Subject: Re: [PATCH v4 1/2] diff --no-index: optionally follow symlinks

From: Dennis Kaarsemaker <[email-masked]>
Date: Sat, 25 Mar 2017 22:30:22 +0100

On Fri, 2017-03-24 at 15:56 -0700, Junio C Hamano wrote:

> diff --git a/diff.c b/diff.c
> > index be11e4ef2b..2afecfb939 100644
> > --- a/diff.c
> > +++ b/diff.c
> > @@ -2815,7 +2815,7 @@ int diff_populate_filespec(struct diff_filespec *s, unsigned int flags)
> >  		s->size = xsize_t(st.st_size);
> >  		if (!s->size)
> >  			goto empty;
> > -		if (S_ISLNK(st.st_mode)) {
> > +		if (S_ISLNK(s->mode)) {
> 
> This change is conceptually wrong.  s->mode (often) comes from the
> index but in this codepath, after finding that s->oid is not valid
> or we want to read from the working tree instead (several lines
> before this part), we are committed to read from the working tree
> and check things with st.st_* fields, not s->mode, when we decide
> what to do with the thing we find on the filesystem, no?

Hmm, true. It just accidentally does the right thing because s->mode
happens to always match the expectations of this code. I will pass on
more information into diff_populate_filespec so an explicit check can
be done here.

> > @@ -2825,6 +2825,10 @@ int diff_populate_filespec(struct diff_filespec *s, unsigned int flags)
> >  			s->should_free = 1;
> >  			return 0;
> >  		}
> > +		if (S_ISLNK(st.st_mode)) {
> > +			stat(s->path, &st);
> > +			s->size = xsize_t(st.st_size);
> > +		}
> >  		if (size_only)
> >  			return 0;
> >  		if ((flags & CHECK_BINARY) &&
> 
> I suspect that this would conflict with a recent topic.  

Possibly. I used the same base commit for the newer versions as that
seems to be your preference. If there is a merge conflict, do you want
me to rebase against current master?

> But more importantly, this inserted code feels doubly wrong.
> 
>  - what allows us to unconditionally do "ah, symbolic link on the
>    disk--find the target of the link, not the symbolic link itself"?
>    We do not seem to be checking '--dereference' around here.

The implicit check above (which you already noted is faulty) allows us
to do this. So fixing the check above will also involve fixing this.

>  - does this code do a reasonable thing when the path is a symbolic
>    link that points at a directory?  what does it mean to grab
>    st.st_size for such a thing (and then go on to open() and xmmap()
>    it)?

No, it does something entirely unreasonable. I hadn't even thought of
testing with symlinks to directories, as my ulterior motive was the
next commit that makes it work with pipes. This will be fixed.

Thanks very much for the thoroughness of your review!

D.