bug#56712: 29.0.50; x-dnd-tests-do-direct-save fails on Cygwin

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Ken Brown <kbrown@cornell.edu> writes:

> On 7/24/2022 9:33 AM, Po Lu wrote:
>> Now done.  Ken, could you please re-run the test and show the detailed
>> error that is signalled?
>
> Thanks.  The error is now clear:
>
> Test x-dnd-tests-do-direct-save condition:
>     (file-missing "Opening output file" "No such file or directory"
>     "//tmp/x-dnd-testvZQUCZ/x-dnd-test-targetKiqqPT")
>
> According to POSIX, the meaning of exactly 2 slashes at the beginning
> of a path name is system dependent.  On Cygwin it's interpreted as a
> UNC path.  That explains the error, but I don't immediately see where
> the extra slash comes from.  (There are plenty of references to UNC
> paths and to "file://" and "file:///" in dnd.el, so maybe there's some
> code in there that incorrectly assumes that MS-Windows is the only
> platform on which exactly 2 initial slashes has a special meaning.)
>
> Ken

Unless system-type can be windows-nt on Cygwin, I don't think that can
happen.  I will look into this, thanks.