overlay.d: create symlink to the disk holding root partition

Issue: coreos/fedora-coreos-tracker#759

Signed-off-by: Nikita Dubrovskii <nikita@linux.ibm.com>

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-root-symlink.service

@@ -0,0 +1,23 @@
+[Unit]
+Description=Create symlink to boot disk
+ConditionPathExists=/etc/initrd-release
+DefaultDependencies=no
+Wants=systemd-udevd.service
+After=systemd-udevd.service
+
+Requires=dev-disk-by\x2dlabel-root.device
+After=dev-disk-by\x2dlabel-root.device
+# And since the boot device may be on multipath; optionally wait for it to
+# appear via the dynamic target.
+After=coreos-multipath-wait.target
+After=coreos-gpt-setup.service
+Before=ignition-disks.service
+
+OnFailure=emergency.target
+OnFailureJobMode=isolate
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/sbin/create-root-symlink
+

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-teardown-initramfs.sh

@@ -208,6 +208,9 @@ main() {
     # clean it up so that no information from outside of the
     # real root is passed on to NetworkManager in the real root
     rm -rf /run/NetworkManager/
+
+    # Remove symlink to disk holding 'root' partition
+    rm -f /dev/disk/by-label/coreos-root-disk
 }
 
 main

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/create-root-symlink.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+# creates a stable device path for the boot disk when there's exactly one such disk,
+# and create no such symlink in other cases (e.g. rootfs RAID)
+# Symlink is not propogated to the real root (removed during coreos-teardown-initramfs)
+
+set -euo pipefail
+
+# force kernel to update info about disks
+for dev in $(lsblk -o NAME --noheadings --nodeps --list --paths); do
+    partx -u ${dev}
+done
+
+udevadm trigger
+udevadm settle
+
+LINK=/dev/disk/coreos-root-disk
+PARTITIONS=()
+# skip multipath disks (with same WWN)
+mapfile -t PARTITIONS < <(lsblk -o WWN,LABEL,NAME,PKNAME --pairs --paths --noheadings | grep 'LABEL="root"' | sed 's/WWN=\"\" //' | sort -u -k1,2)
+
+LENGTH=${#PARTITIONS[@]}
+if [[ ${LENGTH} -eq 1 ]]; then
+    eval ${PARTITIONS[0]}
+    echo "Creating symlink '${LINK}' to '${PKNAME}' (contains 'root' partition '${NAME}')"
+    mkdir -p /dev/disk/
+    ln -s ${PKNAME} ${LINK}
+fi

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/module-setup.sh

@@ -22,6 +22,9 @@ install() {
         diff \
         lsblk \
         sed \
+        sort \
+        grep \
+        partx \
         sgdisk
 
     inst_simple "$moddir/coreos-diskful-generator" \
@@ -30,6 +33,9 @@ install() {
     inst_script "$moddir/coreos-gpt-setup.sh" \
         "/usr/sbin/coreos-gpt-setup"
 
+    inst_script "$moddir/create-root-symlink.sh" \
+        "/usr/sbin/create-root-symlink"
+
     inst_script "$moddir/coreos-ignition-setup-user.sh" \
         "/usr/sbin/coreos-ignition-setup-user"
 
@@ -56,4 +62,7 @@ install() {
         "ignition-diskful.target"
 
     install_ignition_unit coreos-ignition-setup-user.service
+
+    install_ignition_unit coreos-root-symlink.service \
+        ignition-diskful.target
 }