Re: [PATCH v3 04/24] merge-recursive: provide a better label for diff3 common ancestor

[Elijah Newren <newren@gmail.com>]

On Fri, Aug 16, 2019 at 2:33 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Elijah Newren <newren@gmail.com> writes:
>
> > @@ -3507,6 +3507,11 @@ int merge_recursive(struct merge_options *opt,
> >       struct commit *merged_common_ancestors;
> >       struct tree *mrtree;
> >       int clean;
> > +     int num_merge_bases;
> > +     struct strbuf merge_base_abbrev = STRBUF_INIT;
> > +
> > +     if (!opt->call_depth)
> > +             assert(opt->ancestor == NULL);
>
> Hmph.  Do we have anything to say on this field when call_depth is
> not zero?  Is it OK for opt->ancestor to be sometimes NULL and non
> NULL some other times?

I was specifically trying to add a check for external callers of
merge_recursive() to make sure they called it correctly.  Since
merge_recursive() sets opt->ancestor before calling itself
recursively, I had to hide the assertion behind an if-check, namely on
call_depth.

We could add an assertion that opt->ancestor != NULL when
opt->call_depth > 0, but it seemed odd to document pre-conditions for
how merge_recursive() calls itself.  Anyway, this code block actually
becomes a bit cleaner later in the series when I create  separate
merge_recursive() and merge_recursive_internal() functions, as the
assertion can just go into merge_recursive() and not be protected by
the opt->call_depth check.

> > @@ -3528,6 +3533,7 @@ int merge_recursive(struct merge_options *opt,
> >                       output_commit_title(opt, iter->item);
> >       }
> >
> > +     num_merge_bases = commit_list_count(ca);
>
> Criss-cross merge with very large number of merge bases is rare, so
> it is OK to count them all, even though we only care about "is it
> zero, is it one, or is it two or more?"
>
> I suspect this does not have to count, though, if we really wanted
> to avoid counting.
>
> >       merged_common_ancestors = pop_commit(&ca);
> >       if (merged_common_ancestors == NULL) {
> >               /* if there is no common ancestor, use an empty tree */
>
> Here is the case where we can already decide the ancestor name for
> the later merge_trees() should be "empty tree".
>
> And if merged_common_ancestors is not NULL, ca may have run out (in
> which case, we only have a single merge base), or ca still has
> another merge base (in which case, we have two or more).  So, if you
> add
>                 ancestor_name = "empty tree";
>         } else if (ca) {
>                 ancestor_name = "merged common ancestors";
>         } else {
>                 ancestor_name = abbrev_name(merged_common_ancestors);
>         }
>
> to that if() statement above, that should be sufficient, no?
>
> opt is used for inner merge in the for() loop, so you would probably
> need another "char *" variable without contaminating opt->ancestor_name
> at this point, and then assign the value in the temporary to the
> opt->ancestor field where the original always assigned "merged
> common ancestors".

Sure, I can make these changes.

> > @@ -3568,10 +3574,23 @@ int merge_recursive(struct merge_options *opt,
> >       if (!opt->call_depth)
> >               repo_read_index(opt->repo);
> >
> > -     opt->ancestor = "merged common ancestors";
> > +     switch (num_merge_bases) {
> > +     case 0:
> > +             opt->ancestor = "<empty tree>";
>
> Also, I do not see a reason why you want angle bra-ket pair around
> "empty tree".  You are already using "merged common ancestors"
> literal phrase without any special marker syntax.

Oh good point; will drop.