=== Attempt to mount a CRC-valid HAMMER2 image with volu_size=0 ===
(crafted by craft_zero.py from a real newfs_hammer2 image; all three volume-header CRCs recomputed)

+ uname -a:
DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul  2 06:02:54 UTC 2026     root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC  x86_64

+ /sbin/mount_hammer2 /dev/vn0@DATA /mnt/h2 :
mount: Invalid argument
RC=1

+ dmesg excerpt (kernel rejection at hammer2_ondisk.c:292):
hammer2_mount: device="/dev/vn0" label="DATA" rdonly=0
hammer2_ondisk: "/dev/vn0" zone=0 id=0 offset=0x0000000000000000 size=0x0000000000000000
hammer2_ondisk: /dev/vn0 has size of 0
hammer2_mount: devstr="/dev/vn0@DATA"
hammer2_mount: device="/dev/vn0" label="DATA" rdonly=0
hammer2_ondisk: "/dev/vn0" zone=0 id=0 offset=0x0000000000000000 size=0x0000000000000000
hammer2_ondisk: /dev/vn0 has size of 0