Re: [PATCH v3 2/4] format-patch: add '--base' option to record base tree info

[Ye Xiaolong <xiaolong.ye@intel.com>]

On Thu, Mar 31, 2016 at 10:38:04AM -0700, Junio C Hamano wrote:
>> diff --git a/builtin/log.c b/builtin/log.c
>> index 0d738d6..03cbab0 100644
>> --- a/builtin/log.c
>> +++ b/builtin/log.c
>> @@ -1185,6 +1185,82 @@ static int from_callback(const struct option *opt, const char *arg, int unset)
>>  	return 0;
>>  }
>>  
>> +struct base_tree_info {
>> +	struct object_id base_commit;
>> +	int nr_patch_id, alloc_patch_id;
>> +	struct object_id *patch_id;
>> +};
>> +
>> +static void prepare_bases(struct base_tree_info *bases,
>> +			  const char *base_commit,
>> +			  struct commit **list,
>> +			  int total)
>> +{
>> +	struct commit *base = NULL, *commit;
>> +	struct rev_info revs;
>> +	struct diff_options diffopt;
>> +	struct object_id *patch_id;
>> +	unsigned char sha1[20];
>> +	int i;
>> +
>> +	diff_setup(&diffopt);
>> +	DIFF_OPT_SET(&diffopt, RECURSIVE);
>> +	diff_setup_done(&diffopt);
>> +
>> +	base = lookup_commit_reference_by_name(base_commit);
>> +	if (!base)
>> +		die(_("Unknown commit %s"), base_commit);
>> +	oidcpy(&bases->base_commit, &base->object.oid);
>> +
>> +	init_revisions(&revs, NULL);
>> +	revs.max_parents = 1;
>> +	base->object.flags |= UNINTERESTING;
>> +	add_pending_object(&revs, &base->object, "base");
>> +	for (i = 0; i < total; i++) {
>> +		list[i]->object.flags |= 0;
>
>What does this statement do, exactly?  Are you clearing some bits
>but not others, and if so which ones?
>
>> +		add_pending_object(&revs, &list[i]->object, "rev_list");
>> +		list[i]->util = (void *)1;
>
>Are we sure commit objects not on the list have their ->util cleared?
>The while() loop below seems to rely on that to correctly filter out
>the ones that are on the list.
>
After some investigation and according to my understanding, the commit
object is allocated through alloc_commit_node->alloc_node, 

void *alloc_commit_node(void)
{
	struct commit *c = alloc_node(&commit_state, sizeof(struct commit));
	c->object.type = OBJ_COMMIT;
	c->index = alloc_commit_index();
	return c;
}

static inline void *alloc_node(struct alloc_state *s, size_t node_size)
{
	void *ret;

	if (!s->nr) {
		s->nr = BLOCKING;
		s->p = xmalloc(BLOCKING * node_size);
	}
	s->nr--;
	s->count++;
	ret = s->p;
	s->p = (char *)s->p + node_size;
	memset(ret, 0, node_size);
	return ret;
}

So the commit->util should be cleared after initialization, and it has
not been touched except above "for" loop in our code execution path, I think
it is safe to rely on it to filter out commits that are on the rev list.

Thanks,
Xiaolong.
>> +	}
>> +
>> +	if (prepare_revision_walk(&revs))
>> +		die(_("revision walk setup failed"));
>> +	/*
>> +	 * Traverse the prerequisite commits list,
>> +	 * get the patch ids and stuff them in bases structure.
>> +	 */
>> +	while ((commit = get_revision(&revs)) != NULL) {
>> +		if (commit->util)
>> +			continue;
>> +		if (commit_patch_id(commit, &diffopt, sha1))
>> +			die(_("cannot get patch id"));
>> +		ALLOC_GROW(bases->patch_id, bases->nr_patch_id + 1, bases->alloc_patch_id);
>> +		patch_id = bases->patch_id + bases->nr_patch_id;
>> +		hashcpy(patch_id->hash, sha1);
>