[udev] /dev/disk/by-path link for scsi disks always has host "0", causing conflicts

> It appears these numbers are pulled from ID_PATH, and I can't figure out where this variable comes from.

From what used to be /lib/udev/path_id, which is now built into libudev-private for performance reasons. You can do that part manually with:

sudo udevadm test-builtin path_id /sys/block/sr0

Reproducible with scsi_debug:

sudo modprobe scsi_debug removable=1 ptype=5

Gives:
P: /devices/pseudo_0/adapter0/host10/target10:0:0/10:0:0:0/block/sr0
N: sr0
E: DEVLINKS=/dev/cdrom /dev/disk/by-id/scsi-353333330000055f0 /dev/disk/by-id/wwn-0x53333330000055f0 /dev/disk/by-path/scsi-0:0:0:0
E: DEVNAME=/dev/sr0
E: DEVPATH=/devices/pseudo_0/adapter0/host10/target10:0:0/10:0:0:0/block/sr0

Debugging notes:

       name = udev_device_get_sysname(parent);
        if (sscanf(name, "%d:%d:%d:%d", &host, &bus, &target, &lun) != 4)
                return NULL;

This works fine, host is "10" in the scsi_debug case above.

But then the code below which does "Rebase host offset to get the local relative number" turns host into a "0" as 10 is the smallest host number on that device. This code is a bit dubious to me, I'll ask Kay.

Discussed with Kay on IRC:

kay | pitti: the host number is a simple kernel *enumeration* which depends on discovery order, it can never be used to compose *predictable* identifier
kay | pitti: that ID_PATH=scsi-0:0:1:0 should never happen

The summary is that path_id should fail instead of computing a non-unique ID_PATH for generic SCSI devices. IOW, the whole handle_scsi_default() should go away.

Fixed upstream: http://cgit.freedesktop.org/systemd/systemd/commit/?id=e98bbfd20

This bug was fixed in the package systemd - 208-8ubuntu2

---------------
systemd (208-8ubuntu2) utopic; urgency=medium

  [ Ben Howard ]
  * 75-persistent-net-generator.rules: Fix matches of HyperV. (LP: #1361272)

  [ Martin Pitt ]
  * Add new autopkgtest boot_and_services which reboots the testbed with
    systemd as init and verifies that the most critical services like lightdm,
    D-BUS, NetworkManager, and cron work. (To be extended in the future)
  * Fix udev path in udev(7). (LP: #1357782)
  * path_id: suppress ID_PATH for devices with an unknown parent device type.
    (LP: #1321816)
  * udev-builtin-keyboard: Allow numeric key codes. (LP: #1247584)
 -- Martin Pitt <email address hidden> Mon, 08 Sep 2014 14:44:35 +0200