Re: [PATCH v6 4/6] rebase: fast-forward --onto in more cases

> 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.
> 
> Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
> Signed-off-by: Denton Liu <liu.denton@gmail.com>
> ---
>   builtin/rebase.c               | 28 +++++++++++++++++++---------
>   t/t3400-rebase.sh              |  2 +-
>   t/t3404-rebase-interactive.sh  |  2 +-
>   t/t3432-rebase-fast-forward.sh |  4 ++--
>   4 files changed, 23 insertions(+), 13 deletions(-)
> 
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index de10b6f5ad..f5aca1eee0 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -895,8 +895,8 @@ 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 = NULL;
> @@ -915,6 +915,18 @@ static int can_fast_forward(struct commit *onto, struct object_id *head_oid,
>   	if (!oideq(merge_base, &onto->object.oid))
>   		goto done;
>   
> +	if (!upstream)
> +		goto done;
> +
> +	free_commit_list(merge_bases);
> +	merge_bases = get_merge_bases(upstream, head);
> +	if (!merge_bases || merge_bases->next) {
> +		goto done;
> +	}

> +
> +	if (!oideq(&onto->object.oid, &merge_bases->item->object.oid))
> +		goto done;
> +
>   	res = 1;
>   
>   done:
> @@ -1688,13 +1700,11 @@ 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.
>   	 */
> -	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)) {
> @@ -1788,7 +1798,7 @@ 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
>