Create /dev/disk/* symlink to boot disk, for use by Ignition config

[bgilbert]

(apologies for writing a JIRA card; actually I copypasted this)

As a user configuring disk partitions via FCC,
I want a stable device path for the boot disk,
so I can write a storage.disks section without hardcoding /dev/sda, /dev/vda, or /dev/nvme0n1.

Acceptance criteria:

• Create /dev/disk/... symlink for disk containing root filesystem when there's exactly one such disk, and create no such symlink in other cases (e.g. rootfs RAID)
• Confirm symlink doesn't propagate to the real root
• Possibly add an FCCT validation to check for symlink being used outside storage.disks.device context (to prevent adding a mount unit pointing to the symlink)
• FCOS and RHCOS docs updated to explicitly resize root partition when rootfs LUKS configured without RAID
• FCOS and RHCOS docs updated to use stable symlink when configuring /var partition on boot disk
• kola test for stable symlink added

[bgilbert]

I don't think udev rules natively have a way to take action based on the properties of a child device. In principle we could shell out to a script, though. However: if we do this with udev, race conditions could be a concern. At some point during ignition-disks, the boot device will no longer have a recognizable rootfs on it, and udev will remove the symlink. Ignition device aliases should handle this, if the symlink is only used to partition the disk. But if the config tries to reuse the symlink e.g. to put a filesystem on the whole-disk device for the original boot disk, it may fail.

We might want to create the symlink programmatically from a systemd service, instead. That would avoid any possible races. Plus, we're used to sequencing systemd units in the initrd, and less used to debugging udev interactions.

[cgwalters]

This is also related to...some tracker issue where we discussed discovering the boot disk and how for us that boils down to our GRUB config hardcoding LABEL=boot. If this is about before the root partition might be moved by Ignition, then we can create this in early boot exactly once, right?

Because this refers to the root label which is FCOS specific it'd live in f-c-c git?

Agree that systemd unit makes the most sense; we can just explicitly sequence it where we want.

[bgilbert]

If this is about before the root partition might be moved by Ignition, then we can create this in early boot exactly once, right?

Right.

Because this refers to the root label which is FCOS specific it'd live in f-c-c git?

Yep.

[jlebon]

Fixed in coreos/fedora-coreos-config#1308.

[dustymabe]

Would be cool to add some docs for this.

[jlebon]

Would be cool to add some docs for this.

Opened coreos/fedora-coreos-docs#353 so we don't lose that.

[dustymabe]

The fix for this went into testing stream release 35.20220213.2.0. Please try out the new release and report issues.

[travier]

I think this one went into stable 35.20220213.3.0. @dustymabe?

[dustymabe]

The fix for this went into stable stream release 35.20220213.3.0.