[PATCH] Add ability for can_write to indicate why the file is not writable

[Rafael Espíndola]

> What do we lose by getting rid of that first case entirely, because
> the second case already has to handle the exact same problems
> regardless? Eg) File exists and can be written to, passing your first
> checks, but when we attempt to produce/copy the temp file and that
> fails because the file has been deleted or been flagged as not
> writable.

+1

Even if producing the temporary is expensive, how common is the
failure? If not common it is probably worth the simplicity to simply
try to copy.

Cheers,
Rafael