Windows: Activating a venv from a MSYS shell spawned from a cmd shell borks your PATH

Bug report

Bug description:

I believe I have found an issue with the activate script installed as part of virtualenv creation. Under a specific set of circumstances (activating the venv from a Windows shell, then spawning a Bash shell, then activating the venv in Bash), it can break your PATH, causing your shell to be unusable until it is restarted.

To reproduce, do the following:

In cmd:

> rem Create and enter a venv normally
> python -m venv .venv
> .venv\Scripts\activate.bat
(.venv) > rem Now we will run Git Bash (which is a distribution of MSYS) from this shell.
(.venv) > "C:\Program Files\Git\git-bash.exe"

In the opened Git Bash terminal:

$ echo $PATH
/c/Users/jsmith/bin:/mingw64/bin:/usr/local/bin:/usr/bin:/bin:/mingw64/bin:/usr/bin:/c/Users/jsmith/bin:/c/path/to/.venv/Scripts: <snip the rest of my Windows PATH>

# Note that the venv is already active, but it's not obvious that it is because there is no prompt
$ command -v python
/c/path/to/.venv/Scripts/python

# Now activate the venv
$ source .venv/Scripts/activate

# Uh oh! Our PATH is broken now
(.venv) $ find
bash: find: command not found

# Observe that PATH no longer includes the MinGW bin dirs (bad) and uses backslashes instead of forward slashes (also bad!)
(.venv) $ echo $PATH
C:\path\to\.venv/Scripts: <snip the rest of Windows path>

It looks like what happens here is that the deactivate function in the activate script sees the _OLD_VIRTUAL_PATH env var (which it inherited from the cmd process) and tries to restore it as the "old PATH". Except oops, this path is Windows-style and is different from what is needed inside Git Bash.

Note that I actually ran into this issue because it was happening in my IDE terminal (specifically CLion); it looks like CLion attempts to activate the venv before starting the terminal. But it's easier to understand and reproduce with cmd.

One possible fix would be to change activate.bat to use a different name for _OLD_VIRTUAL_PATH, so that the bash and cmd scripts won't ever try to use the same variable. Unfortunately, that does leave the door open for some issues, because unlike bash, variables in cmd are always exported to subprocesses (set foo=bar is equivalent to export foo=bar in bash), so this variable will always get inherited by a subprocess. So, you could still run into an issue where activating a venv "reverts" your PATH to the value used by a parent process when it last activated a venv. Oh well, I guess this is what us holdouts get for still using cmd...

CPython versions tested on:

3.11

Operating systems tested on:

Windows

Linked PRs

• gh-157722

Reproduce the Windows cmd and Git Bash sequence from the report, then inspect Lib/venv/scripts/common/activate and the activate.bat script mentioned in the issue. Compare how each script handles _OLD_VIRTUAL_PATH and verify that activating from MSYS preserves the usable shell PATH; the linked gh-157722 indicates work is already underway.