ceph-disk: partprobe should block udev induced BLKRRPART

ceph-disk: partprobe should block udev induced BLKRRPART

Wrap partprobe with flock to stop udev from issuing BLKRRPART because
this is racy and frequently fails with a message like:

    Error: Error informing the kernel about modifications to partition
    /dev/vdc1 -- Device or resource busy.  This means Linux won't know about
    any changes you made to /dev/vdc1 until you reboot -- so you shouldn't
    mount it or use it in any way before rebooting.

Opening a device (/dev/vdc for instance) in write mode indirectly
triggers a BLKRRPART ioctl from udev (starting version 214 and up)
when the device is closed (see below for the udev release note).

However, if udev fails to acquire an exclusive lock (with
flock(fd, LOCK_EX|LOCK_NB); ) the BLKRRPART ioctl is not issued.

https://github.com/systemd/systemd/blob/045e00cf16c47bc516c0823d059b7548f3ce9c7c/src/udev/udevd.c#L1042

Acquiring an exclusive lock before running the process that opens the
device in write mode is therefore an effective way to control this
behavior.

git clone git://anonscm.debian.org/pkg-systemd/systemd.git
systemd/NEWS:
CHANGES WITH 214:

  * As an experimental feature, udev now tries to lock the
       disk device node (flock(LOCK_SH|LOCK_NB)) while it
       executes events for the disk or any of its partitions.
       Applications like partitioning programs can lock the
       disk device node (flock(LOCK_EX)) and claim temporary
       device ownership that way; udev will entirely skip all event
       handling for this disk and its partitions. If the disk
       was opened for writing, the close will trigger a partition
       table rescan in udev's "watch" facility, and if needed
       synthesize "change" events for the disk and all its partitions.
       This is now unconditionally enabled, and if it turns out to
       cause major problems, we might turn it on only for specific
       devices, or might need to disable it entirely. Device Mapper
       devices are excluded from this logic.

Fixes: http://tracker.ceph.com/issues/15176
Signed-off-by: Marius Vollmer <marius.vollmer@redhat com>
Signed-off-by: Loic Dachary <loic@dachary.org>