Re: [PATCH 1/1] commit-reach: properly peel tags

From: Jeff King <peff@peff.net>
To: Junio C Hamano <gitster@pobox.com>
Cc: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>,
	git@vger.kernel.org, Derrick Stolee <dstolee@microsoft.com>
Subject: Re: [PATCH 1/1] commit-reach: properly peel tags
Date: Wed, 12 Sep 2018 17:34:08 -0400	[thread overview]
Message-ID: <20180912213407.GA27598@sigill.intra.peff.net> (raw)
In-Reply-To: <xmqqpnxijtpd.fsf@gitster-ct.c.googlers.com>

On Wed, Sep 12, 2018 at 02:23:42PM -0700, Junio C Hamano wrote:

> > diff --git a/commit-reach.c b/commit-reach.c
> > index 86715c103c..6de72c6e03 100644
> > --- a/commit-reach.c
> > +++ b/commit-reach.c
> > @@ -544,20 +544,31 @@ int can_all_from_reach_with_flag(struct object_array *from,
> >  {
> >  	struct commit **list = NULL;
> >  	int i;
> > +	int nr_commits;
> >  	int result = 1;
> >  
> >  	ALLOC_ARRAY(list, from->nr);
> > +	nr_commits = 0;
> >  	for (i = 0; i < from->nr; i++) {
> > -		list[i] = (struct commit *)from->objects[i].item;
> > +		struct object *from_one = from->objects[i].item;
> >  
> > -		if (parse_commit(list[i]) ||
> > -		    list[i]->generation < min_generation)
> > -			return 0;
> > +		from_one = deref_tag(the_repository, from_one,
> > +				     "a from object", 0);
> > +		if (!from_one || from_one->type != OBJ_COMMIT) {
> > +			from->objects[i].item->flags |= assign_flag;
> 
> I wondered why this is not futzing with "from_one->flags"; by going
> back to the original from->objects[] array, the code is setting the
> flags on the original tag object and not the non-commit object that
> was pointed at by the tag.

Note that from_one may even be NULL.

> > +			continue;
> > +		}
> > +
> > +		list[nr_commits] = (struct commit *)from_one;
> > +		if (parse_commit(list[nr_commits]) ||
> > +		    list[nr_commits]->generation < min_generation)
> > +			return 0; /* is this a leak? */
> > +		nr_commits++;
> >  	}
> 
> In the original code, the flags bits were left unchanged if the loop
> terminated by hitting a commit whose generation is too young (or
> parse_commit() returns non-zero).  With this updated code, flags bit
> can be modified before the code notices the situation and leave the
> function, bypassing the "cleanup" we see below that clears the
> "assign_flag" bits.
> 
> Would it be a problem that we return early without cleaning up?
> 
> Even if we do not call this early return, the assign_flag bits added
> to the original tag in from->objects[i].item won't be cleaned in
> this new code, as "cleanup:" section now loops over the list[] that
> omits the object whose flags was smudged above before the "continue".
> 
> Would it be a problem that we leave the assign_flags without
> cleaning up?

Yeah, I hadn't thought about the bit cleanup when making my original
suggestion. In the original code (before 4fbcca4eff), I think we did set
flags as we iterated through the loop, and we could still do an early
return when we hit "!reachable(...)". But I don't see any cleanup of
assign_flag there at all.

So I guess I'm pretty confused about what the semantics are supposed to
be.

-Peff