Re: [PATCH] odb: do not use "blank" substitute for NULL

> When various *object_info() functions are given an extended object
> info structure as NULL by a caller that does not want any details,
> the code uses a file-scope static blank_oi to pass it down to the
> helper functions they use, to avoid handling NULL specifically.
> 
> The ps/object-read-stream topic graduated to 'master' recently
> however had a bug that assumed that two identically named file-scope
> static variables in two functions are the same, which of course is
> not the case.  This made "git commit" take 0.38 seconds to 1508
> seconds in some case, as reported by Aaron Plattner here:
> 
>    https://lore.kernel.org/git/f4ba7e89-4717-4b36-921f-56537131fd69@nvidia.com/
> 
> We _could_ move the blank_oi variable to a global scope in BSS to
> fix this regression, but explicitly handling the NULL is a much
> safer fix.  It would also reduce the chance of errors that somebody
> accidentally writes into blank_oi, making its contents dirty, which
> potentially will make subsequent calls into the callpath misbehave.
> 
> By explicitly handling NULL input, we no longer have to worry about
> it.

> Reported-by: Aaron Plattner <aplattner@nvidia.com>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>   object-file.c |  8 ++++----
>   odb.c         | 29 +++++++++++++----------------
>   packfile.c    |  3 +--
>   3 files changed, 18 insertions(+), 22 deletions(-)
> 
> diff --git a/object-file.c b/object-file.c
> index 12177a7dd7..e0cce3a62a 100644
> --- a/object-file.c
> +++ b/object-file.c
> @@ -426,7 +426,7 @@ int odb_source_loose_read_object_info(struct odb_source *source,
>   	unsigned long size_scratch;
>   	enum object_type type_scratch;
>   
> -	if (oi->delta_base_oid)
> +	if (oi && oi->delta_base_oid)
>   		oidclr(oi->delta_base_oid, source->odb->repo->hash_algo);
>   
>   	/*
> @@ -437,13 +437,13 @@ int odb_source_loose_read_object_info(struct odb_source *source,
>   	 * return value implicitly indicates whether the
>   	 * object even exists.
>   	 */
> -	if (!oi->typep && !oi->sizep && !oi->contentp) {
> +	if (!oi || (!oi->typep && !oi->sizep && !oi->contentp)) {
>   		struct stat st;
> -		if (!oi->disk_sizep && (flags & OBJECT_INFO_QUICK))
> +		if ((!oi || !oi->disk_sizep) && (flags & OBJECT_INFO_QUICK))
>   			return quick_has_loose(source->loose, oid) ? 0 : -1;
>   		if (stat_loose_object(source->loose, oid, &st, &path) < 0)
>   			return -1;
> -		if (oi->disk_sizep)
> +		if (oi && oi->disk_sizep)
>   			*oi->disk_sizep = st.st_size;
>   		return 0;
>   	}
> diff --git a/odb.c b/odb.c
> index f4cbee4b04..85dc21b104 100644
> --- a/odb.c
> +++ b/odb.c
> @@ -664,34 +664,31 @@ static int do_oid_object_info_extended(struct object_database *odb,
>   				       const struct object_id *oid,
>   				       struct object_info *oi, unsigned flags)
>   {
> -	static struct object_info blank_oi = OBJECT_INFO_INIT;
>   	const struct cached_object *co;
>   	const struct object_id *real = oid;
>   	int already_retried = 0;
>   
> -
>   	if (flags & OBJECT_INFO_LOOKUP_REPLACE)
>   		real = lookup_replace_object(odb->repo, oid);
>   
>   	if (is_null_oid(real))
>   		return -1;
>   
> -	if (!oi)
> -		oi = &blank_oi;
> -
>   	co = find_cached_object(odb, real);
>   	if (co) {
> -		if (oi->typep)
> -			*(oi->typep) = co->type;
> -		if (oi->sizep)
> -			*(oi->sizep) = co->size;
> -		if (oi->disk_sizep)
> -			*(oi->disk_sizep) = 0;
> -		if (oi->delta_base_oid)
> -			oidclr(oi->delta_base_oid, odb->repo->hash_algo);
> -		if (oi->contentp)
> -			*oi->contentp = xmemdupz(co->buf, co->size);
> -		oi->whence = OI_CACHED;
> +		if (oi) {
> +			if (oi->typep)
> +				*(oi->typep) = co->type;
> +			if (oi->sizep)
> +				*(oi->sizep) = co->size;
> +			if (oi->disk_sizep)
> +				*(oi->disk_sizep) = 0;
> +			if (oi->delta_base_oid)
> +				oidclr(oi->delta_base_oid, odb->repo->hash_algo);
> +			if (oi->contentp)
> +				*oi->contentp = xmemdupz(co->buf, co->size);
> +			oi->whence = OI_CACHED;
> +		}
>   		return 0;
>   	}
>   
> diff --git a/packfile.c b/packfile.c
> index 7a16aaa90d..2aa6135c3a 100644
> --- a/packfile.c
> +++ b/packfile.c
> @@ -2095,7 +2095,6 @@ int packfile_store_read_object_info(struct packfile_store *store,
>   				    struct object_info *oi,
>   				    unsigned flags UNUSED)
>   {
> -	static struct object_info blank_oi = OBJECT_INFO_INIT;
>   	struct pack_entry e;
>   	int rtype;
>   
> @@ -2106,7 +2105,7 @@ int packfile_store_read_object_info(struct packfile_store *store,
>   	 * We know that the caller doesn't actually need the
>   	 * information below, so return early.
>   	 */
> -	if (oi == &blank_oi)
> +	if (!oi)
>   		return 0;
>   
>   	rtype = packed_object_info(store->odb->repo, e.p, e.offset, oi);