fix(state-writer): include trailing line in dedupe canonicaliser

A target left behind without a final newline (interrupted previous write, external tampering) would lose its last line in the bash while-read loop, so a proposed write that removed a tail field could falsely dedupe against the truncated remainder. The `|| [[ -n line ]]` tail picks up the unterminated last line. Regression coverage includes the no-trailing-newline case and a backslashes-plus-trailing-spaces round-trip.

Author: d4rken

scripts/lib/state-writer.sh

@@ -50,10 +50,16 @@
 
 # Print $1's content with decided_at= lines filtered out. Used by the
 # dedupe path so a fresh timestamp on an otherwise-identical decision
-# doesn't force a rename.
+# doesn't force a rename. The `|| [[ -n "$line" ]]` tail catches an
+# existing target that's missing its final newline — without it the
+# loop would silently drop the last line and a proposed write that
+# removed a tail field could falsely dedupe against the truncated
+# remainder. (The writer's own output always ends in \n, so this only
+# matters for files left behind by an interrupted write or external
+# tampering.)
 _airplanes_write_state_canonical() {
     local line
-    while IFS= read -r line; do
+    while IFS= read -r line || [[ -n "$line" ]]; do
         case "$line" in
             decided_at=*) continue ;;
         esac

test/test_state_writer.bats

@@ -307,3 +307,45 @@ bar"
     [ "$status" -eq 1 ]
     grep -qx 'service=before' "$TARGET"
 }
+
+@test "dedupe: existing target without trailing newline → tail field still compared" {
+    # An external writer or an interrupted previous run could leave a
+    # target with no final newline. Without the `|| [[ -n line ]]` tail
+    # in the canonicaliser the last line silently disappears from the
+    # comparison, and a proposed write that removed the trailing field
+    # would falsely dedupe against the truncated remainder, preserving
+    # stale content. This case asserts the canonicaliser includes the
+    # final line so the writer detects the difference and renames.
+    printf 'schema_version=1\nservice=mlat\nstate=disabled\nextra=v' \
+        > "$TARGET"
+    local mtime_before
+    mtime_before="$(stat -c %Y "$TARGET")"
+    sleep 1
+    airplanes_write_state "$TARGET" \
+        service=mlat \
+        state=disabled
+    local mtime_after
+    mtime_after="$(stat -c %Y "$TARGET")"
+    [ "$mtime_after" -gt "$mtime_before" ]
+    ! grep -qx 'extra=v' "$TARGET"
+}
+
+@test "dedupe: values with backslashes + trailing spaces preserved verbatim" {
+    # `IFS= read -r` + `printf '%s\n'` round-trips backslashes and
+    # trailing whitespace; canonicalisation must not mangle them, or
+    # else two semantically-identical writes would compare unequal and
+    # break dedupe.
+    airplanes_write_state "$TARGET" \
+        "feed_bin=/usr/bin/x\\trailing " \
+        "decided_at=2026-05-18T10:00:00Z"
+    local mtime_before
+    mtime_before="$(stat -c %Y "$TARGET")"
+    sleep 1
+    airplanes_write_state "$TARGET" \
+        "feed_bin=/usr/bin/x\\trailing " \
+        "decided_at=2026-05-18T10:00:30Z"
+    local mtime_after
+    mtime_after="$(stat -c %Y "$TARGET")"
+    [ "$mtime_after" -eq "$mtime_before" ]
+    grep -qFx 'feed_bin=/usr/bin/x\trailing ' "$TARGET"
+}