bug#75207: Fwd: bug#75207: 29.4; Path conversion from native codepage to UTF-8 fails when Windows is set by default to UTF-8

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 03 Jan 2025 11:49:34 +0000
> From: Michał Lach <michal@0lock.xyz>
> Cc: Eli Zaretskii <eliz@gnu.org>
> 
> Forgot to CC the bug report mail.
> 
> > Begin forwarded message:
> > 
> > From: <michal@0lock.xyz>
> > Subject: RE: bug#75207: 29.4; Path conversion from native codepage to UTF-8 fails when Windows is set by default to UTF-8
> > Date: 3 January 2025 at 02:48:53 CET
> > To: "'Eli Zaretskii'" <eliz@gnu.org>
> > Reply-To: <michal@0lock.xyz>
> > 
> > M-: (getenv "ENU") -> nil
> > M-: current-locale-environment -> "ENG"
> > M-: w32-ansi-code-page -> 65001
> > M-: (default-value 'buffer-file-coding-system) -> iso-latin-1-dos

OK.  I think I see the problem (and it is not specific to UTF-8
codepage), but just to be sure, please show some more values:

  M-: w32-multibyte-code-page RET
  M-: locale-coding-system RET
  M-: file-name-coding-system RET
  M-: default-file-name-coding-system RET

> > Here is the repro.
> > 1. Put a path to your "PATH" environmental variable with some diacritic 
> > character (ł in my case, maybe it won't work for some)
> > 2. M-: exec-path returns gibberish
> > 
> > Here, "Michał" becomes "MichaÅ‚", you can get a similar result if you do 
> > MultiByteToWideChar using Windows-1252 codepage on a UTF-8 path.

We think that PATH is encoded in Windows-1252 codepage, and the
question is why and where do we err.  The above additional values I
ask about might help answer that question.

> > I've digged around and it looks like codepage_for_filenames (src/w32.c) at 
> > somepoint returns the Windows-1252 codepage.
> > This is then passed to MultiByteToWideChar() and the scenario that I 
> > described above happens.
> > I've checked this hypothesis with API Monitor and this is what actually 
> > happens, I can attach a trace if you will find it useful.

Not necessary for now, thanks.

If I send you a C-level patch, are you able to build Emacs after
patching it, preferably the master branch of our Git repository?