From: Johannes Schindelin <Johannes.Schindelin@gmx.de> To: Alban Gruin <alban.gruin@gmail.com> Cc: "SZEDER Gábor" <szeder.dev@gmail.com>, git@vger.kernel.org, "Elijah Newren" <newren@gmail.com>, "Ævar Arnfjörð" <avarab@gmail.com>, "Eugeniu Rosca" <erosca@de.adit-jv.com>, "Junio C Hamano" <gitster@pobox.com>, "Jeff King" <peff@peff.net>, "Eugeniu Rosca" <roscaeugeniu@gmail.com>, "Phillip Wood" <phillip.wood@dunelm.org.uk> Subject: Re: [PATCH v3] rebase -i: stop checking out the tip of the branch to rebase Date: Wed, 5 Feb 2020 15:31:08 +0100 (CET) [thread overview] Message-ID: <nycvar.QRO.7.76.6.2002051531000.3718@tvgsbejvaqbjf.bet> (raw) In-Reply-To: <20200124150500.15260-1-alban.gruin@gmail.com> [-- Attachment #1: Type: text/plain, Size: 8130 bytes --] Hi Alban, On Fri, 24 Jan 2020, Alban Gruin wrote: > One of the first things done when using a sequencer-based > rebase (ie. `rebase -i', `rebase -r', or `rebase -m') 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. After this, the tip of the branch is not even modified. > The `am' backend, on the other hand, does not check out the branch. > > On big repositories, it's a performance penalty: with `rebase -i', the > user may have to wait before editing the todo list while git is > extracting the branch silently, and "quiet" rebases will be slower than > `am'. > > 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. At this point, I am a bit puzzled as a reader: why can we just drop this? My immediate reaction was: isn't this required to switch to a new branch when `switch_to` is non-`NULL`? So I went digging a little. The `prepare_branch_to_be_rebased()` call was introduced in 53bbcfbde7c (rebase -i: implement the main part of interactive rebase as a builtin, 2018-09-27). And looking at the `git-rebase--interactive` part of that patch, it becomes relatively obvious that we inherited this behavior from the shell scripting days. 2c58483a598 (rebase -i: rewrite setup_reflog_action() in C, 2018-08-10) converted the `setup_reflog_action` function (which oddly enough not only set up the reflog action, but also switched to a new branch if so configured). That function was introduced in d48f97aa854 (rebase: reindent function git_rebase__interactive, 2018-03-23), but that was _still_ not the commit that introduced that "let's check out the upstream" behavior. It goes _all_ the way back to 1b1dce4bae7 (Teach rebase an interactive mode, 2007-06-25). Except that back then, it was only done when a branch name was provided (`git rebase -i <upstream> <branch-to-switch-to>`). So it behaved correctly. The problem was introduced in 71786f54c41 (rebase: factor out reference parsing, 2011-02-06), as it substituted the `if test ! -z "$1"` with `if test ! -z "$switch_to"`, relying on the command-line parsing of `git-rebase.sh`. But wait! Wait, wait, wait! `switch_to` is still only set in that incantation where we provide a branch name _in addition to_ an upstream commit. Ah, I think I slowly see where this is going. The problem is actually 2ec33cdd19b (rebase--interactive: don't require what's rebased to be a branch, 2010-03-14) which failed to realize that essentially the entire `git checkout` was necessary to accommodate the subsequent call a mere 8 lines further down from that `checkout`: git symbolic-ref HEAD > "$DOTEST"/head-name 2> /dev/null || echo "detached HEAD" > "$DOTEST"/head-name So that 2ec33cdd19b commit could have saved itself a lot of trouble by realizing what the role of that `git checkout` is, and should have pulled that `head-name` logic into that conditional instead of _actually_ switching to a new branch. Now, let's see what the C code does to determine "head-name". Indeed, it is already handled in the option parsing, in that monster of a function called `cmd_rebase()`. And yes, I think that `head_name` should also be mentioned in this commit message, as something like git rebase -i <base> <branch-to-switch-to> should eventually indeed switch to the specified branch, and this here patch does _not_ break that promise. This is my only concern with this patch, though, therefore: Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de> Thanks, Dscho > 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. > > Unscientific performance measurements, performed on linux.git, are as > follow: > > Before this patch: > > $ time git rebase -m --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50 > > real 0m8,940s > user 0m6,830s > sys 0m2,121s > > After this patch: > > $ time git rebase -m --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50 > > real 0m1,834s > user 0m0,916s > sys 0m0,206s > > Reported-by: SZEDER Gábor <szeder.dev@gmail.com> > Signed-off-by: Alban Gruin <alban.gruin@gmail.com> > --- > > Added a line in the first paragraph to make it clear that the `am' > backend is not affected. > > 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 > >
next prev parent reply other threads:[~2020-02-05 14:31 UTC|newest] Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-01-08 21:43 Unreliable 'git rebase --onto' Eugeniu Rosca 2020-01-08 22:35 ` SZEDER Gábor 2020-01-09 0:55 ` Elijah Newren 2020-01-09 15:03 ` SZEDER Gábor 2020-01-09 17:53 ` Elijah Newren 2020-01-21 19:18 ` [PATCH v1] rebase -i: stop checking out the tip of the branch to rebase Alban Gruin 2020-01-21 20:07 ` Elijah Newren 2020-01-22 20:24 ` Junio C Hamano 2020-01-22 20:47 ` Junio C Hamano 2020-01-24 14:45 ` Alban Gruin 2020-01-24 14:45 ` [PATCH v2] " Alban Gruin 2020-01-24 14:55 ` Alban Gruin 2020-01-24 18:12 ` Junio C Hamano 2020-01-24 15:05 ` [PATCH v3] " Alban Gruin 2020-01-24 18:30 ` Junio C Hamano 2020-02-05 14:31 ` Johannes Schindelin [this message] 2020-01-24 17:11 ` [PATCH v2] " Andrei Rybak 2020-01-09 11:13 ` Unreliable 'git rebase --onto' Eugeniu Rosca [not found] ` <CABPp-BHsyMOz+hi7EYoAnAWfzms7FRfwqCoarnu8H+vyDoN6SQ@mail.gmail.com> 2020-01-09 10:53 ` Eugeniu Rosca 2020-01-09 18:05 ` Elijah Newren 2020-01-10 0:06 ` Eugeniu Rosca 2020-01-10 2:35 ` Elijah Newren
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