Re: [RFC WIP PATCH v8 11/13] rebase: eliminate side-effects from can_fast_forward()

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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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
>
>