Subject: Re: [PATCH v2 2/4] midx-write.c: factor out common want_included_pack() routine

From: Patrick Steinhardt <[email-masked]>
Date: Tue, 2 Apr 2024 13:47:38 +0200

On Mon, Apr 01, 2024 at 05:16:38PM -0400, Taylor Blau wrote:
> When performing a 'git multi-pack-index repack', the MIDX machinery
> tries to aggregate MIDX'd packs together either to (a) fill the given
> `--batch-size` argument, or (b) combine all packs together.
> 
> In either case (using the `midx-write.c::fill_included_packs_batch()` or
> `midx-write.c::fill_included_packs_all()` function, respectively), we
> evaluate whether or not we want to repack each MIDX'd pack, according to
> whether or it is loadable, kept, cruft, or non-empty.
> 
> Between the two `fill_included_packs_` callers, they both care about the
> same conditions, except for `fill_included_packs_batch()` which also
> cares that the pack is non-empty.
> 
> We could extract two functions (say, `want_included_pack()` and a
> `_nonempty()` variant), but this is not necessary. For the case in
> `fill_included_packs_all()` which does not check the pack size, we add
> all of the pack's objects assuming that the pack meets all other
> criteria. But if the pack is empty in the first place, we add all of its
> zero objects, so whether or not we "accept" or "reject" it in the first
> place is irrelevant.
> 
> This change improves the readability in both `fill_included_packs_`
> functions.
> 
> Signed-off-by: Taylor Blau <[email-masked]>
> ---
>  midx-write.c | 32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/midx-write.c b/midx-write.c
> index 5242d2a724..906efa2169 100644
> --- a/midx-write.c
> +++ b/midx-write.c
> @@ -1349,6 +1349,24 @@ static int compare_by_mtime(const void *a_, const void *b_)
>  	return 0;
>  }
>  
> +static int want_included_pack(struct repository *r,
> +			      struct multi_pack_index *m,
> +			      int pack_kept_objects,
> +			      uint32_t pack_int_id)
> +{
> +	struct packed_git *p;
> +	if (prepare_midx_pack(r, m, pack_int_id))
> +		return 0;
> +	p = m->packs[pack_int_id];
> +	if (!pack_kept_objects && p->pack_keep)
> +		return 0;
> +	if (p->is_cruft)
> +		return 0;
> +	if (open_pack_index(p) || !p->num_objects)
> +		return 0;
> +	return 1;
> +}
> +
>  static int fill_included_packs_all(struct repository *r,
>  				   struct multi_pack_index *m,
>  				   unsigned char *include_pack)
> @@ -1359,11 +1377,7 @@ static int fill_included_packs_all(struct repository *r,
>  	repo_config_get_bool(r, "repack.packkeptobjects", &pack_kept_objects);
>  
>  	for (i = 0; i < m->num_packs; i++) {
> -		if (prepare_midx_pack(r, m, i))
> -			continue;
> -		if (!pack_kept_objects && m->packs[i]->pack_keep)
> -			continue;
> -		if (m->packs[i]->is_cruft)
> +		if (!want_included_pack(r, m, pack_kept_objects, i))
>  			continue;
>  
>  		include_pack[i] = 1;
> @@ -1410,13 +1424,7 @@ static int fill_included_packs_batch(struct repository *r,
>  		struct packed_git *p = m->packs[pack_int_id];
>  		size_t expected_size;

I was briefly wondering whether `m->packs[pack_int_id]` could change
after the above assignment. But there's a loop a bit further up here
that already calls `prepare_midx_pack()`, so this shouldn't happen.

> -		if (!p)
> -			continue;
> -		if (!pack_kept_objects && p->pack_keep)
> -			continue;
> -		if (p->is_cruft)
> -			continue;
> -		if (open_pack_index(p) || !p->num_objects)
> +		if (!want_included_pack(r, m, pack_kept_objects, pack_int_id))
>  			continue;

Another thing I wondered was whether it hurts performance that we now
call `prepare_midx_pack()` twice. But this shouldn't matter either as we
exit early from that function in case `m->packs[pack_int_id]` is already
populated.

So this patch looks good to me.

Patrick

>  		expected_size = st_mult(p->pack_size,
> -- 
> 2.44.0.330.g158d2a670b4
> 
>