[Bug 1424734] [NEW] Incorrect multipath device names in /etc/fstab

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Problem:
-------

The multipath installation inserts incorrect device names on /etc/fstab.

The multipath partitions have different device names during the installation 
and on the installed system (specifically, the disk-partition separator).

During installation: /dev/mapper/mpathXpY
On installed system: /dev/mapper/mpathX-partY


Impact:
------

The multipath partitions in /etc/fstab are not mountable/usable -- any partition
that is not specified by UUID (at least swap).

The system only boots because the root partition is specified by UUID, thus
not using the entry in /etc/fstab. 
The swap partition is not made effective as swap.

This is the /etc/fstab from installation:

	~ # grep mapper /target/etc/fstab
	/dev/mapper/mpath0p2 /               ext4    errors=remount-ro 0       1
	/dev/mapper/mpath0p3 none            swap    sw              0       0

And this is the environment in the installed system.

	$ grep mapper /etc/fstab
	/dev/mapper/mpath0p2 /               ext4    errors=remount-ro 0       1
	/dev/mapper/mpath0p3 none            swap    sw              0       0

	$ ls -1 /dev/mapper
	control
	mpath0
	mpath0-part1
	mpath0-part2
	mpath0-part3

	$ mount | grep ' / '
	/dev/mapper/mpath0-part2 on / type ext4 (rw,errors=remount-ro)

	$ grep root=UUID /proc/cmdline 
	BOOT_IMAGE=/boot/vmlinux-3.18.0-13-generic root=UUID=35c634b0-f667-48a8-84f0-a571aa96d103 ro

	$ cat /proc/swaps 
	Filename                                Type            Size    Used    Priority
	$


Cause:
-----

The multipath block devices are created/named by different programs on
installation and installed system, and each one uses a different disk-partition separator.

During installation: libparted (via parted-server)
On installed system: kpartx (via udev rules)

On libparted, The 'p' separator is hard-coded in these functions:
[libparted/arch/linux.c]

	_dm_add_partition():
		if (asprintf (&vol_name, "%sp%d", dev_name, part->num) == -1)

	_device_get_part_path():
		snprintf (result, result_len, "%sp%d", devpath, num);
 
On kpartx udev rules, the '-part' separator is used:
[/lib/udev/rules.d/95-kpartx.rules]

	ENV{DM_STATE}=="ACTIVE", ENV{DM_UUID}=="mpath-*", \
		RUN+="/sbin/kpartx -a -p -part /dev/$name"



Solution proposal:
-----------------

Ideally, one side or another should be 'fixed' (either libparted or
kpartx rules).

I wrote this contained work-around (patch attached) to a partman's finish.d/ trigger,
standing between each side, as I'm not sure of the fallout of either change.

It intercepts and changes the multipath device names inserted in
/etc/fstab.

It patches '/lib/partman/finish.d/40fstab_hd_entries' because that file already
has a 'case' including '/dev/mapper', so it seemed an appropriate place. 
The change is /in/ the addfstab() function, because the specified device must 
actually exist until it's called, as 'mapdevfs $dev' is an argument.


With it applied, this is the new /etc/fstab during installation:

	~ # grep mapper /target/etc/fstab 
	/dev/mapper/mpath0-part2 /               ext4    errors=remount-ro 0       1
	/dev/mapper/mpath0-part3 none            swap    sw              0       0


And on the installed system, the swap is available/functional now:

	$ grep mapper /etc/fstab
	/dev/mapper/mpath0-part2 /               ext4    errors=remount-ro 0       1
	/dev/mapper/mpath0-part3 none            swap    sw              0       0

	$ mount | grep ' / '
	/dev/mapper/mpath0-part2 on / type ext4 (rw,errors=remount-ro)

	$ cat /proc/swaps 
	Filename                                Type            Size    Used    Priority
	/dev/mapper/mpath0-part3                partition       5492672 0       -1


Steps to reproduce:
------------------

You can use qemu/kvm if you don't have multipath hardware handy, just specify
2 drives with the same serial number.  This is an example for ppc64el, easily
changeable for amd64 (change qemu-system & -M)

$ qemu-img create -f qcow2 vivid-multipath.qcow2 128g

$ qemu-system-ppc64 -M pseries \
	-enable-kvm \
	-nographic -nodefaults \
	-monitor stdio -serial pty \
	-m 2G \
	-net nic -net user \
	-drive file=vivid-multipath.qcow2,serial=abcdef \
	-drive file=vivid-multipath.qcow2,serial=abcdef \
	-kernel netboot/vmlinux \
	-initrd netboot/initrd.gz \
	-append 'disk-detect/multipath/enable=true grub-installer/multipath=true'

Note: to boot the installed system, remove the -kernel/-initrd/-append options.
      if the system takes long to boot in the initrd, use the 'debug' option
      in the kernel command line (current work-around for another problem).

** Affects: partman-target (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: patch
-- 
Incorrect multipath device names in /etc/fstab
https://bugs.launchpad.net/bugs/1424734
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