Subject: Re: [PATCH] rebase --merge: optionally skip upstreamed commits

From: Taylor Blau <[email-masked]>
Date: Mon, 9 Mar 2020 20:10:15 -0600

Hi Jonathan,

This patch makes good sense to me. I left a few notes below, but they
are relatively minor, and this seems to be all in a good direction.

As a (somewhat) interesting aside, this feature would be useful to me
outside of partial clones, since I often have this workflow in my local
development wherein 'git rebase' spends quite a bit of time comparing
patches on my branch to everything new upstream.

On Mon, Mar 09, 2020 at 01:55:23PM -0700, Jonathan Tan wrote:
> When rebasing against an upstream that has had many commits since the
> original branch was created:
>
>  O -- O -- ... -- O -- O (upstream)
>   \
>    -- O (my-dev-branch)
>
> because "git rebase" attempts to exclude commits that are duplicates of
> upstream ones, it must read the contents of every novel upstream commit,
> in addition to the tip of the upstream and the merge base.

This sentence is a little confusing if you skip over the graph, since it
reads: "When rebasing against an because ... because ...". It may be
clearer if you swap the order of the last two clauses to instead be:

  it must read the contents of every novel upstream commit, in addition to
  the tip of the upstream and the merge base, because "git rebase"
  attempts to exclude commits that are duplicates of upstream ones.

> This can be a significant performance hit, especially in a partial
> clone, wherein a read of an object may end up being a fetch.
>
> Add a flag to "git rebase" to allow suppression of this feature. This
> flag only works when using the "merge" backend.
>
> This flag changes the behavior of sequencer_make_script(), called from
> do_interactive_rebase() <- run_rebase_interactive() <-
> run_specific_rebase() <- cmd_rebase(). With this flag, limit_list()
> (indirectly called from sequencer_make_script() through
> prepare_revision_walk()) will no longer call cherry_pick_list(), and
> thus PATCHSAME is no longer set. Refraining from setting PATCHSAME both
> means that the intermediate commits in upstream are no longer read (as
> shown by the test) and means that no PATCHSAME-caused skipping of
> commits is done by sequencer_make_script(), either directly or through
> make_script_with_merges().

This all sounds good to me.

> Signed-off-by: Jonathan Tan <[email-masked]>
> ---
> More improvements for partial clone, but this is a benefit for
> non-partial-clone as well, hence the way I wrote the commit message (not
> focusing too much on partial clone) and the documentation.
>
> I've chosen --skip-already-present and --no-skip-already-present to
> reuse the language already existing in the documentation and to avoid a
> double negative (e.g. --avoid-checking-if-already-present and
> --no-avoid-checking-if-already-present) but this causes some clumsiness
> in the documentation and in the code. Any suggestions for the name are
> welcome.
>
> I've only implemented this for the "merge" backend since I think that
> there is an effort to migrate "rebase" to use the "merge" backend by
> default, and also because "merge" uses diff internally which already has
> the (per-commit) blob batch prefetching.

This also makes sense to me.

> ---
>  Documentation/git-rebase.txt | 12 +++++-
>  builtin/rebase.c             | 10 ++++-
>  sequencer.c                  |  3 +-
>  sequencer.h                  |  2 +-
>  t/t3402-rebase-merge.sh      | 77 ++++++++++++++++++++++++++++++++++++
>  5 files changed, 100 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
> index 0c4f038dd6..f73a82b4a9 100644
> --- a/Documentation/git-rebase.txt
> +++ b/Documentation/git-rebase.txt
> @@ -318,6 +318,15 @@ See also INCOMPATIBLE OPTIONS below.
>  +
>  See also INCOMPATIBLE OPTIONS below.
>
> +--skip-already-present::
> +--no-skip-already-present::
> +	Skip commits that are already present in the new upstream.
> +	This is the default.

I believe that you mean '--skip-already-present' is the default, here,
but the placement makes it ambiguous, since it is in a paragraph with a
header that contains both the positive and negated version of this flag.

Maybe this could changed to: s/This/--skip-already-present/'.

> ++
> +If the skip-if-already-present feature is unnecessary or undesired,
> +`--no-skip-already-present` may improve performance since it avoids
> +the need to read the contents of every commit in the new upstream.
> +
>  --rerere-autoupdate::
>  --no-rerere-autoupdate::
>  	Allow the rerere mechanism to update the index with the
> @@ -866,7 +875,8 @@ Only works if the changes (patch IDs based on the diff contents) on
>  'subsystem' did.
>
>  In that case, the fix is easy because 'git rebase' knows to skip
> -changes that are already present in the new upstream.  So if you say
> +changes that are already present in the new upstream (unless
> +`--no-skip-already-present` is given). So if you say

Extremely minor nit: there is a whitespace change on this line where the
original has two spaces between the '.' and 'So', and the new version
has only one.

>  (assuming you're on 'topic')
>  ------------
>      $ git rebase subsystem
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 6154ad8fa5..943211e5bb 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -88,13 +88,15 @@ struct rebase_options {
>  	struct strbuf git_format_patch_opt;
>  	int reschedule_failed_exec;
>  	int use_legacy_rebase;
> +	int skip_already_present;
>  };
>
>  #define REBASE_OPTIONS_INIT {			  	\
>  		.type = REBASE_UNSPECIFIED,	  	\
>  		.flags = REBASE_NO_QUIET, 		\
>  		.git_am_opts = ARGV_ARRAY_INIT,		\
> -		.git_format_patch_opt = STRBUF_INIT	\
> +		.git_format_patch_opt = STRBUF_INIT,	\
> +		.skip_already_present =	1		\
>  	}
>
>  static struct replay_opts get_replay_opts(const struct rebase_options *opts)
> @@ -373,6 +375,7 @@ static int run_rebase_interactive(struct rebase_options *opts,
>  	flags |= opts->rebase_cousins > 0 ? TODO_LIST_REBASE_COUSINS : 0;
>  	flags |= opts->root_with_onto ? TODO_LIST_ROOT_WITH_ONTO : 0;
>  	flags |= command == ACTION_SHORTEN_OIDS ? TODO_LIST_SHORTEN_IDS : 0;
> +	flags |= opts->skip_already_present ? TODO_LIST_SKIP_ALREADY_PRESENT : 0;
>
>  	switch (command) {
>  	case ACTION_NONE: {
> @@ -1507,6 +1510,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>  		OPT_BOOL(0, "reschedule-failed-exec",
>  			 &reschedule_failed_exec,
>  			 N_("automatically re-schedule any `exec` that fails")),
> +		OPT_BOOL(0, "skip-already-present", &options.skip_already_present,
> +			 N_("skip changes that are already present in the new upstream")),

I scratched my head a little bit about why we weren't using OPT_BIT and
&flags directly here, but it matches the pattern in the surrounding, so
I think that 'OPT_BOOL' and target '&options.skip_already_present' here.

>  		OPT_END(),
>  	};
>  	int i;
> @@ -1840,6 +1845,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>  			      "interactive or merge options"));
>  	}
>
> +	if (!options.skip_already_present && !is_interactive(&options))
> +		die(_("--no-skip-already-present does not work with the 'am' backend"));
> +
>  	if (options.signoff) {
>  		if (options.type == REBASE_PRESERVE_MERGES)
>  			die("cannot combine '--signoff' with "
> diff --git a/sequencer.c b/sequencer.c
> index ba90a513b9..752580c017 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -4797,12 +4797,13 @@ int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
>  	int keep_empty = flags & TODO_LIST_KEEP_EMPTY;
>  	const char *insn = flags & TODO_LIST_ABBREVIATE_CMDS ? "p" : "pick";
>  	int rebase_merges = flags & TODO_LIST_REBASE_MERGES;
> +	int skip_already_present = !!(flags & TODO_LIST_SKIP_ALREADY_PRESENT);
>
>  	repo_init_revisions(r, &revs, NULL);
>  	revs.verbose_header = 1;
>  	if (!rebase_merges)
>  		revs.max_parents = 1;
> -	revs.cherry_mark = 1;
> +	revs.cherry_mark = skip_already_present;

:-). All of that plumbing just to poke at this variable. Looks good to
me.

>  	revs.limited = 1;
>  	revs.reverse = 1;
>  	revs.right_only = 1;
> diff --git a/sequencer.h b/sequencer.h
> index 393571e89a..39bb12f624 100644
> --- a/sequencer.h
> +++ b/sequencer.h
> @@ -149,7 +149,7 @@ int sequencer_remove_state(struct replay_opts *opts);
>   * `--onto`, we do not want to re-generate the root commits.
>   */
>  #define TODO_LIST_ROOT_WITH_ONTO (1U << 6)
> -
> +#define TODO_LIST_SKIP_ALREADY_PRESENT (1U << 7)

This was another spot that I thought could maybe be turned into an enum,
but it's clearly not the fault of your patch, and could easily be turned
into #leftoverbits.

>  int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
>  			  const char **argv, unsigned flags);
> diff --git a/t/t3402-rebase-merge.sh b/t/t3402-rebase-merge.sh
> index a1ec501a87..9b52739a10 100755
> --- a/t/t3402-rebase-merge.sh
> +++ b/t/t3402-rebase-merge.sh
> @@ -162,4 +162,81 @@ test_expect_success 'rebase --skip works with two conflicts in a row' '
>  	git rebase --skip
>  '
>
> +test_expect_success '--no-skip-already-present' '
> +	git init repo &&
> +
> +	# O(1-10) -- O(1-11) -- O(0-10) master
> +	#        \
> +	#         -- O(1-11) -- O(1-12) otherbranch
> +
> +	printf "Line %d\n" $(test_seq 1 10) >repo/file.txt &&
> +	git -C repo add file.txt &&
> +	git -C repo commit -m "base commit" &&
> +
> +	printf "Line %d\n" $(test_seq 1 11) >repo/file.txt &&
> +	git -C repo commit -a -m "add 11" &&
> +
> +	printf "Line %d\n" $(test_seq 0 10) >repo/file.txt &&
> +	git -C repo commit -a -m "add 0 delete 11" &&
> +
> +	git -C repo checkout -b otherbranch HEAD^^ &&
> +	printf "Line %d\n" $(test_seq 1 11) >repo/file.txt &&
> +	git -C repo commit -a -m "add 11 in another branch" &&
> +
> +	printf "Line %d\n" $(test_seq 1 12) >repo/file.txt &&
> +	git -C repo commit -a -m "add 12 in another branch" &&
> +
> +	# Regular rebase fails, because the 1-11 commit is deduplicated
> +	test_must_fail git -C repo rebase --merge master 2> err &&
> +	test_i18ngrep "error: could not apply.*add 12 in another branch" err &&
> +	git -C repo rebase --abort &&
> +
> +	# With --no-skip-already-present, it works
> +	git -C repo rebase --merge --no-skip-already-present master
> +'
> +
> +test_expect_success '--no-skip-already-present refrains from reading unneeded blobs' '
> +	git init server &&
> +
> +	# O(1-10) -- O(1-11) -- O(1-12) master
> +	#        \
> +	#         -- O(0-10) otherbranch
> +
> +	printf "Line %d\n" $(test_seq 1 10) >server/file.txt &&
> +	git -C server add file.txt &&
> +	git -C server commit -m "merge base" &&
> +
> +	printf "Line %d\n" $(test_seq 1 11) >server/file.txt &&
> +	git -C server commit -a -m "add 11" &&
> +
> +	printf "Line %d\n" $(test_seq 1 12) >server/file.txt &&
> +	git -C server commit -a -m "add 12" &&
> +
> +	git -C server checkout -b otherbranch HEAD^^ &&
> +	printf "Line %d\n" $(test_seq 0 10) >server/file.txt &&
> +	git -C server commit -a -m "add 0" &&
> +
> +	test_config -C server uploadpack.allowfilter 1 &&
> +	test_config -C server uploadpack.allowanysha1inwant 1 &&
> +
> +	git clone --filter=blob:none "file://$(pwd)/server" client &&
> +	git -C client checkout origin/master &&
> +	git -C client checkout origin/otherbranch &&
> +
> +	# Sanity check to ensure that the blobs from the merge base and "add
> +	# 11" are missing
> +	git -C client rev-list --objects --all --missing=print >missing_list &&
> +	MERGE_BASE_BLOB=$(git -C server rev-parse master^^:file.txt) &&
> +	ADD_11_BLOB=$(git -C server rev-parse master^:file.txt) &&
> +	grep "\\?$MERGE_BASE_BLOB" missing_list &&
> +	grep "\\?$ADD_11_BLOB" missing_list &&
> +
> +	git -C client rebase --merge --no-skip-already-present origin/master &&
> +
> +	# The blob from the merge base had to be fetched, but not "add 11"
> +	git -C client rev-list --objects --all --missing=print >missing_list &&
> +	! grep "\\?$MERGE_BASE_BLOB" missing_list &&
> +	grep "\\?$ADD_11_BLOB" missing_list
> +'
> +
>  test_done
> --
> 2.25.1.481.gfbce0eb801-goog

The tests look good to me. Thanks for working on this!

Thanks,
Taylor