pack-bitmap: return multiple packs via reuse_partial_packfile_from_bitmap()

Further prepare for enabling verbatim pack-reuse over multiple packfiles
by changing the signature of reuse_partial_packfile_from_bitmap() to
populate an array of `struct bitmapped_pack *`'s instead of a pointer to
a single packfile.

Since the array we're filling out is sized dynamically[^1], add an
additional `size_t *` parameter which will hold the number of reusable
packs (equal to the number of elements in the array).

Note that since we still have not implemented true multi-pack reuse,
these changes aren't propagated out to the rest of the caller in
builtin/pack-objects.c.

In the interim state, we expect that the array has a single element, and
we use that element to fill out the static `reuse_packfile` variable
(which is a bog-standard `struct packed_git *`). Future commits will
continue to push this change further out through the pack-objects code.

[^1]: That is, even though we know the number of packs which are
  candidates for pack-reuse, we do not know how many of those
  candidates we can actually reuse.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

builtin/pack-objects.c

@@ -3940,14 +3940,19 @@ static int pack_options_allow_reuse(void)
 
 static int get_object_list_from_bitmap(struct rev_info *revs)
 {
+	struct bitmapped_pack *packs = NULL;
+	size_t packs_nr = 0;
+
 	if (!(bitmap_git = prepare_bitmap_walk(revs, 0)))
 		return -1;
 
 	if (pack_options_allow_reuse())
-		reuse_partial_packfile_from_bitmap(bitmap_git, &reuse_packfile,
+		reuse_partial_packfile_from_bitmap(bitmap_git, &packs,
+						   &packs_nr,
 						   &reuse_packfile_bitmap);
 
-	if (reuse_packfile) {
+	if (packs) {
+		reuse_packfile = packs[0].p;
 		reuse_packfile_objects = bitmap_popcount(reuse_packfile_bitmap);
 		if (!reuse_packfile_objects)
 			BUG("expected non-empty reuse bitmap");

pack-bitmap.c

@@ -2001,7 +2001,8 @@ static int bitmapped_pack_cmp(const void *va, const void *vb)
 }
 
 void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
-					struct packed_git **packfile_out,
+					struct bitmapped_pack **packs_out,
+					size_t *packs_nr_out,
 					struct bitmap **reuse_out)
 {
 	struct repository *r = the_repository;
@@ -2069,7 +2070,8 @@ void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
 	 * need to be handled separately.
 	 */
 	bitmap_and_not(result, reuse);
-	*packfile_out = packs[0].p;
+	*packs_out = packs;
+	*packs_nr_out = packs_nr;
 	*reuse_out = reuse;
 }
 

pack-bitmap.h

@@ -78,8 +78,9 @@ int test_bitmap_hashes(struct repository *r);
 struct bitmap_index *prepare_bitmap_walk(struct rev_info *revs,
 					 int filter_provided_objects);
 uint32_t midx_preferred_pack(struct bitmap_index *bitmap_git);
-void reuse_partial_packfile_from_bitmap(struct bitmap_index *,
-					struct packed_git **packfile,
+void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
+					struct bitmapped_pack **packs_out,
+					size_t *packs_nr_out,
 					struct bitmap **reuse_out);
 int rebuild_existing_bitmaps(struct bitmap_index *, struct packing_data *mapping,
 			     kh_oid_map_t *reused_bitmaps, int show_progress);