Subject: Re: [PATCH v2 3/4] sha1_file: consolidate storage-agnostic object fns

From: Junio C Hamano <[email-masked]>
Date: Thu, 15 Jun 2017 10:50:46 -0700

Jonathan Tan <[email-masked]> writes:

> Looking at the 3 primary functions (sha1_object_info_extended,
> read_object, has_sha1_file_with_flags), they independently implement
> mechanisms such as object replacement, retrying the packed store after
> failing to find the object in the packed store then the loose store, and
> being able to mark a packed object as bad and then retrying the whole
> process. Consolidating these mechanisms would be a great help to
> maintainability.
>
> Therefore, consolidate all 3 functions by extending
> sha1_object_info_extended() to support the functionality needed by all 3
> functions, and then modifying the other 2 to use
> sha1_object_info_extended().

This is a rather "ugly" looking patch ;-) but I followed what
has_sha1_file_with_flags() and read_object() do before and after
this change, and I think this patch is a no-op wrt their behaviour
(which is a good thing).

But I have a very mixed feelings on one aspect of the resulting
sha1_object_info_extended().

>  int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi, unsigned flags)
>  {
> ...
>  	if (!find_pack_entry(real, &e)) {
>  		/* Most likely it's a loose object. */
> -		if (!sha1_loose_object_info(real, oi, flags)) {
> +		if (oi && !sha1_loose_object_info(real, oi, flags)) {
>  			oi->whence = OI_LOOSE;
>  			return 0;
>  		}
> +		if (!oi && has_loose_object(real))
> +			return 0;

This conversion is not incorrect per-se.  

We can see that has_sha1_file_with_flags() after this patch still
calls has_loose_object().  But it bothers me that there is no hint
to future developers to warn that a rewrite of the above like this
is incorrect:

        if (!find_pack_entry(read, &e)) {
                /* Most likely it's a loose object. */
       +        struct object_info dummy_oi;
       +        if (!oi) {
       +                memset(&dummy_oi, 0, sizeof(dummy_oi);
       +                oi = &dummy_oi;
       +        }
       -        if (oi && !sha1_loose_object_info(real, oi, flags)) {
       +        if (!sha1_loose_object_info(real, oi, flags)) {
                        oi->whence = OI_LOOSE;
                        return 0;
                }
       -        if (!oi && has_loose_object(real))
       -                return 0;

It used to be very easy to see that has_sha1_file_with_flags() will
call has_loose_object() when it does not find the object in a pack,
which will result in the loose object file freshened.  In the new
code, it is very subtle to see that---it will happen when the caller
passes oi == NULL, and has_sha1_file_with_flags() is such a caller,
but it is unclear if there are other callers of this "consolidated"
sha1_object_info_extended() that passes oi == NULL, and if they do
also want to freshen the loose object file when they do so.

> @@ -3480,18 +3491,12 @@ int has_sha1_pack(const unsigned char *sha1)
>  
>  int has_sha1_file_with_flags(const unsigned char *sha1, int flags)
>  {
> -	struct pack_entry e;
> +	int f = OBJECT_INFO_SKIP_CACHED |
> +		((flags & HAS_SHA1_QUICK) ? OBJECT_INFO_QUICK : 0);
>  
>  	if (!startup_info->have_repository)
>  		return 0;
> -	if (find_pack_entry(sha1, &e))
> -		return 1;
> -	if (has_loose_object(sha1))
> -		return 1;
> -	if (flags & HAS_SHA1_QUICK)
> -		return 0;
> -	reprepare_packed_git();
> -	return find_pack_entry(sha1, &e);
> +	return !sha1_object_info_extended(sha1, NULL, f);
>  }

I would have preferred to see the new variable not to be called 'f',
as that makes it unclear what it is (is that a callback function
pointer?).  In this case, uyou are forcing the flag bits passed
down, so perhaps you can reuse the same variable?  

If you allocated a separate variable because
has_sha1_file_with_flags() and sha1_object_info_extended() take flag
bits from two separate vocabularies, that is a valid reasoning, but
if that is the case, then I would have named 'f' to reflect that
fact that this is different from parameter 'flag' that is defined in
the has_sha1_file_with_flags() world, but a different thing that is
defined in sha1_object_info_extended() world, e.g. "soie_flag" or
something like that.

Thanks.