midx: teach midx_fanout_add_midx_fanout() about incremental MIDXs

The function `midx_fanout_add_midx_fanout()` is used to help construct
the fanout table when generating a MIDX by reusing data from an existing
MIDX.

Prepare this function to work with incremental MIDXs by making a few
changes:

  - The bounds checks need to be adjusted to start object lookups taking
    into account the number of objects in the previous MIDX layer (i.e.,
    by starting the lookups at position `m->num_objects_in_base` instead
    of position 0).

  - Likewise, the bounds checks need to end at `m->num_objects_in_base`
    objects after `m->num_objects`.

  - Finally, `midx_fanout_add_midx_fanout()` needs to recur on earlier
    MIDX layers when dealing with an incremental MIDX chain by calling
    itself when given a MIDX with a non-NULL `base_midx`.

Note that after 0c5a62f (midx-write.c: do not read existing MIDX with
`packs_to_include`, 2024-06-11), we do not use this function with an
existing MIDX (incremental or not) when generating a MIDX with
--stdin-packs, and likewise for incremental MIDXs.

But it is still used when adding the fanout table from an incremental
MIDX when generating a non-incremental MIDX (without --stdin-packs, of
course).

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

Author: ttaylorr

midx-write.c

@@ -196,7 +196,7 @@ static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
 				      struct pack_midx_entry *e,
 				      uint32_t pos)
 {
-	if (pos >= m->num_objects)
+	if (pos >= m->num_objects + m->num_objects_in_base)
 		return 1;
 
 	nth_midxed_object_oid(&e->oid, m, pos);
@@ -247,12 +247,16 @@ static void midx_fanout_add_midx_fanout(struct midx_fanout *fanout,
 					uint32_t cur_fanout,
 					int preferred_pack)
 {
-	uint32_t start = 0, end;
+	uint32_t start = m->num_objects_in_base, end;
 	uint32_t cur_object;
 
+	if (m->base_midx)
+		midx_fanout_add_midx_fanout(fanout, m->base_midx, cur_fanout,
+					    preferred_pack);
+
 	if (cur_fanout)
-		start = ntohl(m->chunk_oid_fanout[cur_fanout - 1]);
-	end = ntohl(m->chunk_oid_fanout[cur_fanout]);
+		start += ntohl(m->chunk_oid_fanout[cur_fanout - 1]);
+	end = m->num_objects_in_base + ntohl(m->chunk_oid_fanout[cur_fanout]);
 
 	for (cur_object = start; cur_object < end; cur_object++) {
 		if ((preferred_pack > -1) &&