Ceph

In ObjectCacher::bh_read_finish, we have a nonsense snippet

      } else {
        bh->bl.substr_of(bl,
                         oldpos-bh->start(),
                         bh->length());
        mark_clean(bh);
      }

Here we're trying to map the offsets of an incoming OSD op reply's bufferlist with our BufferHeads. But oldpos==bh->start at all times. The correct offset calculation is "bh->start()-start" ("start" being a function parameter corresponding to bl's position within the object).

The patch is obviously simple enough, but I want tests verifying it as well.