Subject: Re: [PATCH 2/2] builtin/repack.c: implement support for `--cruft-max-size`

From: Patrick Steinhardt <[email-masked]>
Date: Fri, 8 Sep 2023 13:21:44 +0200

On Thu, Sep 07, 2023 at 05:52:04PM -0400, Taylor Blau wrote:
[snip]
> @@ -125,17 +133,39 @@ static void mark_packs_for_deletion_1(struct string_list *names,
>  		if (len < hexsz)
>  			continue;
>  		sha1 = item->string + len - hexsz;
> -		/*
> -		 * Mark this pack for deletion, which ensures that this
> -		 * pack won't be included in a MIDX (if `--write-midx`
> -		 * was given) and that we will actually delete this pack
> -		 * (if `-d` was given).
> -		 */
> -		if (!string_list_has_string(names, sha1))
> -			item->util = (void*)1;
> +
> +		if (pack_is_retained(item)) {
> +			item->util = NULL;
> +		} else if (!string_list_has_string(names, sha1)) {
> +			/*
> +			 * Mark this pack for deletion, which ensures
> +			 * that this pack won't be included in a MIDX
> +			 * (if `--write-midx` was given) and that we
> +			 * will actually delete this pack (if `-d` was
> +			 * given).
> +			 */
> +			item->util = DELETE_PACK;
> +		}

I find the behaviour of this function a tad surprising as it doesn't
only mark a pack for deletion, but it also marks a pack as not being
retained anymore. Shouldn't we rather:

    if (pack_is_retained(item)) {
        // Theoretically speaking we shouldn't even do this bit here as
        // we _un_mark the pack for deletion. But at least we shouldn't
        // be removing the `RETAIN_PACK` bit, I'd think.
        item->util &= ~DELETE_PACK; 
    } else if (!string_list_has_string(names, sha1)) {
        // And here we shouldn't discard the `RETAIN_PACK` bit either.
        item->util |= DELETE_PACK;
    }

>  	}
>  }
>  
> +static void retain_cruft_pack(struct existing_packs *existing,
> +			      struct packed_git *cruft)
> +{
> +	struct strbuf buf = STRBUF_INIT;
> +	struct string_list_item *item;
> +
> +	strbuf_addstr(&buf, pack_basename(cruft));
> +	strbuf_strip_suffix(&buf, ".pack");
> +
> +	item = string_list_lookup(&existing->cruft_packs, buf.buf);
> +	if (!item)
> +		BUG("could not find cruft pack '%s'", pack_basename(cruft));
> +
> +	item->util = (void*)RETAIN_PACK;
> +	strbuf_release(&buf);
> +}
> +

Similarly, should we handle potentially pre-existing bits gracefully and
`item->util |= RETAIN_PACK`?

>  static void mark_packs_for_deletion(struct existing_packs *existing,
>  				    struct string_list *names)
>  
> @@ -217,6 +247,8 @@ static void collect_pack_filenames(struct existing_packs *existing,
>  	}
>  
>  	string_list_sort(&existing->kept_packs);
> +	string_list_sort(&existing->non_kept_packs);
> +	string_list_sort(&existing->cruft_packs);
>  	strbuf_release(&buf);
>  }
>  
> @@ -799,6 +831,72 @@ static void remove_redundant_bitmaps(struct string_list *include,
>  	strbuf_release(&path);
>  }
>  
> +static int existing_cruft_pack_cmp(const void *va, const void *vb)
> +{
> +	struct packed_git *a = *(struct packed_git **)va;
> +	struct packed_git *b = *(struct packed_git **)vb;
> +
> +	if (a->pack_size < b->pack_size)
> +		return -1;
> +	if (a->pack_size > b->pack_size)
> +		return 1;
> +	return 0;
> +}
> +
> +static void collapse_small_cruft_packs(FILE *in, unsigned long max_size,

We might want to use `size_t` to denote file sizes instead of `unsigned
long`.

> +				       struct existing_packs *existing)
> +{
> +	struct packed_git **existing_cruft, *p;
> +	struct strbuf buf = STRBUF_INIT;
> +	unsigned long total_size = 0;

Here, as well.

> +	size_t existing_cruft_nr = 0;
> +	size_t i;
> +
> +	ALLOC_ARRAY(existing_cruft, existing->cruft_packs.nr);
> +
> +	for (p = get_all_packs(the_repository); p; p = p->next) {
> +		if (!(p->is_cruft && p->pack_local))
> +			continue;
> +
> +		strbuf_reset(&buf);
> +		strbuf_addstr(&buf, pack_basename(p));
> +		strbuf_strip_suffix(&buf, ".pack");
> +
> +		if (!string_list_has_string(&existing->cruft_packs, buf.buf))
> +			continue;
> +
> +		if (existing_cruft_nr >= existing->cruft_packs.nr)
> +			BUG("too many cruft packs (found %"PRIuMAX", but knew "
> +			    "of %"PRIuMAX")",
> +			    (uintmax_t)existing_cruft_nr + 1,
> +			    (uintmax_t)existing->cruft_packs.nr);
> +		existing_cruft[existing_cruft_nr++] = p;
> +	}
> +
> +	QSORT(existing_cruft, existing_cruft_nr, existing_cruft_pack_cmp);
> +
> +	for (i = 0; i < existing_cruft_nr; i++) {
> +		off_t proposed;

This should also be `size_t` given that `st_add` returns `size_t` and
not `off_t`.

> +		p = existing_cruft[i];
> +		proposed = st_add(total_size, p->pack_size);
> +
> +		if (proposed <= max_size) {
> +			total_size = proposed;
> +			fprintf(in, "-%s\n", pack_basename(p));
> +		} else {
> +			retain_cruft_pack(existing, p);
> +			fprintf(in, "%s\n", pack_basename(p));
> +		}

It's a bit funny that we re-check whether we have exceeded the maximum
size in subsequente iterations once we hit the limit, but it arguably
makes the logic a bit simpler.

> +	}
> +
> +	for (i = 0; i < existing->non_kept_packs.nr; i++)
> +		fprintf(in, "-%s.pack\n",
> +			existing->non_kept_packs.items[i].string);
> +
> +	strbuf_release(&buf);
> +}
> +
>  static int write_cruft_pack(const struct pack_objects_args *args,
>  			    const char *destination,
>  			    const char *pack_prefix,
> @@ -846,10 +944,18 @@ static int write_cruft_pack(const struct pack_objects_args *args,
>  	in = xfdopen(cmd.in, "w");
>  	for_each_string_list_item(item, names)
>  		fprintf(in, "%s-%s.pack\n", pack_prefix, item->string);
> -	for_each_string_list_item(item, &existing->non_kept_packs)
> -		fprintf(in, "-%s.pack\n", item->string);
> -	for_each_string_list_item(item, &existing->cruft_packs)
> -		fprintf(in, "-%s.pack\n", item->string);
> +	if (args->max_pack_size && !cruft_expiration) {
> +		unsigned long max_pack_size;
> +		if (!git_parse_ulong(args->max_pack_size, &max_pack_size))
> +			return error(_("could not parse --cruft-max-size: '%s'"),
> +				     args->max_pack_size);
> +		collapse_small_cruft_packs(in, max_pack_size, existing);
> +	} else {
> +		for_each_string_list_item(item, &existing->non_kept_packs)
> +			fprintf(in, "-%s.pack\n", item->string);
> +		for_each_string_list_item(item, &existing->cruft_packs)
> +			fprintf(in, "-%s.pack\n", item->string);
> +	}

If I understand correctly, we only collapse small cruft packs in case
we're not expiring any objects at the same time. Is there an inherent
reason why? I would imagine that it can indeed be useful to expire
objects contained in cruft packs and then have git-repack(1) recombine
whatever is left into larger packs.

If the reason is basically "it's complicated" then that is fine with me,
we can still implement the functionality at a later point in time. But
until then I think that we should let callers know that the two options
are incompatible with each other by producing an error when both are
passed.

Patrick