Re: [PATCH v3 6/9] multi-pack-index: implement 'expire' verb

[Jonathan Tan <jonathantanmy@google.com>]

> From: Derrick Stolee <dstolee@microsoft.com>
> 
> The 'git multi-pack-index expire' command looks at the existing
> mult-pack-index, counts the number of objects referenced in each
> pack-file, deletes the pack-fils with no referenced objects, and
> rewrites the multi-pack-index to no longer reference those packs.

Thanks - this was quite straightforwardly written.

> @@ -745,7 +761,10 @@ int write_midx_file(const char *object_dir)
>  			  midx_name);
>  	}
>  
> -	packs.m = load_multi_pack_index(object_dir, 1);
> +	if (m)
> +		packs.m = m;
> +	else
> +		packs.m = load_multi_pack_index(object_dir, 1);

If we already loaded the m, we can just pass it in - OK.

> +	if (packs_to_drop && packs_to_drop->nr) {
> +		int drop_index = 0;
> +		int missing_drops = 0;
> +
> +		for (i = 0; i < packs.nr && drop_index < packs_to_drop->nr; i++) {
> +			int cmp = strcmp(packs.info[i].pack_name,
> +					 packs_to_drop->items[drop_index].string);
> +
> +			if (!cmp) {
> +				drop_index++;
> +				packs.info[i].expired = 1;
> +			} else if (cmp > 0) {
> +				error(_("did not see pack-file %s to drop"),
> +				      packs_to_drop->items[drop_index].string);
> +				drop_index++;
> +				missing_drops++;
> +				i--;
> +			} else {
> +				packs.info[i].expired = 0;
> +			}
> +		}
> +
> +		if (missing_drops) {
> +			result = 1;
> +			goto cleanup;
> +		}
> +	}

This takes into account that packfiles can shift while we run this
command, I see. Other than that, this is a common pattern - how we
iterate through 2 sorted arrays, one a subsequence of each other.

And indeed packs_to_drop is a sorted list, because we use
string_list_insert() below.

>  	ALLOC_ARRAY(pack_perm, packs.nr);
>  	for (i = 0; i < packs.nr; i++) {
> -		pack_perm[packs.info[i].orig_pack_int_id] = i;
> +		if (packs.info[i].expired) {
> +			dropped_packs++;
> +			pack_perm[packs.info[i].orig_pack_int_id] = PACK_EXPIRED;
> +		} else {
> +			pack_perm[packs.info[i].orig_pack_int_id] = i - dropped_packs;
> +		}

Here...

>  	}
>  
> -	for (i = 0; i < packs.nr; i++)
> -		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
> +	for (i = 0; i < packs.nr; i++) {
> +		if (!packs.info[i].expired)
> +			pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
> +	}

...and here and elsewhere, we have to contend with the fact that
packs.info has pack_info that we don't want to write. I think it would
be slightly better to filter out the expired ones from packs.info, and
then when generating pack_perm, first memset it to 0xff. This way, we
wouldn't have to check expiry everywhere. But I don't feel too strongly
about this.

>  int expire_midx_packs(const char *object_dir)
>  {
> -	return 0;
> +	uint32_t i, *count, result = 0;
> +	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
> +	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
> +
> +	if (!m)
> +		return 0;
> +
> +	count = xcalloc(m->num_packs, sizeof(uint32_t));
> +	for (i = 0; i < m->num_objects; i++) {
> +		int pack_int_id = nth_midxed_pack_int_id(m, i);
> +		count[pack_int_id]++;
> +	}
> +
> +	for (i = 0; i < m->num_packs; i++) {
> +		char *pack_name;
> +
> +		if (count[i])
> +			continue;
> +
> +		if (prepare_midx_pack(m, i))
> +			continue;
> +
> +		if (m->packs[i]->pack_keep)
> +			continue;
> +
> +		pack_name = xstrdup(m->packs[i]->pack_name);
> +		close_pack(m->packs[i]);
> +		FREE_AND_NULL(m->packs[i]);
> +
> +		string_list_insert(&packs_to_drop, m->pack_names[i]);
> +		unlink_pack_path(pack_name, 0);
> +		free(pack_name);
> +	}
> +
> +	free(count);
> +
> +	if (packs_to_drop.nr)
> +		result = write_midx_internal(object_dir, m, &packs_to_drop);
> +
> +	string_list_clear(&packs_to_drop, 0);
> +	return result;
>  }

This is as I expected - unlink all the files we don't want, and even
though much of the midx hasn't changed, we still need to write it
because it has a new list of packfiles.

> +test_expect_success 'expire removes unreferenced packs' '
> +	(
> +		cd dup &&
> +		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
> +		refs/heads/A
> +		^refs/heads/C
> +		EOF
> +		git multi-pack-index write &&
> +		ls .git/objects/pack | grep -v -e pack-[AB] >expect &&
> +		git multi-pack-index expire &&
> +		ls .git/objects/pack >actual &&
> +		test_cmp expect actual &&
> +		ls .git/objects/pack/ | grep idx >expect-idx &&
> +		test-tool read-midx .git/objects | grep idx >actual-midx &&
> +		test_cmp expect-idx actual-midx
> +	)
> +'

Maybe add a fsck at the end for sanity's sake. Also, I think that
preservation of .keep packfiles is an important feature, and maybe worth
a test.