Re: [PATCH v2 3/4] builtin/pack-objects.c: ensure included `--stdin-packs` exist

> pack does not pass the `is_pack_valid()` check.
> 
> The `--stdin-packs` mode of `pack-objects` is not quite prepared to
> handle this. To prepare it for this change, do the following two things:
> 
>   - Ensure provided packs pass the `is_pack_valid()` check when
>     collecting the caller-provided packs into the "included" and
>     "excluded" lists.
> 

>   - Gracefully handle any _invalid_ packs being passed to
>     `want_object_in_pack()`.
> 
> Calling `is_pack_valid()` early on makes it substantially less likely
> that we will have to deal with a pack going away, since we'll have an
> open file descriptor on its contents much earlier.
> 
> But even packs with open descriptors can become invalid in the future if
> we (a) hit our open descriptor limit, forcing us to close some open
> packs, and (b) one of those just-closed packs has gone away in the
> meantime.
> 
> `add_object_entry_from_pack()` depends on having a non-NULL
> `*found_pack`, since it passes that pointer to `packed_object_info()`,
> meaning that we would SEGV if the pointer became NULL (like we propose
> to do in `want_object_in_pack()` in the following patch).
> 
> But avoiding calling `packed_object_info()` entirely is OK, too, since
> its only purpose is to identify which objects in the included packs are
> commits, so that they can form the tips of the advisory traversal used
> to discover the object namehashes.
> 
> Failing to do this means that at worst we will produce lower-quality
> deltas, but it does not prevent us from generating the pack as long as
> we can find a copy of each object from the disappearing pack in some
> other part of the repository.
> 

> Co-authored-by: Victoria Dye <vdye@github.com>
> Signed-off-by: Taylor Blau <me@ttaylorr.com>
> ---
>  builtin/pack-objects.c | 35 ++++++++++++++++++++---------------
>  1 file changed, 20 insertions(+), 15 deletions(-)
> 
> diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
> index ec3193fd95..ffeaecd1d8 100644
> --- a/builtin/pack-objects.c
> +++ b/builtin/pack-objects.c
> @@ -3201,10 +3201,8 @@ static int add_object_entry_from_pack(const struct object_id *oid,
>  				      uint32_t pos,
>  				      void *_data)
>  {
> -	struct rev_info *revs = _data;
> -	struct object_info oi = OBJECT_INFO_INIT;
>  	off_t ofs;
> -	enum object_type type;
> +	enum object_type type = OBJ_NONE;
>  
>  	display_progress(progress_state, ++nr_seen);
>  
> @@ -3215,20 +3213,25 @@ static int add_object_entry_from_pack(const struct object_id *oid,
>  	if (!want_object_in_pack(oid, 0, &p, &ofs))
>  		return 0;
>  
> -	oi.typep = &type;
> -	if (packed_object_info(the_repository, p, ofs, &oi) < 0)
> -		die(_("could not get type of object %s in pack %s"),
> -		    oid_to_hex(oid), p->pack_name);
> -	else if (type == OBJ_COMMIT) {
> -		/*
> -		 * commits in included packs are used as starting points for the
> -		 * subsequent revision walk
> -		 */
> -		add_pending_oid(revs, NULL, oid, 0);
> +	if (p) {
> +		struct rev_info *revs = _data;
> +		struct object_info oi = OBJECT_INFO_INIT;
> +
> +		oi.typep = &type;
> +		if (packed_object_info(the_repository, p, ofs, &oi) < 0) {
> +			die(_("could not get type of object %s in pack %s"),
> +			    oid_to_hex(oid), p->pack_name);
> +		} else if (type == OBJ_COMMIT) {
> +			/*
> +			 * commits in included packs are used as starting points for the
> +			 * subsequent revision walk
> +			 */
> +			add_pending_oid(revs, NULL, oid, 0);
> +		}
> +
> +		stdin_packs_found_nr++;
>  	}
>  
> -	stdin_packs_found_nr++;
> -
>  	create_object_entry(oid, type, 0, 0, 0, p, ofs);
>  
>  	return 0;
> @@ -3346,6 +3349,8 @@ static void read_packs_list_from_stdin(void)
>  		struct packed_git *p = item->util;
>  		if (!p)
>  			die(_("could not find pack '%s'"), item->string);
> +		if (!is_pack_valid(p))
> +			die(_("packfile %s cannot be accessed"), p->pack_name);
>  	}
>  
>  	/*