refact(updater): read df in POSIX format instead of counting columns backwards

df -P prints one line per filesystem whatever the device name is called, which
is what the counting from the end worked around. $4 says what $(NF-2) meant.

Four readers: disk_usage (-Ph), check_free_space (-P -B1), check_boot_reserve
(-Pk) and do-update-lxc.sh (-P -B1). init.sh and resources.sh keep --output=,
which asks for a single column and cannot wrap either.

The two that read a number now check that they got one. Without it a df that
does not know -P would print its usage to stderr and leave the parse empty, and
an empty string reads as zero free space in check_free_space, as a division by
zero in check_boot_reserve and as a skipped guard in do-update-lxc.sh. They
raise 100 naming the flag instead.

That is the open question about this change: -P is POSIX and GNU has it, but
the updater and the image factory are Alpine images with no coreutils, so their
df is busybox's, and busybox's support for -P is not verified here. The
installer factory already reads df -Pk (create_iso.sh:67) and is Debian-based,
so it proves nothing for the other two. One command in the factory settles it:

  ./tool.sh shell -> df -Pk /

The suite is at 57. Taking -P out of any of the four turns six cases red, and
dropping either value check turns one more - the mock answers a df without -P
the way a real one does, with the device name on a line of its own.

Signed-off-by: Simon Walz <contact@simonwalz.de>

Author: simonwalz

system/cuos/do-update-lxc.sh

@@ -30,9 +30,12 @@ if [[ "${IMAGE_VERSION_STRING}" == "$(cat /etc/image)" ]]; then
   raise_info 22 "No new image version available. Exiting."
 fi
 
-# Fields counted from the end: df puts a device name that does not fit on a
-# line of its own, and then the columns are one line further down.
-AVAIL_BYTES=$(df -B1 "/" | awk 'END { print $(NF-2) }')
+# -P: POSIX output, one line per filesystem. Without it df puts a device name
+# that does not fit on a line of its own and the columns move.
+AVAIL_BYTES=$(df -P -B1 "/" | awk 'NR == 2 { print $4 }')
+if [[ ! "${AVAIL_BYTES}" =~ ^[0-9]+$ ]]; then
+  raise 21 "Could not read the free space of /: df -P -B1 gave \"${AVAIL_BYTES}\""
+fi
 REQUIRED_BYTES=$((2 * 1024 * 1024 * 1024))
 if [ "$AVAIL_BYTES" -le "$REQUIRED_BYTES" ]; then
   report_info "cuos:update:no_space" "Not enough free disk space available"

updater/perform_update.sh

@@ -42,11 +42,11 @@ step() {
   echo "==> $*"
 }
 
-# Fields counted from the end: df puts a device name that does not fit on a
-# line of its own.
+# -P: POSIX output, one line per filesystem. Without it df puts a device name
+# that does not fit on a line of its own and the columns move.
 disk_usage() {
-  df -h "${1:-/}" \
-    | awk 'END { printf "%s of %s used, %s free\n", $(NF-3), $(NF-4), $(NF-2) }'
+  df -Ph "${1:-/}" \
+    | awk 'NR == 2 { printf "%s of %s used, %s free\n", $3, $2, $4 }'
 }
 
 report_disk_usage() {
@@ -101,7 +101,9 @@ check_environment() {
 check_free_space() {
   local avail_bytes required_bytes
 
-  avail_bytes="$(df -B1 "${TARGET_ROOT}" | awk 'END { print $(NF-2) }')"
+  avail_bytes="$(df -P -B1 "${TARGET_ROOT}" | awk 'NR == 2 { print $4 }')"
+  [[ "${avail_bytes}" =~ ^[0-9]+$ ]] \
+    || raise 100 "Could not read the free space of ${TARGET_ROOT}: df -P -B1 gave \"${avail_bytes}\""
   required_bytes="$((2 * 1024 * 1024 * 1024))"
 
   if [[ "${avail_bytes}" -le "${required_bytes}" ]]; then
@@ -119,13 +121,14 @@ check_free_space() {
 # 40 and not 50 percent: the second slot's kernel and initrd are a future
 # build, and they will not be smaller than today's.
 #
-# df -k, not -B1: in the factory this is busybox's df, which has no GNU
-# block-size options. Fields from the end, for a wrapped device name.
+# df -Pk, not -B1: in the factory this is busybox's df, which has no GNU
+# block-size options. -P keeps the columns on one line.
 check_boot_reserve() {
   local size used percent
 
-  read -r size used <<<"$(df -k "${TARGET_BOOT}" \
-    | awk 'END { print $(NF-4), $(NF-3) }')"
+  read -r size used <<<"$(df -Pk "${TARGET_BOOT}" | awk 'NR == 2 { print $2, $3 }')"
+  [[ "${size}" =~ ^[0-9]+$ && "${used}" =~ ^[0-9]+$ && "${size}" -gt 0 ]] \
+    || raise 100 "Could not read the usage of ${TARGET_BOOT}: df -Pk gave \"${size} ${used}\""
   percent="$(( used * 100 / size ))"
 
   if (( percent > 40 )); then

updater/perform_update.test.sh

@@ -65,31 +65,37 @@ btrfs() {
   esac
 }
 
+# POSIX output (df -P): one line per filesystem, however long the device name.
+# MOCK_DF_UNSUPPORTED stands for a df that does not know -P: usage to stderr,
+# nothing on stdout.
 df() {
-  case "${1}" in
-    -h)
-      echo "Filesystem                Size      Used Available Use% Mounted on"
-      echo "/dev/mapper/loop0p3       1.3G    733.8M    522.3M  58% ${2}"
+  local path="${*: -1}"
+
+  if [[ -n "${MOCK_DF_UNSUPPORTED-}" ]]; then
+    echo "df: unrecognized option: P" >&2
+    return 1
+  fi
+
+  case "$*" in
+    *-Ph*)
+      echo "Filesystem      Size  Used Avail Capacity Mounted on"
+      echo "/dev/disk/by-uuid/1a2b3c4d-0000-1111-2222-334455667788 1.3G 733.8M 522.3M 58% ${path}"
       ;;
-    -k)
+    *-Pk*)
       # The default is the boot partition of the x86_64 build of 2026-09-10:
       # 68.4 MiB of 252.7 MiB with one slot installed.
-      echo "Filesystem     1K-blocks      Used Available Use% Mounted on"
-      if [[ -n "${MOCK_DF_WRAPS-}" ]]; then
-        echo "/dev/disk/by-uuid/1a2b3c4d-0000-1111-2222-334455667788"
-        echo "               ${MOCK_SIZE_KB-258812} ${MOCK_USED_KB-70041} 188771  27% ${2}"
-      else
-        echo "/dev/mapper/loop0p2 ${MOCK_SIZE_KB-258812} ${MOCK_USED_KB-70041} 188771  27% ${2}"
-      fi
+      echo "Filesystem     1024-blocks      Used Available Capacity Mounted on"
+      echo "/dev/disk/by-uuid/1a2b3c4d-0000-1111-2222-334455667788 ${MOCK_SIZE_KB-258812} ${MOCK_USED_KB-70041} 188771 27% ${path}"
+      ;;
+    *-P*)
+      echo "Filesystem     1B-blocks       Used  Available Capacity Mounted on"
+      echo "/dev/disk/by-uuid/1a2b3c4d-0000-1111-2222-334455667788 1446256640 769654784 ${MOCK_AVAIL_BYTES-5368709120} 53% ${path}"
       ;;
     *)
+      # Without -P the device name gets a line of its own and the columns move.
       echo "Filesystem     1B-blocks       Used  Available Use% Mounted on"
-      if [[ -n "${MOCK_DF_WRAPS-}" ]]; then
-        echo "/dev/disk/by-uuid/1a2b3c4d-0000-1111-2222-334455667788"
-        echo "               1446256640  769654784 ${MOCK_AVAIL_BYTES-5368709120}  53% ${2}"
-      else
-        echo "/dev/mapper/loop0p3 1446256640  769654784 ${MOCK_AVAIL_BYTES-5368709120}  53% ${2}"
-      fi
+      echo "/dev/disk/by-uuid/1a2b3c4d-0000-1111-2222-334455667788"
+      echo "               1446256640  769654784 ${MOCK_AVAIL_BYTES-5368709120}  53% ${path}"
       ;;
   esac
 }
@@ -132,15 +138,15 @@ expect "product_name: neither" "CuOS" product_name
 export CONFIG_PATH="${WORK_DIR}/absent.json"
 expect "product_name: no config at all" "CuOS" product_name
 
+# The mock's device name is longer than the column, which is what -P is for.
 expect_rc "check_free_space: 5 GB is enough" 0 rc_of check_free_space
 MOCK_AVAIL_BYTES="1073741824"
 expect_rc "check_free_space: 1 GB is not" 101 rc_of check_free_space
 MOCK_AVAIL_BYTES="5368709120"
-# df puts a device name too long for the column on a line of its own, which
-# moves the columns of the line that carries them.
-MOCK_DF_WRAPS=1
-expect_rc "check_free_space: wrapped df line" 0 rc_of check_free_space
-MOCK_DF_WRAPS=""
+MOCK_DF_UNSUPPORTED=1
+expect_rc "check_free_space: a df without -P is not guessed at" 100 \
+  rc_of check_free_space
+MOCK_DF_UNSUPPORTED=""
 
 export IMAGE="ghcr.io/cuos-dev/cuos-system:development"
 export T_FILE_IMAGE="${WORK_DIR}/image"
@@ -200,9 +206,10 @@ MOCK_USED_KB=41000
 expect_rc "check_boot_reserve: 41% leaves the second slot too little" 115 \
   rc_of check_boot_reserve
 unset MOCK_SIZE_KB MOCK_USED_KB
-MOCK_DF_WRAPS=1
-expect_rc "check_boot_reserve: wrapped df line" 0 rc_of check_boot_reserve
-MOCK_DF_WRAPS=""
+MOCK_DF_UNSUPPORTED=1
+expect_rc "check_boot_reserve: a df without -P is not guessed at" 100 \
+  rc_of check_boot_reserve
+MOCK_DF_UNSUPPORTED=""
 
 expect_rc "check_environment: everything mounted" 0 rc_of check_environment
 MOCK_UNMOUNTED="${TARGET_ROOT}"