Re: [PATCH 03/10] merge-ort: implement unique_path() helper

[Elijah Newren <newren@gmail.com>]

On Wed, Dec 30, 2020 at 6:16 AM Derrick Stolee <stolee@gmail.com> wrote:
>
> On 12/18/2020 12:51 AM, Elijah Newren via GitGitGadget wrote:
> > From: Elijah Newren <newren@gmail.com>
> >
> > Implement unique_path(), based on the one from merge-recursive.c.  It is
> > simplified, however, due to: (1) using strmaps, and (2) the fact that
> > merge-ort lets the checkout codepath handle possible collisions with the
> > working tree means that other code locations don't have to.
> >
> > Signed-off-by: Elijah Newren <newren@gmail.com>
> > ---
> >  merge-ort.c | 25 ++++++++++++++++++++++++-
> >  1 file changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/merge-ort.c b/merge-ort.c
> > index d300a02810e..1adc27a11bc 100644
> > --- a/merge-ort.c
> > +++ b/merge-ort.c
> > @@ -343,11 +343,34 @@ static void path_msg(struct merge_options *opt,
> >       strbuf_addch(sb, '\n');
> >  }
> >
> > +/* add a string to a strbuf, but converting "/" to "_" */
> > +static void add_flattened_path(struct strbuf *out, const char *s)
> > +{
> > +     size_t i = out->len;
> > +     strbuf_addstr(out, s);
> > +     for (; i < out->len; i++)
> > +             if (out->buf[i] == '/')
> > +                     out->buf[i] = '_';
> > +}
> > +
>
> Thank you for pointing out that you based your code on merge-recursive.c.
> I see that this implementation is identical to the one there. I question
> whether this causes collisions in a problematic way, when "a/b/c" and
> "a_b_c" both exist in a tree.
>
> To avoid such a problem, we'd likely need to also expand "_" to "__" or
> similar. This might never actually affect any users because of the
> strange filename matches _and_ we need to be in a directory/file conflict.
>
> And maybe it's not a hole at all? If it is, we can consider patching or
> at least documenting the problem.

add_flattened_path() can certainly result in a collision, regardless
of whether the char *s parameter has any '/' characters in it.  For
example, if you are trying to get a unique path associated with
builtin/commit-graph.c due to changes from the 'next' branch side of
the merge, and builtin/commit-graph.c~next already exists, then you
have a collision.  It's actually pretty rare that the parameter would
have any '/' characters at all, since it's pretty rare for me to see
folks (other than Junio) use hierarchical branch names.  But if the
branch name were ds/line-log-on-bloom, then the provisional filename
would be builtin/commit-graph.c~ds_line-log-on-bloom.  The '/' to '_'
conversion exists just to make sure our new file remains in the same
directory as where the conflict that caused us to need a new unique
path occurred.

But unique_path() does NOT end immediately after calling
add_flattened_path() and there is no collision possible in the return
from unique_path(), because it ends with a
    while (strmap_contains(existing_paths, newpath.buf)) {
loop that modifies the resulting path until it finds one that doesn't
collide with an existing path.