Re: [PATCH 12/13] pack-bitmap: implement BLOB_LIMIT filtering

Jeff King [off-list ref] writes:

Just as the previous commit implemented BLOB_NONE, we can support
BLOB_LIMIT filters by looking at the sizes of any blobs in the result
and unsetting their bits as appropriate. This is slightly more expensive
than BLOB_NONE, but still produces a noticeable speedup (these results
are on git.git):

  Test                                        HEAD~2            HEAD
  ------------------------------------------------------------------------------------
  5310.7: rev-list count with blob:none       1.80(1.77+0.02)   0.22(0.20+0.02) -87.8%
  5310.8: rev-list count with blob:limit=1k   1.99(1.96+0.03)   0.29(0.25+0.03) -85.4%

That's respectable improvement.  packed_object_info() that asks only
for the inflated size is quite cheap when the object is a delta, and
hopefully we have more deltified blobs than deflated ones.

+static unsigned long get_size_by_pos(struct bitmap_index *bitmap_git,
+				     uint32_t pos)
+{
+	struct packed_git *pack = bitmap_git->pack;
+	unsigned long size;
+	struct object_info oi = OBJECT_INFO_INIT;
+
+	oi.sizep = &size;
+
+	if (pos < pack->num_objects) {
+		struct revindex_entry *entry = &pack->revindex[pos];
+		if (packed_object_info(the_repository, pack,
+				       entry->offset, &oi) < 0) {
+			struct object_id oid;
+			nth_packed_object_oid(&oid, pack, entry->nr);
+			die(_("unable to get size of %s"), oid_to_hex(&oid));
+		}
+	} else {
+		struct eindex *eindex = &bitmap_git->ext_index;
+		struct object *obj = eindex->objects[pos - pack->num_objects];
+		if (oid_object_info_extended(the_repository, &obj->oid, &oi, 0) < 0)
+			die(_("unable to get size of %s"), oid_to_hex(&obj->oid));
+	}
+
+	return size;
+}