docs: add Windows support info

Change-Id: I82a1bec3a29d622c76b5709b96bbe8bff8aa427f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/247573
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

2 files changed, 145 insertions, 0 deletions

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 * [repo Manifest Format](./docs/manifest-format.md)
 * [repo Hooks](./docs/repo-hooks.md)
 * [Submitting patches](./SUBMITTING_PATCHES.md)
+* Running Repo in [Microsoft Windows](./docs/windows.md)
 
 ## Install
 
diff --git a/docs/windows.md b/docs/windows.md
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+# Microsoft Windows Details
+
+Repo is primarily developed on Linux with a lot of users on macOS.
+Windows is, unfortunately, not a common platform.
+There is support in repo for Windows, but there might be some rough edges.
+
+Keep in mind that Windows in general is "best effort" and "community supported".
+That means we don't actively test or verify behavior, but rely heavily on users
+to report problems back to us, and to contribute fixes as needed.
+
+[TOC]
+
+## Windows
+
+We only support Windows 10 or newer.
+This is largely due to symlinks not being available in older versions, but it's
+also due to most developers not using Windows.
+
+We will never add code specific to older versions of Windows.
+It might work, but it most likely won't, so please don't bother asking.
+
+## Symlinks
+
+Repo will use symlinks heavily internally.
+On *NIX platforms, this isn't an issue, but Windows makes it a bit difficult.
+
+There are some documents out there for how to do this, but usually the easiest
+answer is to run your shell as an Administrator and invoke repo/git in that.
+
+This isn't a great solution, but Windows doesn't make this easy, so here we are.
+
+### Launch Git Bash
+
+If you install Git Bash (see below), you can launch that with appropriate
+permissions so that all programs "just work".
+
+* Open the Start Menu (i.e. press the ⊞ key).
+* Find/search for "Git Bash".
+* Right click it and select "Run as administrator".
+
+*** note
+**NB**: This environment is only needed when running `repo`, or any specific `git`
+command that might involve symlinks (e.g. `pull` or `checkout`).
+You do not need to run all your commands in here such as your editor.
+***
+
+### Symlinks with GNU tools
+
+If you want to use `ln -s` inside of the default Git/bash shell, you might need
+to export this environment variable:
+```sh
+$ export MSYS="winsymlinks:nativestrict"
+```
+
+Otherwise `ln -s` will copy files and not actually create a symlink.
+This also helps `tar` unpack symlinks, so that's nice.
+
+### References
+
+* https://github.com/git-for-windows/git/wiki/Symbolic-Links
+* https://blogs.windows.com/windowsdeveloper/2016/12/02/symlinks-windows-10/
+
+## Python
+
+You should make sure to be running Python 3.6 or newer under Windows.
+Python 2 might work, but due to already limited platform testing, you should
+only run newer Python versions.
+See our [Python Support](./python-support.md) document for more details.
+
+You can grab the latest Windows installer here:<br>
+https://www.python.org/downloads/release/python-3
+
+## Git
+
+You should install the most recent version of Git for Windows:<br>
+https://git-scm.com/download/win
+
+When installing, make sure to turn on "Enable symbolic links" when prompted.
+
+If you've already installed Git for Windows, you can simply download the latest
+installer from above and run it again.
+It should safely upgrade things in situ for you.
+This is useful if you want to switch the symbolic link option after the fact.
+
+## Shell
+
+We don't have a specific requirement for shell environments when running repo.
+Most developers use MinTTY/bash that's included with the Git for Windows install
+(so see above for installing Git).
+
+Command & Powershell & the Windows Terminal probably work.
+Who knows!
+
+## FAQ
+
+### repo upload always complains about allowing hooks or using --no-verify!
+
+When using `repo upload` in projects that have custom repohooks, you might get
+an error like the following:
+```sh
+$ repo upload
+ERROR: You must allow the pre-upload hook or use --no-verify.
+```
+
+This can be confusing as you never get prompted.
+[MinTTY has a bug][mintty] that breaks isatty checking inside of repo which
+causes repo to never interactively prompt the user which means the upload check
+always fails.
+
+You can workaround this by manually granting consent when uploading.
+Simply add the `--verify` option whenever uploading:
+```sh
+$ repo upload --verify
+```
+
+You will have to specify this flag every time you upload.
+
+[mintty]: https://github.com/mintty/mintty/issues/56
+
+### repohooks always fail with an close_fds error.
+
+When using the [reference repohooks project][repohooks] included in AOSP,
+you might see errors like this when running `repo upload`:
+```sh
+$ repo upload
+ERROR: Traceback (most recent call last):
+  ...
+  File "C:\...\lib\subprocess.py", line 351, in __init__
+    raise ValueError("close_fds is not supported on Windows "
+ValueError: close_fds is not supported on Windows platforms if you redirect stdin/stderr/stdout
+
+Failed to run main() for pre-upload hook; see traceback above.
+```
+
+This error shows up when using Python 2.
+You should upgrade to Python 3 instead (see above).
+
+If you already have Python 3 installed, make sure it's the default version.
+Running `python --version` should say `Python 3`, not `Python 2`.
+If you didn't install the Python versions, or don't have permission to change
+the default version, you can probably workaround this by changing `$PATH` in
+your shell so the Python 3 version is found first.
+
+[repohooks]: https://android.googlesource.com/platform/tools/repohooks