Re: [PATCH v9 5/9] rebase: refactor can_fast_forward into goto tower

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

On Sun, Aug 25, 2019 at 06:47:02PM +0530, Pratyush Yadav wrote:
> On 25/08/19 05:12AM, Denton Liu wrote:
> > Before, can_fast_forward was written with an if-else statement. However,
> > in the future, we may be adding more termination cases which would lead
> > to deeply nested if statements.
> > 
> > Refactor to use a goto tower so that future cases can be easily
> > inserted.
> > 
> > Signed-off-by: Denton Liu <liu.denton@gmail.com>
> > ---
> >  builtin/rebase.c | 24 +++++++++++++++---------
> >  1 file changed, 15 insertions(+), 9 deletions(-)
> > 
> > diff --git a/builtin/rebase.c b/builtin/rebase.c
> > index 670096c065..22c4f1ff93 100644
> > --- a/builtin/rebase.c
> > +++ b/builtin/rebase.c
> > @@ -1264,21 +1264,27 @@ static int can_fast_forward(struct commit *onto, 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;
> > +		goto done;
> >  
> >  	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);
> > -	} else {
> > +	if (!merge_bases || merge_bases->next) {
> >  		oidcpy(merge_base, &null_oid);
> > -		res = 0;
> > +		goto done;
> >  	}
> > -	free_commit_list(merge_bases);
> > +
> > +	oidcpy(merge_base, &merge_bases->item->object.oid);
> > +	if (!oideq(merge_base, &onto->object.oid))
> > +		goto done;
> > +
> > +	res = 1;
> > +
> > +done:
> > +	if (merge_bases)
> > +		free_commit_list(merge_bases);
> 
> free_commit_list() returns immediately when a NULL pointer is passed in, 
> so I'm not sure if this check is really necessary. I think it is a 
> reasonable assumption to make that free* functions work well with NULL 
> inputs.

I didn't realise that free_commit_list() would freely accept a NULL
pointer without segfaulting. I'll remove the surrounding if.

> 
> >  	return res && is_linear_history(onto, head);
> >  }
> >  
> 
> Out of curiosity, since you are going with a goto tower, why not do 
> something like:
> 
> done_merge_bases:
> free_commit_list(merge_bases);
> done:
> return res && is_linear_history(onto, head);
> 
> You jump to done_merge_bases after you have initialized merge_bases, and 
> directly to done before initializing it.

I opted to do it this way since I figured it was less complexity to have
a common jump target at the end. Also, in case in the future we decided
to add more logic before merge_bases was initialised, we wouldn't have
to worry about figuring out which target to jump to.

> 
> I'm not advocating for either way, just curious if there is a specific 
> reason to do it your way. Anyway, if you drop the if (merge_bases), then 
> it doesn't really matter I suppose.

I'm pretty impartial as well. I'll drop the if and leave the rest as is
unless someone else feels strongly about this.

> 
> -- 
> Regards,
> Pratyush Yadav