breaks systemd-tmpfiles-setup.service

[mrvn]

When I boot a live filesystem for centos7 that creates a unionfs-fuse for / the systemd-tmpfiles-setup service fails:

unionfs script (set -x output)

+ FUSE_OPT='-s -o default_permissions -o allow_other -o use_ino -o nonempty -o suid -o big_writes -o max_write=131072'
+ UNION_OPT='-o cow,chroot=/unionfs,max_files=32768,noinitgroups'
+ ulimit -n 32768
+ /sbin/modprobe fuse
+ /bin/mkdir -p /unionfs/ramdisk
+ /bin/mount --bind / /unionfs/ramdisk
+ /bin/mkdir -p /unionfs/nfs
+ /bin/mkdir -p /unionfs/union
+ /usr/bin/unionfs -s -o default_permissions -o allow_other -o use_ino -o nonempty -o suid -o big_writes -o max_write=131072 -o cow,chroot=/unionfs,max_files=32768,noinitgroups /ramdisk=RW:/nfs=RO /unionfs/union
+ mount --bind /unionfs /unionfs/union/unionfs
+ mount --bind /dev /unionfs/union/dev
+ cd /unionfs/union
+ /bin/mkdir oldroot
+ /sbin/pivot_root . oldroot
+ exec /sbin/chroot . /lib/systemd/systemd

[FAILED] Failed to start Create Volatile Files and Directories.
See 'systemctl status systemd-tmpfiles-setup.service' for details.

% systemctl status systemd-tmpfiles-setup.service -l
● systemd-tmpfiles-setup.service - Create Volatile Files and Directories
   Loaded: loaded (/usr/lib/systemd/system/systemd-tmpfiles-setup.service; static; vendor preset: disabled)
   Active: failed (Result: exit-code) since Fri 2017-06-23 15:33:13 CEST; 2min 11s ago
     Docs: man:tmpfiles.d(5)
           man:systemd-tmpfiles(8)
  Process: 406 ExecStart=/usr/bin/systemd-tmpfiles --create --remove --boot --exclude-prefix=/dev (code=exited, status=1/FAILURE)
 Main PID: 406 (code=exited, status=1/FAILURE)

Jun 23 15:33:13 beo-01 systemd[1]: Starting Create Volatile Files and Directories...
Jun 23 15:33:13 beo-01 systemd-tmpfiles[406]: Failed to create directory or subvolume "/var/tmp": Invalid argument
Jun 23 15:33:13 beo-01 systemd-tmpfiles[406]: Failed to create directory or subvolume "/var/lib/machines": Invalid argument
Jun 23 15:33:13 beo-01 systemd-tmpfiles[406]: Failed to create directory or subvolume "/tmp": Invalid argument
Jun 23 15:33:13 beo-01 systemd[1]: systemd-tmpfiles-setup.service: main process exited, code=exited, status=1/FAILURE
Jun 23 15:33:13 beo-01 systemd[1]: Failed to start Create Volatile Files and Directories.
Jun 23 15:33:13 beo-01 systemd[1]: Unit systemd-tmpfiles-setup.service entered failed state.
Jun 23 15:33:13 beo-01 systemd[1]: systemd-tmpfiles-setup.service failed.

Without the unionfs everything works. The interesting thing is that the 3 entries that fail (there are many more) are the only ones where systemd is supposed to create a btrfs subvolume if the FS is btrfs. In strace I do see the ioctl for that, which gives an error. But it seems not the error systemd expects and (from googling old systemd issues) only a ENOTTY error will make it fall back to creating a directory.