Comment 19 for bug 1887558

I attempted to reproduce this issue in a VM and wasn't succesful. I essentially copied the tgtbasedmpaths test but made the backing disk bcache cache before setting up the targets.

1) create vm
$ lxc launch ubuntu-daily:noble --vm n-vm
$ lxc shell n-vm
# apt install -y lsscsi multipath-tools open-iscsi tgt

2) setup virtual multipathd disk
```
targetname="iqn.2016-11.foo.com:target.iscsi"
cwd=$(pwd)
testdir="/mnt/tgtmpathtest"
localhost="127.0.0.1"
portal="${localhost}:3260"
maxpaths=4
backfn="backingfile"
expectwwid="60000000000000000e00000000010001"
testdisk="/dev/disk/by-id/wwn-0x${expectwwid}"

### Setup mpath devices

# Restart tgtd to make sure modules are all loaded
service tgt restart || echo "Failed to restart tgt" >&2

# prep SINGLE test file
truncate --size 100M ${backfn}

make-bcache -C ${backfn}

# create target
tgtadm --lld iscsi --op new --mode target --tid 1 -T "${targetname}"
# allow all tomake-bcache -C ${backfn}
 bind the target
tgtadm --lld iscsi --op bind --mode target --tid 1 -I ALL
# set backing file
tgtadm --lld iscsi --op new --mode logicalunit --tid 1 --lun 1 -b "${cwd}/${backfn}"

# scan for targets (locally)
iscsiadm --mode discovery --type sendtargets --portal ${localhost}

# login
echo "login #1"
iscsiadm --mode node --targetname "${targetname}" --portal ${portal} --login
# duplicate this session (always 1)
for i in $(seq 2 ${maxpaths})
do
    echo "extra login #${i}"
    iscsiadm --mode session -r 1 --op new
done

udevadm settle
sleep 5 # sleep a bit to allow device to be created.
```

And can confirm the disks are using bcache FS by:

# udevadm info /dev/dm-0 | grep ID_FS_TYPE
E: ID_FS_TYPE=bcache

Things look fine and as they should to me
# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sda 8:0 0 10G 0 disk
├─sda1 8:1 0 9G 0 part /
├─sda14 8:14 0 4M 0 part
├─sda15 8:15 0 106M 0 part /boot/efi
└─sda16 259:0 0 913M 0 part /boot
sdb 8:16 0 100M 0 disk
└─mpatha 252:0 0 100M 0 mpath
sdc 8:32 0 100M 0 disk
└─mpatha 252:0 0 100M 0 mpath
sdd 8:48 0 100M 0 disk
└─mpatha 252:0 0 100M 0 mpath
sde 8:64 0 100M 0 disk
└─mpatha 252:0 0 100M 0 mpath
# multipath -ll
mpatha (360000000000000000e00000000010001) dm-0 IET,VIRTUAL-DISK
size=100M features='0' hwhandler='0' wp=rw
|-+- policy='service-time 0' prio=1 status=active
| `- 10:0:0:1 sde 8:64 active ready running
|-+- policy='service-time 0' prio=1 status=enabled
| `- 7:0:0:1 sdb 8:16 active ready running
|-+- policy='service-time 0' prio=1 status=enabled
| `- 8:0:0:1 sdc 8:32 active ready running
`-+- policy='service-time 0' prio=1 status=enabled
  `- 9:0:0:1 sdd 8:48 active ready running
# ls /dev/mapper
control mpatha

I'm not sure if I'm setting up this test incorrectly or not.

In fact if I run `udevadm test` I see the bcache udev rules fail.
# udevadm test /dev/mapper/mpatha
< cut here >
dm-0: /usr/lib/udev/rules.d/69-bcache.rules:21 RUN 'kmod load bcache'
dm-0: /usr/lib/udev/rules.d/69-bcache.rules:22 RUN 'bcache-register $tempnode'
dm-0: /usr/lib/udev/rules.d/69-bcache.rules:26 Importing properties from results of 'bcache-export-cached /dev/dm-0'
dm-0: Starting 'bcache-export-cached /dev/dm-0'
Successfully forked off '(spawn)' as PID 2197.
dm-0: Process 'bcache-export-cached /dev/dm-0' failed with exit code 1.
dm-0: /usr/lib/udev/rules.d/69-bcache.rules:26 Command "bcache-export-cached /dev/dm-0" returned 1 (error), ignoring
< cut here >