Bug#884719: mdadm-grow-continue service doesn't work if --backup-file is specified

[Matthew Gabeler-Lee]

Package: mdadm
Version: 3.4-4+b1
Severity: normal

Trying to use mdadm to grow a raid5 array from 2 to 4 devices, I started it
with the --backup-file argument to ensure integrity.  This seems to have
caused mdadm-grow-continue at ...service to have failed:

● mdadm-grow-continue at md123.service - Manage MD Reshape on /dev/md123
   Loaded: loaded (/lib/systemd/system/mdadm-grow-continue at .service; static; vendor preset: enabled)
   Active: failed (Result: exit-code) since Sun 2017-12-17 16:20:11 EST; 19h ago
 Main PID: 10875 (code=exited, status=2)

Dec 17 16:20:10 hostname.fqdn systemd[1]: Started Manage MD Reshape on /dev/md123.
Dec 17 16:20:11 hostname.fqdn systemd[1]: mdadm-grow-continue at md123.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Dec 17 16:20:11 hostname.fqdn systemd[1]: mdadm-grow-continue at md123.service: Unit entered failed state.
Dec 17 16:20:11 hostname.fqdn systemd[1]: mdadm-grow-continue at md123.service: Failed with result 'exit-code'.

And the grow seemed to freeze at having reshaped a single block.  Manually
running mdadm --grow --continue /dev/md123 --backup-file ...  got things
running again.

-- System Information:
Debian Release: 9.3
  APT prefers stable
  APT policy: (990, 'stable'), (500, 'testing'), (500, 'oldstable'), (490, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.9.0-4-amd64 (SMP w/12 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages mdadm depends on:
ii  debconf [debconf-2.0]  1.5.61
ii  libc6                  2.24-11+deb9u1
ii  lsb-base               9.20161125
ii  udev                   232-25+deb9u1

Versions of packages mdadm recommends:
ii  exim4-daemon-light [mail-transport-agent]  4.89-2+deb9u2
ii  kmod                                       23-2

mdadm suggests no packages.

-- Configuration Files:
/etc/cron.daily/mdadm changed [not included]

-- debconf information excluded

-- debsums errors found:
debsums: changed file /lib/systemd/system/mdmonitor.service (from mdadm package)