bug#56443: 28.1; Mac segmentation fault with ffap

[Mattias Engdegård]

8 juli 2022 kl. 12.45 skrev Eli Zaretskii <eliz@gnu.org>:

> Fine with me, but please update the comment to this effect.

Certainly.

> But why does the original call to file_name_case_insensitive_err fail
> in this case with err > 0 on macOS, and what is the value of err?

It fails because the argument, "mailto:some@mail.address";, is not an existing 
file and so pathconf(filename, _PC_CASE_SENSITIVE) returns -1 which means 
"don't know". (If a file of that name is indeed created then it returns 0 here 
because this is a case-insensitive file system.)

Perhaps we shouldn't ask about case-sensitivity for such names but then again 
those are legal file names to open and Ffile_name_insensitive_p doesn't know 
the intentions of the caller, and in any case the error leads us to try the 
parent directory which doesn't exist so everything works out for the best with 
the patch.

Pushed to master, with a test.