refactor: verify GRUB_CMDLINE_LINUX seds via prepend_grub_cmdline_linux helper

Audited the ~10 silent sed -i sites in the installer against the verification-after pattern that landed for sshd_config last session. Triaged each by failure mode.

The two GRUB_CMDLINE_LINUX seds in lib/btrfs.sh have a real silent-failure risk. If /etc/default/grub is missing or malformed and the sed pattern doesn't match, nothing happens. The kernel boots without cryptdevice=. The system can't unlock LUKS at boot. Added prepend_grub_cmdline_linux to lib/common.sh. Same shape as enable_sshd_root_login (sed, then grep, then error if the line wasn't modified). Replaced the two inline seds with helper calls.

The HOOKS= seds in installer/archangel and lib/btrfs.sh (six total) don't need verification. A missing HOOKS= line makes mkinitcpio -P fail loudly downstream, so silent-replace failure can't reach a booted system. Added a one-line audit-rationale comment at each of the three locations so the next reader doesn't re-litigate the decision.

The FILES= sed at lib/btrfs.sh:213 already self-heals via a sed-then-grep-then-append pattern, so no behavior change there. Filed a separate follow-up to lift that pattern into a named helper for clarity.

Bats: 142 → 146. Four new tests in test_common.bats cover normal (empty cmdline, existing cmdline preserved, other lines preserved) and error (missing GRUB_CMDLINE_LINUX line). Lint clean.

1 files changed, 64 insertions, 0 deletions

diff --git a/tests/unit/test_common.bats b/tests/unit/test_common.bats
index 9d267ab..48efd15 100644
--- a/tests/unit/test_common.bats
+++ b/tests/unit/test_common.bats
@@ -405,3 +405,67 @@ Boot0001* ZFSBootMenu"
     ! grep -q 'PermitRootLogin' "$f"
     rm -f "$f"
 }
+
+#############################
+# prepend_grub_cmdline_linux
+#############################
+# prepend_grub_cmdline_linux prepends a string to GRUB_CMDLINE_LINUX
+# in /etc/default/grub. Errors loudly if the line isn't present, since
+# silently doing nothing would leave the kernel without the parameter
+# (e.g. cryptdevice= for LUKS — a missing prefix means the system
+# can't unlock the root partition at boot).
+
+@test "prepend_grub_cmdline_linux prepends to an empty GRUB_CMDLINE_LINUX" {
+    local f
+    f=$(mktemp)
+    printf '%s\n' 'GRUB_CMDLINE_LINUX=""' > "$f"
+
+    prepend_grub_cmdline_linux "cryptdevice=UUID=abc123:root " "$f"
+
+    grep -qF 'GRUB_CMDLINE_LINUX="cryptdevice=UUID=abc123:root "' "$f"
+    rm -f "$f"
+}
+
+@test "prepend_grub_cmdline_linux preserves text already inside GRUB_CMDLINE_LINUX" {
+    local f
+    f=$(mktemp)
+    printf '%s\n' 'GRUB_CMDLINE_LINUX="quiet splash"' > "$f"
+
+    prepend_grub_cmdline_linux "cryptdevice=UUID=abc:root " "$f"
+
+    grep -qF 'GRUB_CMDLINE_LINUX="cryptdevice=UUID=abc:root quiet splash"' "$f"
+    rm -f "$f"
+}
+
+@test "prepend_grub_cmdline_linux preserves other lines in /etc/default/grub" {
+    local f
+    f=$(mktemp)
+    printf '%s\n' \
+        'GRUB_DEFAULT=0' \
+        'GRUB_TIMEOUT=5' \
+        'GRUB_CMDLINE_LINUX=""' \
+        'GRUB_DISABLE_RECOVERY=true' > "$f"
+
+    prepend_grub_cmdline_linux "cryptdevice=UUID=abc:root " "$f"
+
+    grep -qF 'GRUB_DEFAULT=0' "$f"
+    grep -qF 'GRUB_TIMEOUT=5' "$f"
+    grep -qF 'GRUB_CMDLINE_LINUX="cryptdevice=UUID=abc:root "' "$f"
+    grep -qF 'GRUB_DISABLE_RECOVERY=true' "$f"
+    rm -f "$f"
+}
+
+@test "prepend_grub_cmdline_linux errors when GRUB_CMDLINE_LINUX line is absent" {
+    local f
+    f=$(mktemp)
+    printf '%s\n' \
+        'GRUB_DEFAULT=0' \
+        'GRUB_TIMEOUT=5' > "$f"
+
+    error() { echo "ERROR: $*" >&2; return 1; }
+    run prepend_grub_cmdline_linux "cryptdevice=UUID=abc:root " "$f"
+    [ "$status" -ne 0 ]
+    [[ "$output" == *"GRUB_CMDLINE_LINUX"* ]]
+    ! grep -q 'cryptdevice' "$f"
+    rm -f "$f"
+}