docker cp causes umount/mount attempt that corrupts data

[untoldone]

If you mount a volume using this plugin and then try to use docker cp on a file within the container -- even if not from within the mounted volume, you get a message such as:

Error response from daemon: error while mounting volume '/mnt/temp-testing': VolumeDriver.Mount: mount failed, arguments: ["nsenter" "--mount=/rootfs/proc/1/ns/mnt" "--" "mount" "-t" "ext4" "-o" "discard,defaults" "/dev/disk/by-id/google-docker-volume-temp-testing" "/mnt/temp-testing"]
output: mount: /dev/sdb is already mounted or /mnt/temp-testing busy
       /dev/sdb is already mounted on /mnt/temp-testing

Once running docker cp on the container, you'll find the volume has been detached from the GCP instance but the container thinks the volume is still mounted. The forced removal has caused data loss and db corruption in my case.

[untoldone]

Note it appears other volume plugins have had similar issues: NetApp/netappdvp#93

[untoldone]

Related thread on Moby: moby/moby#34665

[untoldone]

For folks that see this and need a fix while waiting for the pull request to be reviewed -- I've pushed a docker image to untoldone/gce-docker