Comment 5 for bug 1682456

To recreate:

1. partition two disks sized the same into two equal partitions (2G disk, two 1G partitions each)

# cat /proc/partitions
major minor #blocks name

 252 32 2097152 vdc
 252 33 1048576 vdc1
 252 34 1047552 vdc2
 252 48 2097152 vdd
 252 49 1048576 vdd1
 252 50 1047552 vdd2

2. sudo apt install mdadm --no-install-recommends

3. sudo bash -x ./create-mirror.sh

4. cat /proc/mdstat # check that the array's are resyncing

5. sudo mdadm --stop /dev/md1

6. test -e /sys/class/block/md1

If the arrays completely resync before attempting --stop; we don't see the failure;
however, if a resync is present then stopping md0 or md1 will succeed but
/sys/class/block/mdX/* contains files even if /dev/mdX is not present.