40ignition-ostree: Initial support for rootfs replacement

This adds basic infrastructure units for "re-provisioning"
the root filesystem.  See:
coreos/fedora-coreos-tracker#94

A unit first detects if the Ignition configuration has a filesystem
with the label `root` - if so, we save the rootfs into RAM, let
`ignition-disks.service` run, then restore it from RAM.

Earlier attempts used the `brd` kernel module which is a RAM-backed
block device so we can just `dd`.  However, this has some limitations,
such as the need to save the full disk in RAM, and the inability for any
other initrd code to use `brd` devices. As well, `brd` doesn't support
discards, so we require at minimum $rootfs_size RAM (e.g. 3G) until
reprovisioning is complete.

Future work here will likely move the `restore` phase into `rpm-ostree`.

Co-authored-by: Jonathan Lebon <jonathan@jlebon.com>

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/ignition-ostree-dracut-rootfs.sh

@@ -0,0 +1,36 @@
+#!/bin/bash
+set -euo pipefail
+
+# This is implementation details of Ignition; in the future, we should figure
+# out a way to ask Ignition directly whether there's a filesystem with label
+# "root" being set up.
+ignition_cfg=/run/ignition.json
+rootdisk=/dev/disk/by-label/root
+saved_sysroot=/run/ignition-ostree-rootfs
+
+case "${1:-}" in
+    detect)
+        wipes_root=$(jq '.storage?.filesystems? // [] | map(select(.label == "root" and .wipeFilesystem == true)) | length' "${ignition_cfg}")
+        if [ "${wipes_root}" = "0" ]; then
+            exit 0
+        fi
+        echo "Detected rootfs replacement in fetched Ignition config: /run/ignition.json"
+        mkdir "${saved_sysroot}"
+        ;;
+    save)
+        mount "${rootdisk}" /sysroot
+        echo "Moving rootfs to RAM..."
+        cp -aT /sysroot "${saved_sysroot}"
+        ;;
+    restore)
+        # This one is in a private mount namespace since we're not "offically" mounting
+        mount "${rootdisk}" /sysroot
+        echo "Restoring rootfs from RAM..."
+        cd "${saved_sysroot}"
+        find . -mindepth 1 -maxdepth 1 -exec mv -t /sysroot {} \;
+        chattr +i $(ls -d /sysroot/ostree/deploy/*/deploy/*/)
+        ;;
+    *)
+        echo "Unsupported operation: ${1:-}" 1>&2; exit 1
+        ;;
+esac

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/ignition-ostree-rootfs-detect.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Ignition OSTree: detect rootfs replacement
+DefaultDependencies=false
+After=ignition-fetch.service
+Before=ignition-disks.service
+Before=initrd-root-fs.target
+Before=sysroot.mount
+ConditionKernelCommandLine=ostree
+
+# This stage requires udevd to detect disks
+Requires=systemd-udevd.service
+After=systemd-udevd.service
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+EnvironmentFile=/run/ignition.env
+ExecStart=/usr/libexec/ignition-ostree-dracut-rootfs detect

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/ignition-ostree-rootfs-restore.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=Ignition OSTree: restore rootfs
+DefaultDependencies=false
+After=ignition-disks.service
+Before=ignition-ostree-growfs.service
+Before=ignition-ostree-mount-firstboot-sysroot.service
+
+ConditionKernelCommandLine=ostree
+ConditionPathIsDirectory=/run/ignition-ostree-rootfs
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+EnvironmentFile=/run/ignition.env
+# So we can transiently mount sysroot
+MountFlags=slave
+ExecStart=/usr/libexec/ignition-ostree-dracut-rootfs restore

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/ignition-ostree-rootfs-save.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Ignition OSTree: save rootfs
+DefaultDependencies=false
+After=ignition-ostree-rootfs-detect.service
+Before=ignition-disks.service
+ConditionKernelCommandLine=ostree
+ConditionPathIsDirectory=/run/ignition-ostree-rootfs
+# Any services looking at mounts need to order after this
+# because it causes device re-probing.
+After=coreos-gpt-setup.service
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+EnvironmentFile=/run/ignition.env
+# So we can transiently mount sysroot
+MountFlags=slave
+ExecStart=/usr/libexec/ignition-ostree-dracut-rootfs save

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/module-setup.sh

@@ -53,7 +53,8 @@ install() {
         rm        \
         sed       \
         sfdisk    \
-        sgdisk
+        sgdisk    \
+        find
 
     for x in mount populate; do
         install_ignition_unit ignition-ostree-${x}-var.service
@@ -66,6 +67,12 @@ install() {
     inst_simple "$moddir/multipath-generator" \
         "$systemdutildir/system-generators/multipath-generator"
 
+    inst_multiple jq chattr
+    inst_script "$moddir/ignition-ostree-dracut-rootfs.sh" "/usr/libexec/ignition-ostree-dracut-rootfs"
+    for x in detect save restore; do
+        install_ignition_unit ignition-ostree-rootfs-${x}.service
+    done
+
     # Disk support
     install_ignition_unit ignition-ostree-mount-firstboot-sysroot.service diskful
     install_ignition_unit ignition-ostree-mount-subsequent-sysroot.service diskful-subsequent