Description of problem:

# modprobe nbd max_part=16
# qemu-nbd -c /dev/nbd0 f23.qcow2 

# lsblk --fs /dev/nbd0
NAME     FSTYPE LABEL UUID MOUNTPOINT
nbd0                       
├─nbd0p1                   
├─nbd0p2                   
└─nbd0p3                   

the problem is that udev db does not contain ID_FS_* data for the partitions, but blkid is able to detect filesytems on the device, for example:

# blkid -p /dev/nbd0p1 -o udev
ID_FS_UUID=06b7c95e-edd0-4bf8-9053-6c8c04a4ceb2
ID_FS_UUID_ENC=06b7c95e-edd0-4bf8-9053-6c8c04a4ceb2
ID_FS_VERSION=1.0
ID_FS_TYPE=ext2
ID_FS_USAGE=filesystem
ID_PART_ENTRY_SCHEME=dos
ID_PART_ENTRY_UUID=c6c18c62-01
ID_PART_ENTRY_TYPE=0x83
ID_PART_ENTRY_FLAGS=0x80
ID_PART_ENTRY_NUMBER=1
ID_PART_ENTRY_OFFSET=2048
ID_PART_ENTRY_SIZE=409600
ID_PART_ENTRY_DISK=43:0


How reproducible:

udev during qemu-ndb --connect:

# udevadm monitor 
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

KERNEL[86215.879813] change   /devices/virtual/block/nbd0 (block)
KERNEL[86215.879858] add      /devices/virtual/block/nbd0/nbd0p1 (block)
KERNEL[86215.879902] add      /devices/virtual/block/nbd0/nbd0p2 (block)
KERNEL[86215.879937] add      /devices/virtual/block/nbd0/nbd0p3 (block)
UDEV  [86215.881516] change   /devices/virtual/block/nbd0 (block)
UDEV  [86215.882053] add      /devices/virtual/block/nbd0/nbd0p1 (block)
UDEV  [86215.882375] add      /devices/virtual/block/nbd0/nbd0p2 (block)
UDEV  [86215.882433] add      /devices/virtual/block/nbd0/nbd0p3 (block)


Additional info:

I guess the problem is logic around nbd in /usr/lib/udev/rules.d/99-systemd.rules:

# Ignore nbd devices in the "add" event, with "change" the nbd is ready
ACTION=="add", SUBSYSTEM=="block", KERNEL=="nbd*", ENV{SYSTEMD_READY}="0"

from the monitor it seems that the partitions are added by ADD event, maybe
the rule should be more careful and don't ignore partition...