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

On Thu, Jun 15, 2017 at 10:50:46AM -0700, Junio C Hamano wrote:

quoted

 	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.

I also found this quite subtle. However, I don't think that
has_sha1_file() actually freshens. It's a bit confusing because
has_loose_object() reuses the check_and_freshen() function to do the
lookup, but it actually sets the "freshen" flag to false.

That's why in 33d4221c7 (write_sha1_file: freshen existing objects,
2014-10-15), which introduced the freshening functions and converted
has_loose_object(), the actual write_sha1_file() function switched to
using the freshening functions directly (and obviously sets the freshen
parameter to true).

I actually think all of that infrastructure could become part of
Jonathan's consolidated lookup, too. We would just need:

  1. A QUICK flag to avoid re-reading objects/pack when we don't find
     anything (which it looks like he already has).

  2. A FRESHEN flag to update the mtime of any item that we do find.

I suspect we may also need something like ONLY_LOOSE and ONLY_NONLOCAL
to meet all the callers (e.g., has_loose_object_nonlocal). Those should
be easy to implement, I'd think.

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.

I had the same thoughts (both on the name and the "vocabularies"). IMHO
we should consider allocating the bits from the same set. There's only
one HAS_SHA1 flag, and it has an exact match in OBJECT_INFO_QUICK.

I think this patch might be a bit easier to review if it were broken
down more in a sequence of:

  1. Add features to the consolidated function to support everything
     that function X supports.

  2. Preparatory cleanup around X (e.g., pointing HAS_SHA1_QUICK at
     OBJECT_INFO_QUICK).

  3. Convert X to use the consolidated function.

  4. Repeat for each X we wish to consolidate.

That's going to end up with probably 12 patches instead of one, but I
think it may be a lot easier to communicate the reason for the various
design decisions.

-Peff