Cygwin doesn't work with Git Bash but does with Windows Shell

[embcla]

• I was not able to find an open or closed issue matching what I'm seeing

I am installing a new Win11 laptop, and what I see is that I can't get Git Bash to cooperate with Cygwin.
I installed both as 64bit versions.

Cygwin commands (in this bug exemplified by Python3.9) run fine in windows shell, but cause a heap mismatch in Git Bash.
See attached screenshot.

I really tried everything I could before opening this, but can't really figure this one out.
Previously I used Cygwin32, but it doesn't seem to be available anymore.
Disabled ASLR but made no difference.
Made sure there is only 1 cygwin1.dll on path.
Reinstalled multiple times but still have the same issue.
I think it's Git Bash because everything else works from Windows, so I believe that Git Bash is mangling some linker code and causing this issue.

Setup

• Which version of Git for Windows are you using? Is it 32-bit or 64-bit?

$ git --version --build-options

git version 2.35.1.windows.2
cpu: x86_64
built from commit: 5437f0fd368c7faf1a0b5e1fef048232c1f2a3e6
sizeof-long: 4
sizeof-size_t: 8
shell-path: /bin/sh
feature: fsmonitor--daemon

• Which version of Windows are you running? Vista, 7, 8, 10? Is it 32-bit or 64-bit?

$ cmd.exe /c ver

Microsoft Windows [Version 10.0.22000.556]

• What options did you set as part of the installation? Or did you choose the
  defaults?

> type "C:\Program Files\Git\etc\install-options.txt"

Editor Option: VisualStudioCode
Custom Editor Path:
Default Branch Option:
Path Option: Cmd
SSH Option: OpenSSH
Tortoise Option: false
CURL Option: OpenSSL
CRLF Option: CRLFAlways
Bash Terminal Option: MinTTY
Git Pull Behavior Option: Rebase
Use Credential Manager: Enabled
Performance Tweaks FSCache: Enabled
Enable Symlinks: Disabled
Enable Pseudo Console Support: Disabled
Enable FSMonitor: Disabled

• Any other interesting things about your environment that might be related
  to the issue you're seeing?

Not sure

Details

• Which terminal/shell are you running Git from? e.g Bash/CMD/PowerShell/other

Git Bash

• What commands did you run to trigger this issue? If you can provide a
  Minimal, Complete, and Verifiable example
  this will help us understand the issue.

python3.9

• What did you expect to occur after running these commands?

That I'd get into the python interpreter

• What actually happened instead?

Segfault

$ python3.9
      0 [main] python3.9 (22568) C:\cygwin64\bin\python3.9.exe: *** fatal error - cygheap base mismatch detected - 0x180349408/0x180346408.
This problem is probably due to using incompatible versions of the cygwin DLL.
Search for cygwin1.dll using the Windows Start->Find/Search facility
and delete all but the most recent version.  The most recent version *should*
reside in x:\cygwin\bin, where 'x' is the drive on which you have
installed the cygwin distribution.  Rebooting is also suggested if you
are unable to find another cygwin DLL.

[dscho]

Search for cygwin1.dll using the Windows Start->Find/Search facility

Sadly, the problem is not a duplicate cygwin1.dll but instead msys-2.0.dll, which is a friendly fork of Cygwin. Both DLLs implement a POSIX emulation layer (most notably to emulate fork() and pseudo terminals), and they assume that one is compatible with the other, which they aren't.

But even if you could fix those compatibility problems somehow, the problems would not end there. Internally, Cygwin and MSYS2 are just different enough not to work well with one another. For example, Cygwin's less.exe is unlikely to work well with MSYS2's pseudo terminals, and vice versa.

My best suggestion is to decouple the two POSIX emulation layers by running the Cygwin Python via winpty:

$ winpty python3.9

But then, I wonder why you want to call Cygwin from Git for Windows? We kind of support the other way round, at least a little bit.

Having said that, I always strongly recommend to stay within your favorite ecosystem. For example, why not use Cygwin's Bash and Cygwin's Git when you also want to use Cygwin's Python?

[embcla]

So I was using this system because Cygwin allows me to choose the packages I use on windows (gcc, meson, make, etc).

And to be honest I was just firing Git Bash that got me into a terminal, I did not even know Cygwin came with its own terminal, now I checked and Indeed it's there.

Also on my previous laptop, this just worked: Git Bash provided my main bash environment, and then I'd use Cygwin to install and update packages.

So, for me to better understand: which package is loading and installing MSYS2, Git for Windows?

Also Git Bash makes it transparent to access the local file system and use stuff like ssh.
From Cygwin own terminal I find I need more rummaging trough the file system to find stuff and I do not have a ~/.ssh folder.
So again my ideal way (as in most similar to a transparent Linux) seems to be Git Bash that has all the abstractions.
If that one uses MSYS2 then I'm ok to switch.
Though I'm not sure: when I launch a MSYS2 terminal, it clearly shows MSYS2, while when I launch a Git Bash it shows Mingw64 😕

[dscho]

Though I'm not sure: when I launch a MSYS2 terminal, it clearly shows MSYS2, while when I launch a Git Bash it shows Mingw64 😕

You can call MSYS2's Bash in multiple modes. Just use the MINGW64 one: mingw64.exe.

[dscho]

This should be fixed as of git-for-windows/msys2-runtime#48, which made it into Git for Windows v2.40.0