Subject: Re: [PATCH v5 3/5] rebase: fast-forward --onto in more cases

From: Denton Liu <[email-masked]>
Date: Fri, 19 Apr 2019 10:08:34 -0700

Hi Junio,

On Mon, Apr 15, 2019 at 03:29:24PM -0700, Denton Liu wrote:
> Before, when we had the following graph,
> 
> 	A---B---C (master)
> 	    \
> 	     D (side)
> 
> running 'git rebase --onto master... master side' would result in D
> being always rebased, no matter what. However, the desired behavior is
> that rebase should notice that this is fast-forwardable and do that
> instead.
> 
> Add detection to `can_fast_forward` so that this case can be detected
> and a fast-forward will be performed. First of all, rewrite the function
> to use gotos which simplifies the logic. Next, since the
> 
> 	options.upstream &&
> 	!oidcmp(&options.upstream->object.oid, &options.onto->object.oid)
> 
> conditions were removed in `cmd_rebase`, we reintroduce a substitute in
> `can_fast_forward`. In particular, checking the merge bases of
> `upstream` and `head` fixes a failing case in t3416.
> 
> The abbreviated graph for t3416 is as follows:
> 
> 		    F---G topic
> 		   /
> 	  A---B---C---D---E master
> 
> and the failing command was
> 
> 	git rebase --onto master...topic F topic
> 
> Before, Git would see that there was one merge base (C), and the merge
> and onto were the same so it would incorrectly return 1, indicating that
> we could fast-forward. This would cause the rebased graph to be 'ABCFG'
> when we were expecting 'ABCG'.
> 
> With the additional logic, we detect that upstream and head's merge base
> is F. Since onto isn't F, it means we're not rebasing the full set of
> commits from master..topic. Since we're excluding some commits, a
> fast-forward cannot be performed and so we correctly return 0.
> 
> Add '-f' to test cases that failed as a result of this change because
> they were not expecting a fast-forward so that a rebase is forced.
> 
> While we're at it, remove a trailing whitespace from rebase.c.
> 
> Signed-off-by: Denton Liu <[email-masked]>
> ---
>  builtin/rebase.c               | 40 +++++++++++++++++++++++-----------
>  t/t3400-rebase.sh              |  2 +-
>  t/t3404-rebase-interactive.sh  |  2 +-
>  t/t3432-rebase-fast-forward.sh |  4 ++--
>  4 files changed, 31 insertions(+), 17 deletions(-)
> 
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 77deebc65c..7aa6a090d4 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -895,12 +895,12 @@ static int is_linear_history(struct commit *from, struct commit *to)
>  	return 1;
>  }
>  
> -static int can_fast_forward(struct commit *onto, struct object_id *head_oid,
> -			    struct object_id *merge_base)
> +static int can_fast_forward(struct commit *onto, struct commit *upstream,
> +			    struct object_id *head_oid, struct object_id *merge_base)
>  {
>  	struct commit *head = lookup_commit(the_repository, head_oid);
> -	struct commit_list *merge_bases;
> -	int res;
> +	struct commit_list *merge_bases = NULL;
> +	int res = 0;
>  
>  	if (!head)
>  		return 0;
> @@ -908,12 +908,29 @@ static int can_fast_forward(struct commit *onto, struct object_id *head_oid,
>  	merge_bases = get_merge_bases(onto, head);
>  	if (merge_bases && !merge_bases->next) {
>  		oidcpy(merge_base, &merge_bases->item->object.oid);
> -		res = oideq(merge_base, &onto->object.oid);
> +		if (!oideq(merge_base, &onto->object.oid))
> +			goto done;
>  	} else {
>  		oidcpy(merge_base, &null_oid);
> -		res = 0;
> +		goto done;
>  	}
> +
> +	if (!upstream)
> +		goto done;
> +
>  	free_commit_list(merge_bases);
> +	merge_bases = get_merge_bases(upstream, head);
> +	if (merge_bases && !merge_bases->next) {
> +		if (!oideq(&onto->object.oid, &merge_bases->item->object.oid))
> +			goto done;
> +	} else
> +		goto done;
> +
> +	res = 1;
> +
> +done:
> +	if (merge_bases)
> +		free_commit_list(merge_bases);
>  	return res && is_linear_history(onto, head);
>  }
>  
> @@ -1682,13 +1699,10 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>  
>  	/*
>  	 * Check if we are already based on onto with linear history,
> -	 * but this should be done only when upstream and onto are the same
> -	 * and if this is not an interactive rebase.
> +	 * but this should be done if this is not an interactive rebase.
>  	 */

I forgot to incorporate your comment about this comment block in the
last reroll. I'm not sure if this is worth another reroll so could you
please change the comment block to this:

	/*
	 * Check if we are already based on onto with linear history,
	 * in which case we could fast-forward without replacing the commits
	 * with new commits recreated by replaying their changes. This
	 * optimization must not be done if this is an interactive rebase.
	 */

Thanks,

Denton
> -	if (can_fast_forward(options.onto, &options.orig_head, &merge_base) &&
> -	    !is_interactive(&options) && !options.restrict_revision &&
> -	    options.upstream &&
> -	    !oidcmp(&options.upstream->object.oid, &options.onto->object.oid)) {
> +	if (can_fast_forward(options.onto, options.upstream, &options.orig_head, &merge_base) &&
> +	    !is_interactive(&options) && !options.restrict_revision) {
>  		int flag;
>  
>  		if (!(options.flags & REBASE_FORCE)) {
> @@ -1782,7 +1796,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>  	strbuf_addf(&msg, "%s: checkout %s",
>  		    getenv(GIT_REFLOG_ACTION_ENVIRONMENT), options.onto_name);
>  	if (reset_head(&options.onto->object.oid, "checkout", NULL,
> -		       RESET_HEAD_DETACH | RESET_ORIG_HEAD | 
> +		       RESET_HEAD_DETACH | RESET_ORIG_HEAD |
>  		       RESET_HEAD_RUN_POST_CHECKOUT_HOOK,
>  		       NULL, msg.buf))
>  		die(_("Could not detach HEAD"));
> diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
> index 460d0523be..604d624ff8 100755
> --- a/t/t3400-rebase.sh
> +++ b/t/t3400-rebase.sh
> @@ -295,7 +295,7 @@ test_expect_success 'rebase--am.sh and --show-current-patch' '
>  		echo two >>init.t &&
>  		git commit -a -m two &&
>  		git tag two &&
> -		test_must_fail git rebase --onto init HEAD^ &&
> +		test_must_fail git rebase -f --onto init HEAD^ &&
>  		GIT_TRACE=1 git rebase --show-current-patch >/dev/null 2>stderr &&
>  		grep "show.*$(git rev-parse two)" stderr
>  	)
> diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
> index b60b11f9f2..f054186cc7 100755
> --- a/t/t3404-rebase-interactive.sh
> +++ b/t/t3404-rebase-interactive.sh
> @@ -1066,7 +1066,7 @@ test_expect_success C_LOCALE_OUTPUT 'rebase --edit-todo does not work on non-int
>  	git reset --hard &&
>  	git checkout conflict-branch &&
>  	set_fake_editor &&
> -	test_must_fail git rebase --onto HEAD~2 HEAD~ &&
> +	test_must_fail git rebase -f --onto HEAD~2 HEAD~ &&
>  	test_must_fail git rebase --edit-todo &&
>  	git rebase --abort
>  '
> diff --git a/t/t3432-rebase-fast-forward.sh b/t/t3432-rebase-fast-forward.sh
> index 4f04d67fd7..d0e5b1f3e6 100755
> --- a/t/t3432-rebase-fast-forward.sh
> +++ b/t/t3432-rebase-fast-forward.sh
> @@ -64,9 +64,9 @@ test_expect_success 'add work to upstream' '
>  changes='our and their changes'
>  test_rebase_same_head success --onto B B
>  test_rebase_same_head success --onto B... B
> -test_rebase_same_head failure --onto master... master
> +test_rebase_same_head success --onto master... master
>  test_rebase_same_head failure --fork-point --onto B B
>  test_rebase_same_head failure --fork-point --onto B... B
> -test_rebase_same_head failure --fork-point --onto master... master
> +test_rebase_same_head success --fork-point --onto master... master
>  
>  test_done
> -- 
> 2.21.0.921.gb27c68c4e9
>