent instructions for recreating the test environment! It allowed me to make changes and avoid breaking functionality in Linux!)

I looked at how I could make the tests actually run under Windows, but there were several challenges:

1. Test isolation It might be possible to redesign the tests to run in temp directories, rather than relying on true filesystem isolation. I didn't look closely at that yet, though.
2. Testing across Python versions I'm familiar with a tool called tox that does a fantastic job of setting up testing environments for different Python versions, matrices of dependencies, plugins, etc etc etc. Tox would be a good way to test everything locally instead of relying just on CI. I strongly recommend it, always.

If you're interested, I can take a look at updating the tests so they can run on Windows. I do want you to bless the attempt before I try, though. :grinning:

[anishathalye]

Re this pull request, I will try to review in a couple days, need to test on a Windows machine which adds a bit more overhead.

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I would love for someone to get the tests working on Windows. I'm a long-time macOS/Linux user and don't know much about the current state of things on Windows. Last time I checked, there was some weird behavior related to symlinks. Does seem like a good amount of work to figure this out, though. Likely requires modifying Dotbot code, test code, and CI config.

I don't care quite as much about getting the Windows tests running locally / figuring out isolation etc (though I'd be happy to merge a PR if someone adds this). I care more about getting Windows tests running in CI. The isolation and testing across Python versions becomes much easier there (basically zero effort).

I tried modifying the CI config to switch to Windows, but that alone wasn't enough to get it to work. My half-finished attempt is here, in case that's a helpful starting point: https://github.com/anishathalye/x/tree/windows (in particular, see https://github.com/anishathalye/x/blob/windows/.github/workflows/build.yml). Some example output here: https://github.com/anishathalye/x/runs/5982348889?check_suite_focus=true

[kurtmckee]

Windows used to require admin privileges to create symlinks, but that requirement is dropped if Developer Mode is enabled (or if group policy is modified, I think). Also, Python calls the create-symlink API (whatever it's called) with the correct arguments as of...2019, maybe? Hopefully those changes reduce the weirdness.

I'll try to get the tests working in Windows. I mentioned running the tests locally because I typically target locally-runnable tests, then make CI just run the same test suite (though with a full matrix of OS's). Local and CI testing are both valuable. :+1:

[kurtmckee]

Closing because this work is superseded by the work associated with #309.