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From: Phillip Wood <phillip.wood123@gmail.com>
To: Denton Liu <liu.denton@gmail.com>, phillip.wood@dunelm.org.uk
Cc: Git Mailing List <git@vger.kernel.org>,
	Eric Sunshine <sunshine@sunshineco.com>,
	Junio C Hamano <gitster@pobox.com>,
	Ævar Arnfjörð Bjarmason <avarab@gmail.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Johannes Sixt <j6t@kdbg.org>, SZEDER Gábor <szeder.dev@gmail.com>
Subject: Re: [PATCH v5 3/5] rebase: fast-forward --onto in more cases
Date: Wed, 17 Apr 2019 15:14:15 +0100
Message-ID: <a6e2f500-7106-b828-d4a6-132aceabcc5e@gmail.com> (raw)
In-Reply-To: <20190417064420.GA12818@archbookpro.localdomain>

On 17/04/2019 07:44, Denton Liu wrote:
> 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;

that would be better as
	res = oideq(&onto->object.oid, &merge_bases->item->object.oid);
without the last if

>>                  }
>>          } 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.

If you want to keep the goto approach then refactoring the ifs as 
follows is clearer as it avoids jumping from both arms, each stage is a 
simple single armed if (something) goto done

	merge_bases = get_merge_bases(onto, head);
	if (!merge_bases || merge_bases->next) {
		oidcpy(merge_base, &null_oid);
		goto done;
	}

	oidcpy(merge_base, &merge_bases->item->object.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)
		res = oideq(&onto->object.oid, &merge_bases->item->object.oid);

done:
	if (merge_bases)
		free_commit_list(merge_bases);
	return res && is_linear_history(onto, head);
}

Best Wishes

Phillip
>>
>>> @@ -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
>>>

  reply index

Thread overview: 79+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-23 15:25 [PATCH 0/3] rebase: learn --keep-base Denton Liu
2019-03-23 15:25 ` [PATCH 1/3] rebase: teach rebase --keep-base Denton Liu
2019-03-24  3:46   ` Eric Sunshine
2019-03-24 13:20   ` Junio C Hamano
2019-03-25  0:06     ` Denton Liu
2019-03-25  5:41       ` Denton Liu
2019-04-01 10:45         ` Junio C Hamano
2019-03-24 13:37   ` Junio C Hamano
2019-03-25  5:47   ` Denton Liu
2019-03-25 18:50   ` Johannes Schindelin
2019-03-25 19:29     ` Denton Liu
2019-03-26 13:27       ` Johannes Schindelin
2019-03-23 15:25 ` [PATCH 2/3] t3416: test " Denton Liu
2019-03-23 15:25 ` [PATCH 3/3] git-rebase.txt: document --keep-base option Denton Liu
2019-03-24 13:15 ` [PATCH 0/3] rebase: learn --keep-base Junio C Hamano
2019-03-25  0:04   ` Denton Liu
2019-04-01 10:45     ` Junio C Hamano
2019-03-26 14:35 ` Ævar Arnfjörð Bjarmason
2019-03-26 17:50   ` Denton Liu
2019-03-26 20:35     ` Ævar Arnfjörð Bjarmason
2019-03-26 21:30       ` Denton Liu
2019-03-27 15:39         ` Ævar Arnfjörð Bjarmason
2019-03-28 22:17 ` [PATCH v2] rebase: teach rebase --keep-base Denton Liu
2019-03-28 23:13   ` Ævar Arnfjörð Bjarmason
2019-03-29 15:47   ` Johannes Schindelin
2019-03-29 17:52     ` Denton Liu
2019-04-01 10:46     ` Junio C Hamano
2019-04-01 20:51   ` [PATCH v3 0/4] " Denton Liu
2019-04-01 20:51     ` [PATCH v3 1/4] t3431: add rebase --fork-point tests Denton Liu
2019-04-04 20:28       ` Denton Liu
2019-04-05 11:15       ` SZEDER Gábor
2019-04-08  4:38         ` Junio C Hamano
2019-04-05 14:55       ` Johannes Schindelin
2019-04-05 17:25         ` Denton Liu
2019-04-05 17:51           ` Johannes Sixt
2019-04-05 18:51             ` Johannes Schindelin
2019-04-05 20:19               ` Johannes Schindelin
2019-04-05 21:10                 ` SZEDER Gábor
2019-04-01 20:51     ` [PATCH v3 2/4] t3432: test rebase fast-forward behavior Denton Liu
2019-04-01 20:52     ` [PATCH v3 3/4] rebase: fast-forward --onto in more cases Denton Liu
2019-04-02  1:25       ` Junio C Hamano
2019-04-02  1:48         ` Junio C Hamano
2019-04-02  4:44           ` Denton Liu
2019-04-01 20:52     ` [PATCH v3 4/4] rebase: teach rebase --keep-base Denton Liu
2019-04-05 21:39     ` [PATCH v4 0/4] " Denton Liu
2019-04-05 21:40       ` [PATCH v4 1/4] t3431: add rebase --fork-point tests Denton Liu
2019-04-05 21:40       ` [PATCH v4 2/4] t3432: test rebase fast-forward behavior Denton Liu
2019-04-05 21:40       ` [PATCH v4 3/4] rebase: fast-forward --onto in more cases Denton Liu
2019-04-05 21:40       ` [PATCH v4 4/4] rebase: teach rebase --keep-base Denton Liu
2019-04-15 22:29       ` [PATCH v5 0/5] " Denton Liu
2019-04-15 22:29         ` [PATCH v5 1/5] t3431: add rebase --fork-point tests Denton Liu
2019-04-15 22:29         ` [PATCH v5 2/5] t3432: test rebase fast-forward behavior Denton Liu
2019-04-15 22:29         ` [PATCH v5 3/5] rebase: fast-forward --onto in more cases Denton Liu
2019-04-16  6:26           ` Junio C Hamano
2019-04-16 13:59           ` Phillip Wood
2019-04-17  6:44             ` Denton Liu
2019-04-17 14:14               ` Phillip Wood [this message]
2019-04-19 17:08           ` Denton Liu
2019-04-15 22:29         ` [PATCH v5 4/5] rebase: fast-forward --fork-point " Denton Liu
2019-04-15 22:29         ` [PATCH v5 5/5] rebase: teach rebase --keep-base Denton Liu
2019-04-17 18:01       ` [PATCH v6 0/5] " Denton Liu
2019-04-17 18:01         ` [PATCH v6 1/6] t3431: add rebase --fork-point tests Denton Liu
2019-04-17 18:01         ` [PATCH v6 2/6] t3432: test rebase fast-forward behavior Denton Liu
2019-04-17 18:01         ` [PATCH v6 3/6] rebase: refactor can_fast_forward into goto tower Denton Liu
2019-04-17 18:01         ` [PATCH v6 4/6] rebase: fast-forward --onto in more cases Denton Liu
2019-04-17 19:59           ` Phillip Wood
2019-04-17 18:01         ` [PATCH v6 5/6] rebase: fast-forward --fork-point " Denton Liu
2019-04-17 18:01         ` [PATCH v6 6/6] rebase: teach rebase --keep-base Denton Liu
2019-04-21  8:11         ` [PATCH v7 0/6] rebase: learn --keep-base Denton Liu
2019-04-21  8:11           ` [PATCH v7 1/6] t3431: add rebase --fork-point tests Denton Liu
2019-04-23 23:12             ` Denton Liu
2019-04-21  8:11           ` [PATCH v7 2/6] t3432: test rebase fast-forward behavior Denton Liu
2019-04-21  8:11           ` [PATCH v7 3/6] rebase: refactor can_fast_forward into goto tower Denton Liu
2019-04-21  8:11           ` [PATCH v7 4/6] rebase: fast-forward --onto in more cases Denton Liu
2019-04-21  8:11           ` [PATCH v7 5/6] rebase: fast-forward --fork-point " Denton Liu
2019-04-21  8:11           ` [PATCH v7 6/6] rebase: teach rebase --keep-base Denton Liu
2019-04-06 19:44     ` [PATCH v3 0/4] " Ævar Arnfjörð Bjarmason
2019-04-06 20:38       ` Denton Liu
2019-04-13 21:10         ` Ævar Arnfjörð Bjarmason

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