Re: [PATCH v1] rebase -i: stop checking out the tip of the branch to rebase

[Elijah Newren <newren@gmail.com>]

Hi Alban,

// Adding Phillip and Johannes since they know the sequencer internals
very well.

On Tue, Jan 21, 2020 at 11:21 AM Alban Gruin <alban.gruin@gmail.com> wrote:
>
> One of the first things done by the interactive rebase is to make a todo
> list.  This requires knowledge of the commit range to rebase.  To get
> the oid of the last commit of the range, the tip of the branch to rebase
> is checked out with prepare_branch_to_be_rebased(), then the oid of the
> HEAD is read.  On big repositories, it's a performance penalty: the user
> may have to wait before editing the todo list while git is extracting the
> branch silently (because git-checkout is silenced here).  After this,
> the head of the branch is not even modified.
>
> Since we already have the oid of the tip of the branch in
> `opts->orig_head', it's useless to switch to this commit.
>
> This removes the call to prepare_branch_to_be_rebased() in
> do_interactive_rebase(), and adds a `orig_head' parameter to
> get_revision_ranges().  prepare_branch_to_be_rebased() is removed as it
> is no longer used.
>
> This introduces a visible change: as we do not switch on the tip of the
> branch to rebase, no reflog entry is created at the beginning of the
> rebase for it.

Oh, sweet, thanks for digging in.  I had also dug in just after the
report, but not quite far enough as I still had failing tests and I
was feeling a bit stretched thin on other projects so I punted hoping
that SZEDER would post something.  Looks like the orig_head thing was
probably what I was missing.

I was a little surprised that there wasn't any regression test that
needed to be modified, as it reminded me of a previous conversation
about excessive work in the interactive backend[1], but after looking
it up that was apparently about too many calls to commit rather than
too many calls to checkout.

[1] https://lore.kernel.org/git/nycvar.QRO.7.76.6.1811121614190.39@tvgsbejvaqbjf.bet/

> Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
> Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
> ---
>
> Notes:
>     Improvements brought by this patch:
>
>     Before:
>
>     $ time git rebase -m --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50
>
>     real    0m8,940s
>     user    0m6,830s
>     sys     0m2,121s
>
>     After:
>
>     $ time git rebase -m --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50
>
>     real    0m1,834s
>     user    0m0,916s
>     sys     0m0,206s

Nice...do we want to mention this in the commit message proper too?

>
>     Both tests have been performed on a 5400 RPM SATA III hard drive.
>
>  builtin/rebase.c | 18 +++++-------------
>  sequencer.c      | 14 --------------
>  sequencer.h      |  3 ---
>  3 files changed, 5 insertions(+), 30 deletions(-)
>
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 8081741f8a..6154ad8fa5 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -246,21 +246,17 @@ static int edit_todo_file(unsigned flags)
>  }
>
>  static int get_revision_ranges(struct commit *upstream, struct commit *onto,
> -                              const char **head_hash,
> +                              struct object_id *orig_head, const char **head_hash,
>                                char **revisions, char **shortrevisions)
>  {
>         struct commit *base_rev = upstream ? upstream : onto;
>         const char *shorthead;
> -       struct object_id orig_head;
> -
> -       if (get_oid("HEAD", &orig_head))
> -               return error(_("no HEAD?"));
>
> -       *head_hash = find_unique_abbrev(&orig_head, GIT_MAX_HEXSZ);
> +       *head_hash = find_unique_abbrev(orig_head, GIT_MAX_HEXSZ);
>         *revisions = xstrfmt("%s...%s", oid_to_hex(&base_rev->object.oid),
>                                                    *head_hash);
>
> -       shorthead = find_unique_abbrev(&orig_head, DEFAULT_ABBREV);
> +       shorthead = find_unique_abbrev(orig_head, DEFAULT_ABBREV);
>
>         if (upstream) {
>                 const char *shortrev;
> @@ -314,12 +310,8 @@ static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
>         struct replay_opts replay = get_replay_opts(opts);
>         struct string_list commands = STRING_LIST_INIT_DUP;
>
> -       if (prepare_branch_to_be_rebased(the_repository, &replay,
> -                                        opts->switch_to))
> -               return -1;
> -
> -       if (get_revision_ranges(opts->upstream, opts->onto, &head_hash,
> -                               &revisions, &shortrevisions))
> +       if (get_revision_ranges(opts->upstream, opts->onto, &opts->orig_head,
> +                               &head_hash, &revisions, &shortrevisions))
>                 return -1;
>
>         if (init_basic_state(&replay,
> diff --git a/sequencer.c b/sequencer.c
> index b9dbf1adb0..4dc245d7ec 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -3715,20 +3715,6 @@ static int run_git_checkout(struct repository *r, struct replay_opts *opts,
>         return ret;
>  }
>
> -int prepare_branch_to_be_rebased(struct repository *r, struct replay_opts *opts,
> -                                const char *commit)
> -{
> -       const char *action;
> -
> -       if (commit && *commit) {
> -               action = reflog_message(opts, "start", "checkout %s", commit);
> -               if (run_git_checkout(r, opts, commit, action))
> -                       return error(_("could not checkout %s"), commit);
> -       }
> -
> -       return 0;
> -}
> -
>  static int checkout_onto(struct repository *r, struct replay_opts *opts,
>                          const char *onto_name, const struct object_id *onto,
>                          const char *orig_head)
> diff --git a/sequencer.h b/sequencer.h
> index 9f9ae291e3..74f1e2673e 100644
> --- a/sequencer.h
> +++ b/sequencer.h
> @@ -190,9 +190,6 @@ void commit_post_rewrite(struct repository *r,
>                          const struct commit *current_head,
>                          const struct object_id *new_head);
>
> -int prepare_branch_to_be_rebased(struct repository *r, struct replay_opts *opts,
> -                                const char *commit);
> -
>  #define SUMMARY_INITIAL_COMMIT   (1 << 0)
>  #define SUMMARY_SHOW_AUTHOR_DATE (1 << 1)
>  void print_commit_summary(struct repository *repo,
> --
> 2.24.1

The code looks reasonable to me, but I'm still not completely familiar
with all the rebase and sequencer code so I'm hoping Phillip or
Johannes can give a thumbs up.

Thanks for digging into this and figuring out the bits that I missed
when I tried.


Elijah