rpi-resizerootfs: switch the root filesystem resizing away from a systemd oneshot service

Switch away from using a systemd service for the initial root resize.
Instead, we resize the root partition and filesystem in the initrd.

To simplify things, the initrd script will check whether it should resize
the partition on every boot. It does this by checking if the entire disk
(ignoring an empty 4MB) is in use.  However, the scripts themselves are
deleted from the system after the initrd is generated. After the image
is installed, the resize script should exist only in the initrd. When the
kernel gets upgraded (eg, for a security update) or a new initrd is generated
due to a package install, the new initrd will not contain the resize script.
At that point, nothing will remain from the image's initial resize
bootstrapping process.

This process (but not the scripts) is similar to what cloud-initramfs-growroot
does. However, that particular package has an indirect dependency on Python,
and we don't necessarily want that overhead in our images just for resizing.

raspi_master.yaml

@@ -59,6 +59,16 @@ steps:
       __OTHER_APT_ENABLE__
     unless: rootfs_unpacked
 
+  - copy-file: /etc/initramfs-tools/hooks/rpi-resizerootfs
+    src: rootfs/etc/initramfs-tools/hooks/rpi-resizerootfs
+    perm: 0755
+    unless: rootfs_unpacked
+
+  - copy-file: /etc/initramfs-tools/scripts/local-bottom/rpi-resizerootfs
+    src: rootfs/etc/initramfs-tools/scripts/local-bottom/rpi-resizerootfs
+    perm: 0755
+    unless: rootfs_unpacked
+
   - chroot: /
     shell: |
       apt-get update
@@ -97,10 +107,9 @@ steps:
       mkdir -p "${ROOT?}/etc/systemd/system/basic.target.requires/"
       ln -s /etc/systemd/system/rpi-set-sysconf.service "${ROOT?}/etc/systemd/system/basic.target.requires/rpi-set-sysconf.service"
 
-      install -m 755 -o root -g root rootfs/usr/sbin/rpi-resizerootfs "${ROOT?}/usr/sbin/rpi-resizerootfs"
-      install -m 644 -o root -g root rootfs/etc/systemd/system/rpi-resizerootfs.service "${ROOT?}/etc/systemd/system/"
-      mkdir -p "${ROOT?}/etc/systemd/system/systemd-remount-fs.service.requires/"
-      ln -s /etc/systemd/system/rpi-resizerootfs.service "${ROOT?}/etc/systemd/system/systemd-remount-fs.service.requires/rpi-resizerootfs.service"
+      # Resize script is now in the initrd for first boot; no need to ship it.
+      rm -f "${ROOT?}/etc/initramfs-tools/hooks/rpi-resizerootfs"
+      rm -f "${ROOT?}/etc/initramfs-tools/scripts/local-bottom/rpi-resizerootfs"
 
       install -m 644 -o root -g root rootfs/etc/systemd/system/rpi-reconfigure-raspi-firmware.service "${ROOT?}/etc/systemd/system/"
       mkdir -p "${ROOT?}/etc/systemd/system/multi-user.target.requires/"

rootfs/etc/initramfs-tools/hooks/rpi-resizerootfs

@@ -0,0 +1,23 @@
+#!/bin/sh
+set -e
+
+#
+# List the soft prerequisites here.  This is a space separated list of
+# names, of scripts that are in the same directory as this one, that
+# must be run before this one can be.
+#
+PREREQS=""
+case $1 in
+    prereqs) echo "$PREREQS"; exit 0;;
+esac
+
+. /usr/share/initramfs-tools/hook-functions
+
+# XXX: tail and realpath are included by default, right?
+#copy_exec /usr/bin/realpath
+#copy_exec /usr/bin/tail
+copy_exec /sbin/blkid
+copy_exec /bin/lsblk
+copy_exec /sbin/sfdisk
+copy_exec /sbin/partprobe
+copy_exec /sbin/resize2fs

rootfs/etc/initramfs-tools/scripts/local-bottom/rpi-resizerootfs

@@ -0,0 +1,50 @@
+#!/bin/sh
+set -e
+
+#
+# List the soft prerequisites here.  This is a space separated list of
+# names, of scripts that are in the same directory as this one, that
+# must be run before this one can be.
+#
+PREREQS=""
+case $1 in
+    prereqs) echo "$PREREQS"; exit 0;;
+esac
+
+. /scripts/functions
+
+# Given the root partition, get the underlying device and partition number
+rootpart=$(realpath $ROOT)
+rootpart_nr=$(blkid -sPART_ENTRY_NUMBER -o value -p $rootpart)
+rootdev="/dev/$(lsblk -no pkname "$rootpart")"
+
+# Check if there's free space on the device (note: we align the first
+# partition at 4MB, so there's always at least 3MB free)
+free_space="$(sfdisk -qF $rootdev | tail -n1 | grep -v [^0-9]3M)"
+if test -z "$free_space"; then
+    # Great, we already resized; nothing left to do!
+    exit 0
+fi
+
+log_begin_msg "$0 resizing $ROOT"
+
+# Unmount for safety
+umount "${rootmnt}"
+
+# Expand the partition size to fill the entire device
+sfdisk -f $rootdev -N $rootpart_nr <<EOF
+,+
+EOF
+
+wait_for_udev 5
+
+# Now resize the filesystem
+partprobe $rootdev
+resize2fs $rootpart
+
+# Remount root
+if ! mount -r ${FSTYPE:+-t "${FSTYPE}"} ${ROOTFLAGS} "${ROOT}" "${rootmnt?}"; then
+    panic "Failed to mount ${ROOT} as root file system."
+fi
+
+log_end_msg

rootfs/etc/systemd/system/rpi-resizerootfs.service

@@ -1,13 +0,0 @@
-[Unit]
-Description=resize root file system
-Before=local-fs-pre.target
-DefaultDependencies=no
-
-[Service]
-Type=oneshot
-TimeoutSec=infinity
-ExecStart=/usr/sbin/rpi-resizerootfs
-ExecStart=/bin/systemctl --no-reload disable %n
-
-[Install]
-RequiredBy=local-fs-pre.target

rootfs/usr/sbin/rpi-resizerootfs

@@ -1,22 +0,0 @@
-#!/bin/sh
-
-rootpart="$(findmnt -n -o SOURCE /)"
-rootdev="/dev/$(lsblk -no pkname "$rootpart")"
-
-flock $rootdev sfdisk -f $rootdev -N 2 <<EOF
-,+
-EOF
-
-sleep 5
-
-udevadm settle
-
-sleep 5
-
-flock $rootdev partprobe $rootdev
-
-mount -o remount,rw $rootpart
-
-resize2fs $rootpart
-
-exit 0