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From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Ævar Arnfjörð Bjarmason  <avarab@gmail.com>
Cc: git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>,
	Denton Liu <liu.denton@gmail.com>,
	Eric Sunshine <sunshine@sunshineco.com>,
	Johannes Sixt <j6t@kdbg.org>, SZEDER Gábor <szeder.dev@gmail.com>
Subject: Re: [RFC WIP PATCH v8 11/13] rebase: eliminate side-effects from can_fast_forward()
Date: Fri, 17 May 2019 23:16:10 +0200 (DST)
Message-ID: <nycvar.QRO.7.76.6.1905172301210.46@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <20190508001252.15752-12-avarab@gmail.com>

[-- Attachment #1: Type: text/plain, Size: 4622 bytes --]

Hi Ævar,

On Wed, 8 May 2019, Ævar Arnfjörð Bjarmason wrote:

> The can_fast_forward() function is potentially much more expensive
> than is_interactive() since it e.g. might need to call
> is_linear_history().
>
> So reversing the two looks like an obvious improvement, but doing so
> reveals a previously hidden caveat: We need the can_fast_forward()
> function to populate data used later, namely the "merge_bases"
> variable. This has been the case since it was added in
> 9a48a615b4 ("builtin rebase: try to fast forward when possible",
> 2018-09-04).
>
> So let's refactor it into two functions. Now we'll always call
> populate_merge_bases(), and then only call can_fast_forward() if
> is_interactive() is false, making this both faster in pathological
> cases, and more importantly easier to follow.

True.

We might want to mention, though, what exactly that this "pathological
case" is: rebasing commits onto an unrelated history.

> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index ae6b9b42b8..cb5d7fcb53 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -878,24 +878,30 @@ static int is_linear_history(struct commit *from, struct commit *to)
>  	return 1;
>  }
>
> -static int can_fast_forward(struct commit *onto, struct commit *upstream,
> +static void populate_merge_bases(struct commit *head, struct commit *onto,
> +				 struct commit_list *merge_bases,
> +				 struct object_id *merge_base)
> +{
> +	merge_bases = get_merge_bases(onto, head);

Hmm. This overrides whatever was passed in via the parameter. Did you mean
to make the parameter of type `struct commit_list **`, i.e. a *pointer to
a pointer*?

> +	if (!merge_bases || merge_bases->next) {
> +		oidcpy(merge_base, &null_oid);
> +		return;
> +	}
> +	oidcpy(merge_base, &merge_bases->item->object.oid);
> +}
> +
> +static int can_fast_forward(struct commit *head,
> +			    struct commit *onto, struct commit *upstream,
>  			    struct commit *restrict_revision,
> -			    struct object_id *head_oid, struct object_id *merge_base)
> +			    struct object_id *head_oid,
> +			    struct commit_list *merge_bases,
> +			    struct object_id *merge_base)
>  {
> -	struct commit *head = lookup_commit(the_repository, head_oid);
> -	struct commit_list *merge_bases = NULL;
>  	int res = 0;
>
>  	if (!head)
>  		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);

Do we even use `merge_bases` after this anymore? I guess not. So we should
get rid of this function parameter at this point.

>  	if (!oideq(merge_base, &onto->object.oid))

Uh oh. With this patch, `merge_base` can be `NULL` at this point, namely
when trying to rebase commits onto an unrelated history.

>  		goto done;
>
> @@ -1154,6 +1160,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>  		OPT_END(),
>  	};
>  	int i;
> +	struct commit *head_commit;
> +	struct commit_list *merge_bases = NULL;
>
>  	if (argc == 2 && !strcmp(argv[1], "-h"))
>  		usage_with_options(builtin_rebase_usage,
> @@ -1703,9 +1711,14 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>  	 * with new commits recreated by replaying their changes. This
>  	 * optimization must not be done if this is an interactive rebase.
>  	 */
> -	if (can_fast_forward(options.onto, options.upstream, options.restrict_revision,
> -		    &options.orig_head, &merge_base) &&
> -	    !is_interactive(&options)) {
> +	head_commit = lookup_commit(the_repository, &options.orig_head);

Hmm. I do not see this line (or any equivalent code) removed in this
patch. Is it possible that we do not need to (re-?)initialize
`head_commit` at this point?

I guess I should read the entire patch series because that `head_commit`
isn't there in the worktree I am looking at (oh, if only I had an easy way
to review code, but I have to review static patches instead, not even in a
proper IDE... ;-)). Will try to find time to do that next week.

Ciao,
Dscho

> +	if (head_commit)
> +		populate_merge_bases(head_commit, options.onto, merge_bases,
> +				     &merge_base);
> +	if (!is_interactive(&options) &&
> +	    can_fast_forward(head_commit, options.onto, options.upstream,
> +			     options.restrict_revision, &options.orig_head,
> +			     merge_bases, &merge_base)) {
>  		int flag;
>
>  		if (!(options.flags & REBASE_FORCE)) {
> --
> 2.21.0.1020.gf2820cf01a
>
>

  reply index

Thread overview: 98+ 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
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
2019-05-08  0:12           ` [RFC WIP PATCH v8 00/13] learn --keep-base & more Ævar Arnfjörð Bjarmason
2019-05-08  3:57             ` Junio C Hamano
2019-07-19 19:14               ` Junio C Hamano
2019-07-19 21:01                 ` Denton Liu
2019-05-08  0:12           ` [RFC WIP PATCH v8 01/13] t3431: add rebase --fork-point tests Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 02/13] t3432: test rebase fast-forward behavior Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 03/13] t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 04/13] t3432: test for --no-ff's interaction with fast-forward Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 05/13] rebase: refactor can_fast_forward into goto tower Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 06/13] rebase: fast-forward --onto in more cases Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 07/13] rebase: fast-forward --fork-point " Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 08/13] rebase: teach rebase --keep-base Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 09/13] rebase tests: test linear branch topology Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 10/13] rebase: don't rebase linear topology with --fork-point Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 11/13] rebase: eliminate side-effects from can_fast_forward() Ævar Arnfjörð Bjarmason
2019-05-17 21:16             ` Johannes Schindelin [this message]
2019-05-08  0:12           ` [RFC WIP PATCH v8 12/13] rebase: add a should_fast_forward() utility function Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 13/13] WIP: can_fast_forward() support for --preserve-merges and --rebase-merges Ævar Arnfjörð Bjarmason
2019-05-17 22:02             ` Johannes Schindelin
2019-04-06 19:44     ` [PATCH v3 0/4] rebase: teach rebase --keep-base Ævar Arnfjörð Bjarmason
2019-04-06 20:38       ` Denton Liu
2019-04-13 21:10         ` Ævar Arnfjörð Bjarmason

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