Re: [PATCH 02/10] merge-recursive.c: call a new unpack_trees_options_init() function

[Elijah Newren <newren@gmail.com>]

On Mon, Oct 4, 2021 at 7:49 AM Ævar Arnfjörð Bjarmason <avarab@gmail.com> wrote:
>
> On Mon, Oct 04 2021, Elijah Newren wrote:
>
> > On Sun, Oct 3, 2021 at 5:46 PM Ævar Arnfjörð Bjarmason <avarab@gmail.com> wrote:
> >>
> >> In the preceding commit we introduced a new UNPACK_TREES_OPTIONS_INIT
> >> macro, but "merge-recursive.c" could not be converted to it since
> >> it (re-)initializes a "struct unpack_trees_options" on the heap.
> >>
> >> In order to convert use the macro as the source of truth for
> >> initialization we need to not only convert the initialization in
> >> unpack_trees_start(), but also call the new
> >> unpack_trees_options_init() just after the CALLOC_ARRAY() in
> >> merge_start().
> >
> > Um...why?
>
> Replied below.
>
> >> When we call merge_trees() we'll call merge_start() followed by
> >> merge_trees_internal(), and it will call unpack_trees_start() before
> >> it does much of anything. So it's covered by an initialization in
> >> unpack_trees_start().
> >>
> >> But when merge_recursive() is called it will call merge_start()
> >> followed by merge_recursive_internal(), which won't call
> >> unpack_trees_start() until its own call call to
> >> merge_trees_internal(), but in the meantime it might end up using that
> >> calloc'd "struct unpack_trees_options".
> >
> > Nothing in merge-recursive.c usings unpack_opts before
> > unpack_trees_start() or after unpack_trees_finish().  If anyone
> > attempts to use it elsewhere, that would itself be a bug.  So, I'd
> > replace the above three paragraphs with:
> >
> > "Change the initialization of opt->priv_unpack_opts from using memset
> > to 0, with unpack_trees_options_init()."
> >
> > or something like that, and then drop your change to merge_start().
>
> Sure, I'll defer to you about being confident about that. I didn't want
> to leave a copy if it hanging without the proper initialization in case
> there were new callers.
>
> >> This was OK before, as setup_unpack_trees_porcelain() would call
> >> strvec_init(), and our "struct dir_struct" in turn is OK with being
> >> NULL'd. Let's convert the former to macro initialization, we'll deal
> >> with the latter in a subsequent commit.
> >
> > This is quite confusing; it's really hard to understand how this
> > relates to the rest of the commit message.  I have to read the code to
> > see what you're doing, and then write my own commit message in my head
> > because the wording in this paragraph still doesn't parse.
> >
> > I'd make the change strvec_init/STRVEC_INIT changes be a separate
> > patch.  I suspect it'll be easier to write the commit message for it
> > as a standalone commit as well.
>
> Sure, FWIW I had it split up locally, and figured it would be easier to
> explain if the API usage change came with the initialization change that
> made it possible.
>
> >>
> >> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> >> ---
> >>  merge-recursive.c | 3 ++-
> >>  unpack-trees.c    | 8 ++++++--
> >>  unpack-trees.h    | 5 ++++-
> >>  3 files changed, 12 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/merge-recursive.c b/merge-recursive.c
> >> index e594d4c3fa1..d24a4903f1d 100644
> >> --- a/merge-recursive.c
> >> +++ b/merge-recursive.c
> >> @@ -405,7 +405,7 @@ static int unpack_trees_start(struct merge_options *opt,
> >>         struct tree_desc t[3];
> >>         struct index_state tmp_index = { NULL };
> >>
> >> -       memset(&opt->priv->unpack_opts, 0, sizeof(opt->priv->unpack_opts));
> >> +       unpack_trees_options_init(&opt->priv->unpack_opts);
> >>         if (opt->priv->call_depth)
> >>                 opt->priv->unpack_opts.index_only = 1;
> >>         else
> >> @@ -3704,6 +3704,7 @@ static int merge_start(struct merge_options *opt, struct tree *head)
> >>
> >>         CALLOC_ARRAY(opt->priv, 1);
> >>         string_list_init_dup(&opt->priv->df_conflict_file_set);
> >> +       unpack_trees_options_init(&opt->priv->unpack_opts);
> >
> > Drop this hunk.
>
> Speaking of splitting things up in more understandable ways: If we're
> going to hard rely on the interaction between merge_start() and
> unpack_trees_start() wouldn't it make sense to lead with a change that
> dropped that memset, which if this invariant holds is also redundant to
> the CALLOC() of opt->priv in merge_start() in the pre-image.

It is not redundant, and that would be a very confusing change.
merge-recursive is so named because its main driving function contains
recursive calls to itself.  merge_start() is not involved in that
recursion.  For readability, we should initialize unpack_opts before
each use.