refactor: lift FILES= keyfile sed to ensure_initramfs_files helper

btrfs.sh's configure_btrfs_initramfs had a six-line inline that ensured mkinitcpio.conf's FILES= line listed the LUKS keyfile: sed-replace the existing FILES= line, then grep + append as a fallback when no FILES= line existed. The pattern is mkinitcpio-specific and self-healing rather than error-on-miss (FILES= is optional in mkinitcpio.conf, so missing means "no extra files," not a broken config).

I lifted the block into ensure_initramfs_files in lib/common.sh next to prepend_grub_cmdline_linux, then collapsed the btrfs.sh call site to a single ensure_initramfs_files line. Added three bats tests for the three cases (FILES= present and empty, FILES= present with a different value, FILES= absent entirely). Bats: 174 → 177.

No behavior change. The helper's logic matches the inline byte-for-byte: same sed pattern, same grep fallback, same final state.

3 files changed, 65 insertions, 5 deletions

diff --git a/installer/lib/btrfs.sh b/installer/lib/btrfs.sh
index 0715ba7..0a34be0 100644
--- a/installer/lib/btrfs.sh
+++ b/installer/lib/btrfs.sh
@@ -212,11 +212,7 @@ configure_luks_initramfs() {
     # Include keyfile in initramfs for testing mode (unattended boot)
     if [[ "${TESTING:-}" == "yes" ]]; then
         info "Testing mode: embedding keyfile in initramfs"
-        sed -i "s|^FILES=.*|FILES=($LUKS_KEYFILE)|" $MNTPOINT/etc/mkinitcpio.conf
-        # If FILES line doesn't exist, add it
-        if ! grep -q "^FILES=" $MNTPOINT/etc/mkinitcpio.conf; then
-            echo "FILES=($LUKS_KEYFILE)" >> $MNTPOINT/etc/mkinitcpio.conf
-        fi
+        ensure_initramfs_files "$LUKS_KEYFILE" "$MNTPOINT/etc/mkinitcpio.conf"
     fi
 
     # Create crypttab.initramfs for sd-encrypt (used by multi-disk LUKS)
diff --git a/installer/lib/common.sh b/installer/lib/common.sh
index e5f7246..2cd4798 100644
--- a/installer/lib/common.sh
+++ b/installer/lib/common.sh
@@ -328,3 +328,20 @@ prepend_grub_cmdline_linux() {
     grep -qF "GRUB_CMDLINE_LINUX=\"${addition}" "$config_file" \
         || error "GRUB_CMDLINE_LINUX not modified in $config_file (line missing or pattern unmatched)"
 }
+
+#############################
+# Initramfs Configuration
+#############################
+
+# Ensure mkinitcpio.conf's FILES= line lists the given value. Replaces
+# an existing FILES= line, or appends one if absent. Self-healing
+# rather than error-on-miss: FILES= is optional in mkinitcpio.conf, so
+# a missing line means "no extra files," not a broken config.
+ensure_initramfs_files() {
+    local value="$1"
+    local config_file="$2"
+    sed -i "s|^FILES=.*|FILES=(${value})|" "$config_file"
+    if ! grep -q "^FILES=" "$config_file"; then
+        echo "FILES=(${value})" >> "$config_file"
+    fi
+}
diff --git a/tests/unit/test_common.bats b/tests/unit/test_common.bats
index abe3938..8ce7280 100644
--- a/tests/unit/test_common.bats
+++ b/tests/unit/test_common.bats
@@ -473,3 +473,50 @@ Boot0001* ZFSBootMenu"
     ! grep -q 'cryptdevice' "$f"
     rm -f "$f"
 }
+
+#############################
+# ensure_initramfs_files
+#############################
+# ensure_initramfs_files sets mkinitcpio.conf's FILES= line to the
+# given value, replacing an existing line or appending one if absent.
+# Self-healing rather than error-on-miss: FILES= is optional, so a
+# missing line means "no extra files," not a broken config.
+
+@test "ensure_initramfs_files replaces an empty FILES= line" {
+    local f
+    f=$(mktemp)
+    printf '%s\n' 'FILES=()' > "$f"
+
+    ensure_initramfs_files "/etc/luks.key" "$f"
+
+    grep -qF 'FILES=(/etc/luks.key)' "$f"
+    rm -f "$f"
+}
+
+@test "ensure_initramfs_files replaces a FILES= line that lists a different value" {
+    local f
+    f=$(mktemp)
+    printf '%s\n' 'FILES=(/etc/old-key)' > "$f"
+
+    ensure_initramfs_files "/etc/luks.key" "$f"
+
+    grep -qF 'FILES=(/etc/luks.key)' "$f"
+    ! grep -qF '/etc/old-key' "$f"
+    rm -f "$f"
+}
+
+@test "ensure_initramfs_files appends FILES= when the line is absent" {
+    local f
+    f=$(mktemp)
+    printf '%s\n' \
+        'MODULES=()' \
+        'BINARIES=()' \
+        'HOOKS=(base udev)' > "$f"
+
+    ensure_initramfs_files "/etc/luks.key" "$f"
+
+    grep -qF 'FILES=(/etc/luks.key)' "$f"
+    grep -qF 'MODULES=()' "$f"
+    grep -qF 'HOOKS=(base udev)' "$f"
+    rm -f "$f"
+}