Re: [PATCH v4 2/4] format-patch: add '--base' option to record base tree info

>> +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;
>> +	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;
>> +	revs.topo_order = 1;
>> +	for (i = 0; i < total; i++) {
>> +		if (!in_merge_bases(base, list[i]) || base == list[i])
>> +			die(_("base commit should be the ancestor of revision list"));
>
>This check looks overly expensive, but I do not think of a more
>efficient way to do this, given that "All the commits from our
>series must reach the specified base" is what you seem to want.
>
>My understanding is that if base=P is given and you are doing
>"format-patch Z..C" in this picture:
>
>    Q---P---Z---B---*---C
>     \             /
>      .-----------A
>

>your list would become A, B and C, and you want to detect that P is
>not an ancestor of A.  merge_bases_many() computes a wrong thing for
>this use case, and you'd need to go one-by-one.
>
>Unless there is some clever trick to take advantage of the previous
>traversal you made in order to find out A, B and C are the commits
>that are part of your series somehow.
>
>Anybody with clever ideas?
>