Re: [PATCH v3 2/5] fetch-pack: load tip_oids eagerly iff needed

From: "René Scharfe" <l.s.r@web.de>
To: Jeff King <peff@peff.net>
Cc: Git List <git@vger.kernel.org>,
	Junio C Hamano <gitster@pobox.com>,
	Jonathan Tan <jonathantanmy@google.com>
Subject: Re: [PATCH v3 2/5] fetch-pack: load tip_oids eagerly iff needed
Date: Fri, 5 Oct 2018 22:13:34 +0200	[thread overview]
Message-ID: <c75fa650-d2b4-9979-a1f2-25d75c6f447b@web.de> (raw)
In-Reply-To: <20181004220711.GA28287@sigill.intra.peff.net>

Am 05.10.2018 um 00:07 schrieb Jeff King:
> On Thu, Oct 04, 2018 at 05:09:39PM +0200, René Scharfe wrote:
> 
>> tip_oids_contain() lazily loads refs into an oidset at its first call.
>> It abuses the internal (sub)member .map.tablesize of that oidset to
>> check if it has done that already.
>>
>> Determine if the oidset needs to be populated upfront and then do that
>> instead.  This duplicates a loop, but simplifies the existing one by
>> separating concerns between the two.
> 
> I like this approach much better than what I showed earlier. But...
> 
>> diff --git a/fetch-pack.c b/fetch-pack.c
>> index 3b317952f0..53914563b5 100644
>> --- a/fetch-pack.c
>> +++ b/fetch-pack.c
>> @@ -526,23 +526,6 @@ static void add_refs_to_oidset(struct oidset *oids, struct ref *refs)
>>  		oidset_insert(oids, &refs->old_oid);
>>  }
>>  
>> -static int tip_oids_contain(struct oidset *tip_oids,
>> -			    struct ref *unmatched, struct ref *newlist,
>> -			    const struct object_id *id)
>> -{
>> -	/*
>> -	 * Note that this only looks at the ref lists the first time it's
>> -	 * called. This works out in filter_refs() because even though it may
>> -	 * add to "newlist" between calls, the additions will always be for
>> -	 * oids that are already in the set.
>> -	 */
> 
> I don't think the subtle point this comment is making goes away. We're
> still growing the list in the loop that calls tip_oids_contain() (and
> which now calls just oidset_contains). That's OK for the reasons given
> here, but I think that would need to be moved down to this code:
> 
>> +	if (strict) {
>> +		for (i = 0; i < nr_sought; i++) {
>> +			ref = sought[i];
>> +			if (!is_unmatched_ref(ref))
>> +				continue;
>> +
>> +			add_refs_to_oidset(&tip_oids, unmatched);
>> +			add_refs_to_oidset(&tip_oids, newlist);
>> +			break;
>> +		}
>> +	}
> 
> I.e., we need to say here why it's OK to summarize newlist in the
> oidset, even though we're adding to it later.

There is already this comment:

	/* Append unmatched requests to the list */

And that's enough in my eyes.  The refs loop at the top splits the list
into matched ("the list") and unmatched, and the loop below said comment
adds a few more.  I see no subtlety left -- what do I miss?

René