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

[Denton Liu <liu.denton@gmail.com>]

On Tue, Apr 16, 2019 at 02:59:12PM +0100, Phillip Wood wrote:
> Hi Denton
> 
> It's good to see rebase fast-forwarding properly when it should
> 
> On 15/04/2019 23:29, 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 <liu.denton@gmail.com>
> > ---
> >   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);
> >   }
> 
> I had a hard time following all those gotos. When you 'goto done' in both
> branches of an if statement it is hard to work out which cases fall through
> to the rest of the code. If I've understood it correctly I think it is
> clearer as
> 
>         merge_bases = get_merge_bases(onto, head);
>         if (merge_bases && !merge_bases->next) {
>                 oidcpy(merge_base, &merge_bases->item->object.oid);
>                 if (oideq(merge_base, &onto->object.oid) && upstream) {
>                         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))
>                                         res = 1;
>                 }
>         } else {
>                 oidcpy(merge_base, &null_oid);
>         }
> 
>         if (merge_bases)
>                 free_commit_list(merge_bases);
>         return res && is_linear_history(onto, head);
> }
> 
> The nested if's aren't great but I think it is easier to follow

I am pretty impartial between gotos and ifs. If no one else has any
strong opinions between the two, I'll reroll with ifs.

> 
> > @@ -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.
> >   	 */
> > -	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) &&
> 
> This is rather long, perhaps break the argument list

Thanks, will do.

> 
> Best Wishes
> 
> Phillip
> > +	    !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
> >