bug#49723: 28.0.50; Test in coding.c for NUL bytes in filenames is not reliable

[Federico Tedin]

Ok! Here's my first try at this. I ended up skipping the check on
DEFAULT-DIRECTORY since as you mentioned, its value is used with
expand-file-name itself. In the other case, if default-directory is
picked up, then I checked the value of that variable.

expand-file-name.patch
Description: patch

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Federico Tedin <federicotedin@gmail.com>
>> Cc: 49723@debbugs.gnu.org,  Philipp Stephani <phst@google.com>
>> Date: Tue, 14 Sep 2021 21:01:16 +0200
>> 
>> I'm interested in looking into this one since I want to learn more about
>> the C side of the codebase. However, I wasn't able to find a call to
>> expand-file-name in encode_file_name or encode_file_name_1. I did find
>> the null byte check though (CHECK_TYPE + memchr). Maybe I am missing
>> something out.
>
> My description was inaccurate: the expand-file-name call usually
> precedes the call to ENCODE_FILE, it is not part of encode_file_name.
>
>> I assume that a similar check on expand-file-name should be applied to
>> both input arguments, NAME and DEFAULT-DIRECTORY?
>
> I don't think we need that because expand-file-name calls itself on
> DEFAULT-DIRECTORY internally.  But we may need to perform the check on
> default-directory, if we use it inside expand-file-name.