patch, 2 partsmingw: support absolute paths without a drive prefix

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
> 
> When specifying an absolute path without a drive prefix, we convert that
> path internally. Let's make sure that we handle that case properly, too
> ;-)
> 
> This fixes the command
> 
> 	git clone https://github.com/git-for-windows/git \G4W
> 
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>   compat/mingw.c            | 10 +++++++++-
>   t/t5580-clone-push-unc.sh |  2 +-
>   2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/compat/mingw.c b/compat/mingw.c
> index 34b3880b29..4d009901d8 100644
> --- a/compat/mingw.c
> +++ b/compat/mingw.c
> @@ -928,11 +928,19 @@ unsigned int sleep (unsigned int seconds)
>   char *mingw_mktemp(char *template)
>   {
>   	wchar_t wtemplate[MAX_PATH];
> +	int offset = 0;
> +
>   	if (xutftowcs_path(wtemplate, template) < 0)
>   		return NULL;
> +
> +	if (is_dir_sep(template[0]) && !is_dir_sep(template[1]) &&
> +	    iswalpha(wtemplate[0]) && wtemplate[1] == L':') {
> +		/* We have an absolute path missing the drive prefix */

> +		offset = 2;
> +	}
>   	if (!_wmktemp(wtemplate))
>   		return NULL;
> -	if (xwcstoutf(template, wtemplate, strlen(template) + 1) < 0)
> +	if (xwcstoutf(template, wtemplate + offset, strlen(template) + 1) < 0)
>   		return NULL;
>   	return template;
>   }
> diff --git a/t/t5580-clone-push-unc.sh b/t/t5580-clone-push-unc.sh
> index c2b0082296..17c38c33a5 100755
> --- a/t/t5580-clone-push-unc.sh
> +++ b/t/t5580-clone-push-unc.sh
> @@ -29,7 +29,7 @@ case "$UNCPATH" in
>   	;;
>   esac
>   
> -test_expect_failure 'clone into absolute path lacking a drive prefix' '
> +test_expect_success 'clone into absolute path lacking a drive prefix' '
>   	USINGBACKSLASHES="$(echo "$WITHOUTDRIVE"/without-drive-prefix |
>   		tr / \\\\)" &&
>   	git clone . "$USINGBACKSLASHES" &&
> 

> > diff --git a/compat/mingw.c b/compat/mingw.c
> > index 34b3880b29..4d009901d8 100644
> > --- a/compat/mingw.c
> > +++ b/compat/mingw.c
> > @@ -928,11 +928,19 @@ unsigned int sleep (unsigned int seconds)
> >   char *mingw_mktemp(char *template)
> >   {
> >   	wchar_t wtemplate[MAX_PATH];
> > +	int offset = 0;
> > +
> >    if (xutftowcs_path(wtemplate, template) < 0)
> >   		return NULL;
> > +
> > +	if (is_dir_sep(template[0]) && !is_dir_sep(template[1]) &&
> > +	    iswalpha(wtemplate[0]) && wtemplate[1] == L':') {
> > +		/* We have an absolute path missing the drive prefix */
> 
> This comment is true for the source part, template, but I can't find
> where the destination, wtemplate, suddenly gets the drive prefix. As far
> as I can see, xutftowcs_path() just does a plain textual conversion
> without any interpretation of the text as path. Can you explain it?

> On Mon, 10 Dec 2018, Johannes Sixt wrote:
>>> diff --git a/compat/mingw.c b/compat/mingw.c
>>> index 34b3880b29..4d009901d8 100644
>>> --- a/compat/mingw.c
>>> +++ b/compat/mingw.c
>>> @@ -928,11 +928,19 @@ unsigned int sleep (unsigned int seconds)
>>>    char *mingw_mktemp(char *template)
>>>    {
>>>    	wchar_t wtemplate[MAX_PATH];
>>> +	int offset = 0;
>>> +
>>>     if (xutftowcs_path(wtemplate, template) < 0)
>>>    		return NULL;
>>> +
>>> +	if (is_dir_sep(template[0]) && !is_dir_sep(template[1]) &&
>>> +	    iswalpha(wtemplate[0]) && wtemplate[1] == L':') {
>>> +		/* We have an absolute path missing the drive prefix */
>>
>> This comment is true for the source part, template, but I can't find
>> where the destination, wtemplate, suddenly gets the drive prefix. As far
>> as I can see, xutftowcs_path() just does a plain textual conversion
>> without any interpretation of the text as path. Can you explain it?
> 
> It is legal on Windows for such a path to lack the drive prefix, also in
> the wide-character version. So the explanation is: even `wtemplate` won't
> get the drive prefix. It does not need to.

>>>> +	if (is_dir_sep(template[0]) && !is_dir_sep(template[1]) &&
>>>> +	    iswalpha(wtemplate[0]) && wtemplate[1] == L':') {
>>>> +		/* We have an absolute path missing the drive prefix */
>>>
>>> This comment is true for the source part, template, but I can't find
>>> where the destination, wtemplate, suddenly gets the drive prefix. As far
>>> as I can see, xutftowcs_path() just does a plain textual conversion
>>> without any interpretation of the text as path. Can you explain it?
>>
>> It is legal on Windows for such a path to lack the drive prefix, also in
>> the wide-character version. So the explanation is: even `wtemplate` won't
>> get the drive prefix. It does not need to.
>
> I'm sorry, my question was extremely fuzzy. I actually wanted to know
> how the condition that you introduce in this patch can ever be true.
>
> And after looking at the Git for Windows code, I could answer it
> myself: it cannot. Not with this patch alone. In GfW, there is
> additional code in xutftowcs_path() that massages wtemplate to receive
> a drive prefix; but vanilla Git does not have that code, so that
> is_dir_sep(template[0]) and iswalpha(wtemplate[0]) can never be true
> at the same time at this point.

> Am 13.12.18 um 03:48 schrieb Junio C Hamano:
>> So,... what's the conclusion?  The patch in the context of my tree
>> would be a no-op, and we'd need a prerequisite change to the support
>> function to accompany this patch to be effective?
> 
> Correct, that is my conclusion.

> Am 13.12.18 um 07:25 schrieb Johannes Sixt:
> > Am 13.12.18 um 03:48 schrieb Junio C Hamano:
> > > So,... what's the conclusion?  The patch in the context of my tree
> > > would be a no-op, and we'd need a prerequisite change to the support
> > > function to accompany this patch to be effective?
> > 
> > Correct, that is my conclusion.
> 
> Moreover, there is no problem with your tree to begin with. I cloned into a
> destination without a drive letter:
> 
> C:\>git clone R:\j6t\expat.git \Temp\expat
> Cloning into '\Temp\expat'...
> done.
> 
> and it works fine. If I understood the description in patch 1/2 correctly,
> this should have failed.