is_absolute_path incorrect on Windows

Here's a thread that incorrectly started on the security list, but really is
more about functionality. Looking for comments:

The function is_absolute_path() is incorrect on Windows. As it's implemented,
it considers the following to be an absolute path:
* Anything that starts with /
* Anything that starst with \
* Anything alphanumerical, followed by a colon, followed by either / or \

Everything else is treated as relative.

However, it misses the case with for example E:foo, which is a perfectly
valid path on windows. Which isn't absolute *or* relative - it's relative
to the current directory on the E: drive. Which will be the same as the
current directory for the process *if* the process current directory is
on drive E:. In other cases, it's a different directory.

This function is used in the genfile.c functions to read and list files
by admin tools like pgadmin - to make sure we can only open files that are
in our own data directory - by making sure they're either relative, or they're
absolute but rooted in our own data directory. (It rejects anything with ".."
in it already).

The latest step in that thread is this comment from Tom:

> Yeah. I think the fundamental problem is that this code assumes there
> are two kinds of paths: absolute and relative to CWD. But on Windows
> there's really a third kind, relative with a drive letter. I believe
> that is_absolute_path is correct on its own terms, namely to identify a
> fully specified path. If we change it to allow cases that aren't really
> fully specified we will break other uses, such as in make_absolute_path.
>
> I'm inclined to propose adding an additional path test operator, along
> the lines of "has_drive_specifier(path)" (always false on non-Windows),
> and use that where needed to reject relative-with-drive-letter paths.

I think I agree with this point, but we all agreed that we should throw
the question out for the wider audience on -hackers for more comments.

So - comments?

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/