Re: master 95b1eacd47: Fix handling of UNCs in 'parse-colon-path

[Ken Brown]

On 8/24/2022 2:48 PM, Eli Zaretskii wrote:
> > Date: Wed, 24 Aug 2022 14:34:02 -0400
> > From: Ken Brown <kbrown@cornell.edu>
> >
> > On 8/24/2022 12:20 PM, Eli Zaretskii wrote:
> > > branch: master
> > > commit 95b1eacd4750da7329380aabcb383a8f9d96a59b
> > > Author: Eli Zaretskii <eliz@gnu.org>
> > > Commit: Eli Zaretskii <eliz@gnu.org>
> > >
> > >       Fix handling of UNCs in 'parse-colon-path
> > >       
> > >       * lisp/files.el (parse-colon-path): Don't remove the second
> > >       leading slash on systems that support UNCs.  (Bug#57353)
> > >       
> > >       * test/lisp/files-tests.el (files-tests-bug-21454): Update
> > >       expected results.
> > >       (files-colon-path): Add a new test pattern.
> >
> > After this commit I'm getting the following test failure on Cygwin.  I don't
> > have time to look into it now, but I can do so in a few days if the fix isn't
> > obvious.
>
> Ugh, it means the test data needs to be split three-way: one for
> MS-Windows/MS-DOS, another for Cygwin, and one more for the rest.
> Because Cygwin is like Posix systems, but it does want to support
> UNCs.

It's not just the test data that's wrong on Cygwin, but parse-colon-path is 
wrong: It always collapses multiple leading slashes to two.

Posix says that multiple leading slashes are equivalent to one slash *except* in 
the case of exactly two leading slashes.  In that case, the interpretation is 
implementation-dependent.

Cygwin follows Posix and chooses to interpret precisely two leading slashes as 
referring to a UNC path.  In particular, 3 or more slashes should be collapsed 
to 1 slash, as on other Posix systems, while exactly 2 leading slashes should be 
left alone.

Ken