Subject: Re: [PATCH] packfile: avoid access(3p) calls for missing packs

From: Junio C Hamano <[email-masked]>
Date: Mon, 19 May 2025 08:46:25 -0700

Jeff King <[email-masked]> writes:

> The code in prepare_midx_pack() converts a numeric pack id (referenced
> inside the midx) into a "struct packed_git" pointer, caching the results
> in multi_pack_index->packs. That field holds NULL for "we have not
> looked it up yet" or a valid pointer to a packed_git. It probably needs
> to hold a third state: "we tried and failed".

Yeah, this was exactly why I asked "are we having repeated failures
in the same process", and I am happy with this direction.

> Something like this (large untested) patch:
>
> diff --git a/midx.c b/midx.c
> index 3d0015f782..354b1f886c 100644
> --- a/midx.c
> +++ b/midx.c
> @@ -405,7 +405,7 @@ void close_midx(struct multi_pack_index *m)
>  	munmap((unsigned char *)m->data, m->data_len);
>  
>  	for (i = 0; i < m->num_packs; i++) {
> -		if (m->packs[i])
> +		if (m->packs[i] && m->packs[i] != (void *)(intptr_t)-1)
>  			m->packs[i]->multi_pack_index = 0;
>  	}
>  	FREE_AND_NULL(m->packs);
> @@ -458,6 +458,8 @@ int prepare_midx_pack(struct repository *r, struct multi_pack_index *m,
>  
>  	pack_int_id = midx_for_pack(&m, pack_int_id);
>  
> +	if (m->packs[pack_int_id] == (void *)(intptr_t)-1)
> +		return 1;
>  	if (m->packs[pack_int_id])
>  		return 0;
>  
> @@ -482,8 +484,10 @@ int prepare_midx_pack(struct repository *r, struct multi_pack_index *m,
>  	strbuf_release(&pack_name);
>  	strbuf_release(&key);
>  
> -	if (!p)
> +	if (!p) {
> +		m->packs[pack_int_id] = (void *)(intptr_t)-1;
>  		return 1;
> +	}
>  
>  	p->multi_pack_index = 1;
>  	m->packs[pack_int_id] = p;