bug#18310: 24.3.93; relative links don't work in eww and Windows 7

[Eli Zaretskii <eliz@gnu.org>]

> From: joaotavora@gmail.com (João Távora)
> Date: Thu, 21 Aug 2014 11:33:32 +0100
> 
> On Windows 7:
> 
>     emacs -Q
>     M-x eww RET
>     http://www.lispworks.com/documentation/HyperSpec/Front/index.htm RET
> 
> Try to follow any of the relative links on the page, they point to
> something strange like "www.lispworks.comz" (note the final "z") which
> basically breaks all navigation.
> 
> The `shr-url' property at point shows
> 
>     http://www.lispworks.comz:/documentation/HyperSpec/Front/StartPts.htm
> 
> And everything indicates this is a consequence of a previous bug fix of
> mine for bug#17217 [1], which does not manifest itself in my Linux
> box. I'm pretty sure it also did not manifest itself on my old Windows
> XP box.

I'm pretty sure it did happen on XP.

> In that fix, I used the function `expand-file-name' in `shr-expand-url'
> to compute the expanded URL for "totally relative" case of hrefs like
> "../something".
> 
> This new bug seems to be caused by `expand-file-name' insisting on
> producing a valid windows pathname (with drive letter), even though it
> was passed the second argument DEFAULT-DIRECTORY.

That's what expand-file-name is supposed to do: it should produce a
fully-qualified file name that unequivocally describes a file.  That
means the file name must specify the drive letter.

> That is, on my Windows 7 system:
> 
>    (expand-file-name "../bla" "/something/else")
> 
> expands to
> 
>    "z:/something/bla"
> 
> Whereas I intented it to expand to "/something/bla".

"/something/bla" is not a fully-qualified file name, not on Windows.
As long as the drive letter is not specified, the file name is
ambiguous.

> My HOME variable is set to at "z:", but unsetting it does not help
> either.

I don't think it's because of HOME, since there was no "~" in the file
name.  I'm guessing that the default-directory of the buffer where you
used that code was on the z: drive, so Emacs used that to complete the
missing drive letter.

> I don't have time right now to look at the C-code for
> `expand-file-name'.

There's nothing wrong with expand-file-name, please don't waste your
time looking there.  The problem is in the change you made.  You
cannot use expand-file-name on anything but a file name on a local
filesystem, even if the "thing" you have to deal with happens to look
like a file name and uses slashes as separators.  expand-file-name
assumes without testing that its arguments are syntactically valid
local file names, and will produce invalid results if they are not.

Please use url-expand-file-name instead, it does exactly what you
want, and does that portably.