Re: master 6011d39b6a: Fix drag-and-drop of files with multibyte filenames

[Eli Zaretskii <eliz@gnu.org>]

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Sun, 05 Jun 2022 18:00:10 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I don't think I understand this change.  raw-text basically doesn't do
> > any conversion, except if the text includes raw bytes.  Is that the
> > problem here, and if so, how come a file name can include raw bytes in
> > its name?
> 
> Encoding it as `raw-text-unix' is to satisfy the requirement in
> xselect.c that strings returned by selection converters must be
> unibyte.  IOW, it's the same as
> 
>   (string-as-unibyte (expand-file-name value))
> 
> except that we can't use `string-as-unibyte', because it's obsolete.

Then why not encode in UTF-8, for example?

> > And what does "Motif expects this to be STRING, but it treats the data
> > as a sequence of bytes instead of a Latin-1 string" mean in this
> > context?  The difference between raw bytes and Latin-1 strings is only
> > meaningful to Emacs; how does Motif distinguish between them?
> 
> The selection property type STRING means a Latin-1 string, with some
> minor extensions.  See this paragraph under "TEXT Properties" in the
> ICCCM:
> 
>    STRING as a type or a target specifies the ISO Latin-1 character set
>    plus the control characters TAB (octal 11) and NEWLINE (octal
>    12). The spacing interpretation of TAB is context dependent. Other
>    ASCII control characters are explicitly not included in STRING at the
>    present time.
> 
> But Motif doesn't comply with the ICCCM meaning of STRING or use the
> generic TEXT type when converting a drag-and-drop selection to
> FILE_NAME.  It instead expects the type of the selection property to be
> STRING, but the data is treated as raw bytes.

If some program other than Emacs is the target of the drop, raw bytes
produced from raw-text will not be meaningful for it.

I actually don't understand why you don't use ENCODE_FILE for files
and ENCODE_SYSTEM for everything else -- this is the only encoding
which we know to be generally suitable for any operation that calls
low-level C APIs whose implementation is not in Emacs.  Bonus points
for adhering to selection-coding-system when that is non-nil.

Are there any known problems with using these two system encodings in
this case?