Subject: Re: [PATCH v1 06/10] packfile: packed_object_info avoids packed_to_object_type

From: Patrick Steinhardt <[email-masked]>
Date: Wed, 24 Jul 2024 10:36:04 +0200

On Mon, Jul 15, 2024 at 12:35:15AM +0000, Eric Wong wrote:
> For calls the delta base cache, packed_to_object_type calls
> can be omitted.  This prepares us to bypass content_limit for
> non-blob types in the following commit.
> 
> Signed-off-by: Eric Wong <[email-masked]>
> ---
>  packfile.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/packfile.c b/packfile.c
> index b2660e14f9..c2ba6ab203 100644
> --- a/packfile.c
> +++ b/packfile.c
> @@ -1522,7 +1522,7 @@ int packed_object_info(struct repository *r, struct packed_git *p,
>  {
>  	struct pack_window *w_curs = NULL;
>  	off_t curpos = obj_offset;
> -	enum object_type type;
> +	enum object_type type, final_type = OBJ_BAD;
>  	struct delta_base_cache_entry *ent;

I think it might help this patch to move `type` to the scopes where it's
used to demonstrate that all code paths set `final_type` as expected.

>  	/*
> @@ -1534,7 +1534,7 @@ int packed_object_info(struct repository *r, struct packed_git *p,
>  	ent = get_delta_base_cache_entry(p, obj_offset);
>  	if (ent) {
>  		oi->whence = OI_DBCACHED;
> -		type = ent->type;
> +		final_type = type = ent->type;
>  		if (oi->sizep)
>  			*oi->sizep = ent->size;
>  		if (oi->contentp) {
> @@ -1552,6 +1552,7 @@ int packed_object_info(struct repository *r, struct packed_git *p,
>  	} else if (oi->contentp && !oi->content_limit) {
>  		*oi->contentp = unpack_entry(r, p, obj_offset, &type,
>  						oi->sizep);
> +		final_type = type;
>  		if (!*oi->contentp)
>  			type = OBJ_BAD;
>  	} else {
> @@ -1581,6 +1582,7 @@ int packed_object_info(struct repository *r, struct packed_git *p,
>  			if (oi->sizep && *oi->sizep <= oi->content_limit) {
>  				*oi->contentp = unpack_entry(r, p, obj_offset,
>  							&type, oi->sizep);
> +				final_type = type;
>  				if (!*oi->contentp)
>  					type = OBJ_BAD;
>  			} else {
> @@ -1602,17 +1604,17 @@ int packed_object_info(struct repository *r, struct packed_git *p,
>  	}
>  
>  	if (oi->typep || oi->type_name) {
> -		enum object_type ptot;
> -		ptot = packed_to_object_type(r, p, obj_offset,
> -					     type, &w_curs, curpos);
> +		if (final_type < 0)
> +			final_type = packed_to_object_type(r, p, obj_offset,
> +						     type, &w_curs, curpos);

So this is the actual change we're interested in, right? Instead of
unconditionally calling `packed_to_object_type()`, we skip that call in
case we know that we have already figured out the correct object type.

Wouldn't it be easier to manage this with a single `type` variable,
only, and then conditionally call `packed_to_object_type()` only in the
cases where `type != OBJ_OFS_DELTA && type != OBJ_REF_DELTA`? Not sure
whether that would be all that useful though given that the function
already knows to exit without doing anything in case the type is already
properly resolved. So maybe the next patch will enlighten me.

Patrick

>  		if (oi->typep)
> -			*oi->typep = ptot;
> +			*oi->typep = final_type;
>  		if (oi->type_name) {
> -			const char *tn = type_name(ptot);
> +			const char *tn = type_name(final_type);
>  			if (tn)
>  				strbuf_addstr(oi->type_name, tn);
>  		}
> -		if (ptot < 0) {
> +		if (final_type < 0) {
>  			type = OBJ_BAD;
>  			goto out;
>  		}
>