Fix unreasonably small DefaultShellCommandOption size limit

[samhocevar]

The DefaultShellCommandOption registry key has a size limit of 15 characters. This seems unreasonably small especially since DefaultShellArguments has a much larger limit and its contents are not used for a non-interactive shell, so a lot of what is in there has to be duplicated.

For instance, if I wanted to configure OpenSSH to use MSYS2 without creating a helper script, this is what my registry keys would look like:

DefaultShell                 C:\msys64\msys2_shell.cmd
DefaultShellArguments        -defterm -mingw64 -no-start -shell zsh
DefaultShellCommandOption    -defterm -mingw64 -no-start -shell zsh -c
DefaultShellEscapeArguments  0

This PR increases the limit to the same value as DefaultShellArguments, which is currently PATH_MAX / 2 - 1.

[tgauth]

commenting to try to kick-off CI...have had some changes to the trigger recently

[tgauth]

CI looks good - thanks @samhocevar!

[tgauth]

Actually - I'm having trouble repro'ing the scenario in the description with these changes so there may be additional changes needed to get this to work.

The non-interactive session still fails to return the output of the command to the console.

[samhocevar]

Sorry, DefaultShellEscapeArguments should actually be 0 in that case. I have edited my original report.

[samhocevar]

Maybe the following information can help pinpoint the problem? Without modifying OpenSSH, my setup uses the following configuration:

DefaultShell                 C:\msys64\msys2_shell_helper.cmd
DefaultShellArguments        -shell zsh -l
DefaultShellCommandOption    -shell zsh -c
DefaultShellEscapeArguments  0

My C:\msys64\msys2_shell_helper.cmd wrapper contains just the following line:

@call %~dp0\msys2_shell.cmd -defterm -mingw64 -no-start %*

And non-interactive login works like this:

22/08 23:51 sam@potpal🪟~% ssh localhost uname -a
sam@localhost's password:
MINGW64_NT-10.0-22621 potpal 3.4.7.x86_64 2023-08-09 17:32 UTC x86_64 Msys
22/08 23:51 sam@potpal🪟~%

[tgauth]

Thanks @samhocevar - are you running ssh localhost uname -a from a zsh session?

With just the DefaultShell configured, I'm able to execute a non-interactive command.
With both the DefaultShell and DefaultShellCommandOption configured, I'm getting a - : invalid option

[samhocevar]

Thanks for investigating thoroughly!

In my case, it does not seem to matter what the originating shell is, I get the same behaviour from PowerShell (7.1.0-preview.3), MSYS2 (hard to tell the exact version, but mostly current), or WSL (Debian Trixie):

Here is my exact registry configuration for SSHd:

% reg query 'HKLM\Software\OpenSSH'

HKEY_LOCAL_MACHINE\Software\OpenSSH
    DefaultShell    REG_SZ    C:\msys64\msys2_shell_helper.cmd
    DefaultShellArguments    REG_SZ    -shell zsh -l
    DefaultShellEscapeArguments    REG_DWORD    0x0
    DefaultShellCommandOption    REG_SZ    -shell zsh -c

My unmodified Windows SSHd (C:/Windows/System32/OpenSSH/sshd.exe) version is OpenSSH_for_Windows_8.6p1, LibreSSL 3.4.3.

My Zsh version (C:/msys64/usr/bin/zsh.exe) is zsh 5.9 (x86_64-pc-msys).

I am attaching the full contents of C:/msys64/msys2_shell.cmd because that is the file most likely to differ, with the rather inconsistent upgrade paths one may have with MSYS2:

• msys2_shell.cmd.txt
• msys2_shell_helper.cmd.txt

[tgauth]

Thanks for the additional clarifications @samhocevar - am now able to repro the fix locally.

One more question...

This PR increases the limit to the same value as DefaultShellArguments, which is currently PATH_MAX / 2 - 1.

Is DefaultShellArguments set to PATH_MAX / 2 - 1 or just PATH_MAX?