bug#10257: 23.3.1 Cygwin: network drives - file is write protected (false positive)

[Ken Brown <kbrown@cornell.edu>]

On 12/13/2011 9:54 PM, Stefan Monnier wrote:
>> The code I'm talking about is near the beginning of the definition of
>> basic-save-buffer-2' in files.el:
>
> Thanks.
>
>> I'm not (yet) proposing a change.  I'm simply asking what the rationale is
>> for calling `yes-or-no-p' and making the user confirm that s/he wants to try
>> to save the file.
>
> I'm not sure what was the intention, but I know that this code is
> triggered in cases such as:
> - running as root: write will always succeed.
> - saving to a read-only file that you own: while `write' will fail,
>    you can make it succeed by changing the access rights (which is what
>    tempsetmodes is for).
> - saving to a read-only file in a writable dir: write will fail, but
>    you can make it succeed by calling unlink first [Not sure if this
>    works in Emacs right now].
> In all these cases, Emacs is able to write the file, but the read-only
> bit expresses an intention not to modify the file so it makes sense to
> ask for confirmation.

There's a fourth case, and that's what led to the present bug report (by 
Jari):
- saving to a file that is in fact writable, for which file-writable-p 
gives the wrong answer.

> This said, the code you quote should never prevent you from saving
> a file, it should only ask for confirmation (i.e. it might be annoying
> but it shouldn't prevent you from getting your work done).

Jari was annoyed to the point where he felt he couldn't get his work 
done.  He's in a situation where file-writable-p will consistently 
report that a file is not writable, but writing will in fact succeed.  I 
was trying to help him find a way to avoid having to repeatedly confirm 
that he wants to save.

> But other than for the "running as root" case, the above two cases could
> replace the `file-writable-p' test with a `write-region' test:
> file-writable-p is documented (via POSIX's documentation of `access') to
> be an approximation, whereas `write-region' should reliably tell us
> whether we can write to the file.

So that sounds like a solution to Jari's problem.  I'll leave it to him 
to propose a patch along those lines.

Ken