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fix(state-writer): skip atomic rename when content unchanged
Wrappers under Restart=always with a sleep+exit-0 disabled branch (dump978-fa's no_hardware loop, airplanes-mlat's mlat_enabled_false loop) re-classify on every cycle and call airplanes_write_state with the same decision but a fresh decided_at timestamp. The atomic rename was firing every ~1200 cycles/day on a typical no-978-dongle feeder, waking any inotify consumer (e.g. systemd .path units watching the file). Compare the proposed content against the current target ignoring decided_at= lines and skip the rename when they match. Existing on-disk decided_at stays, so consumers see the time of the last material decision rather than the time of the last wrapper restart. AIRPLANES_WRITE_STATE_FORCE=1 keeps the previous always-rename behavior for callers that need it.
1 parent 01ffe62 commit c8020cb

2 files changed

Lines changed: 201 additions & 1 deletion

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‎scripts/lib/state-writer.sh‎

Lines changed: 37 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -20,6 +20,17 @@
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# connected) is observable via systemd liveness, journal output, and
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# external probes.
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#
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# Dedupe: when an activation re-runs the wrapper and produces the same
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# decision (the common case for wrappers that sleep + exit 0 under
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# Restart=always — dump978-fa's no_hardware loop, airplanes-mlat's
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# disabled loop), the writer skips the atomic-rename if the proposed
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# content equals the current file content, ignoring decided_at= (which
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# is by convention a per-activation timestamp every caller refreshes).
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# Net effects: mtime stays at the time of the last *material* change,
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# inotify consumers (e.g. systemd .path units) don't fire on no-op
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# re-runs. AIRPLANES_WRITE_STATE_FORCE=1 disables this and always
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# atomically replaces the target.
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#
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# Readers MUST NOT `source` this file — values are unquoted; arbitrary
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# string content (e.g. user-supplied MLAT_USER) could contain shell
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# metacharacters. Parse line-by-line with KEY=VALUE split on first `=`.
@@ -34,7 +45,21 @@
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# mlat_user=
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#
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# Returns 0 on success, 1 on validation failure or write error
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# (target file is left unchanged on either).
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# (target file is left unchanged on either). Returns 0 with target
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# unchanged when the dedupe path skips the rename.
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# Print $1's content with decided_at= lines filtered out. Used by the
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# dedupe path so a fresh timestamp on an otherwise-identical decision
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# doesn't force a rename.
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_airplanes_write_state_canonical() {
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local line
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while IFS= read -r line; do
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case "$line" in
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decided_at=*) continue ;;
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esac
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printf '%s\n' "$line"
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done < "$1"
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}
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airplanes_write_state() {
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local target="$1"; shift
@@ -81,6 +106,17 @@ airplanes_write_state() {
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return 1
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fi
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# Dedupe path: same decision as already on disk → skip the rename.
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# The mode-preserving chmod is also skipped — the existing file's
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# mode reflects an earlier successful write that already set 0644.
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if [[ "${AIRPLANES_WRITE_STATE_FORCE:-}" != "1" ]] \
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&& [[ -f "$target" && -r "$target" ]] \
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&& [[ "$(_airplanes_write_state_canonical "$tmp")" \
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== "$(_airplanes_write_state_canonical "$target")" ]]; then
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rm -f "$tmp"
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return 0
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fi
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chmod 0644 "$tmp" || { rm -f "$tmp"; return 1; }
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mv -f "$tmp" "$target" || { rm -f "$tmp"; return 1; }
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return 0

‎test/test_state_writer.bats‎

Lines changed: 164 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -143,3 +143,167 @@ bar"
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grep -qx 'service=second' "$TARGET"
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! grep -qx 'service=first' "$TARGET"
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}
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# --- Dedupe path ---
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# The writer skips the atomic rename when the proposed content matches
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# the current target ignoring decided_at= lines. This kills inotify
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# wakeups on no-op wrapper re-runs (Restart=always + sleep + exit 0)
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# without changing the on-disk content readers see.
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@test "dedupe: identical content with same decided_at → no rename, mtime preserved" {
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=enabled \
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reason=ok \
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decided_at=2026-05-18T10:00:00Z
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local mtime_before
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mtime_before="$(stat -c %Y "$TARGET")"
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sleep 1 # widen mtime resolution window
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=enabled \
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reason=ok \
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decided_at=2026-05-18T10:00:00Z
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local mtime_after
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mtime_after="$(stat -c %Y "$TARGET")"
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[ "$mtime_after" -eq "$mtime_before" ]
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}
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@test "dedupe: identical content with fresh decided_at → no rename" {
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# Every wrapper restart cycle re-computes decided_at. Without
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# ignoring it the file would be atomically replaced ~1200 times/day
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# on a feeder where the daemon's classify produces the same result
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# cycle after cycle.
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=disabled \
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reason=mlat_enabled_false \
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decided_at=2026-05-18T10:00:00Z
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local mtime_before
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mtime_before="$(stat -c %Y "$TARGET")"
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sleep 1
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=disabled \
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reason=mlat_enabled_false \
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decided_at=2026-05-18T10:00:30Z
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local mtime_after
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mtime_after="$(stat -c %Y "$TARGET")"
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[ "$mtime_after" -eq "$mtime_before" ]
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# Old decided_at stays — readers see the time of the last material
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# decision, not the time of the last wrapper restart.
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grep -qx 'decided_at=2026-05-18T10:00:00Z' "$TARGET"
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}
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@test "dedupe: changed non-timestamp field → full rename happens" {
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=enabled \
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reason=ok \
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decided_at=2026-05-18T10:00:00Z
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local mtime_before
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mtime_before="$(stat -c %Y "$TARGET")"
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sleep 1
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=disabled \
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reason=mlat_enabled_false \
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decided_at=2026-05-18T10:00:30Z
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local mtime_after
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mtime_after="$(stat -c %Y "$TARGET")"
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[ "$mtime_after" -gt "$mtime_before" ]
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grep -qx 'state=disabled' "$TARGET"
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grep -qx 'reason=mlat_enabled_false' "$TARGET"
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grep -qx 'decided_at=2026-05-18T10:00:30Z' "$TARGET"
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}
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@test "dedupe: added field → full rename happens" {
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=disabled \
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decided_at=2026-05-18T10:00:00Z
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local mtime_before
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mtime_before="$(stat -c %Y "$TARGET")"
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sleep 1
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=disabled \
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decided_at=2026-05-18T10:00:30Z \
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new_field=v1
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local mtime_after
234+
mtime_after="$(stat -c %Y "$TARGET")"
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[ "$mtime_after" -gt "$mtime_before" ]
236+
grep -qx 'new_field=v1' "$TARGET"
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}
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@test "dedupe: removed field → full rename happens" {
240+
airplanes_write_state "$TARGET" \
241+
service=airplanes-mlat \
242+
state=enabled \
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reason=ok \
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decided_at=2026-05-18T10:00:00Z \
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extra=v
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local mtime_before
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mtime_before="$(stat -c %Y "$TARGET")"
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sleep 1
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airplanes_write_state "$TARGET" \
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service=airplanes-mlat \
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state=enabled \
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reason=ok \
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decided_at=2026-05-18T10:00:30Z
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local mtime_after
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mtime_after="$(stat -c %Y "$TARGET")"
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[ "$mtime_after" -gt "$mtime_before" ]
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! grep -qx 'extra=v' "$TARGET"
258+
}
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260+
@test "dedupe: reordered fields → full rename happens (order is semantic)" {
261+
# KEY=VALUE order is part of the writer's contract (caller-provided
262+
# order). A reorder is a content change.
263+
airplanes_write_state "$TARGET" \
264+
service=foo \
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state=enabled \
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decided_at=2026-05-18T10:00:00Z
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local mtime_before
268+
mtime_before="$(stat -c %Y "$TARGET")"
269+
sleep 1
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airplanes_write_state "$TARGET" \
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state=enabled \
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service=foo \
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decided_at=2026-05-18T10:00:30Z
274+
local mtime_after
275+
mtime_after="$(stat -c %Y "$TARGET")"
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[ "$mtime_after" -gt "$mtime_before" ]
277+
}
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279+
@test "dedupe: AIRPLANES_WRITE_STATE_FORCE=1 always renames" {
280+
airplanes_write_state "$TARGET" \
281+
service=foo \
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state=enabled \
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decided_at=2026-05-18T10:00:00Z
284+
local mtime_before
285+
mtime_before="$(stat -c %Y "$TARGET")"
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sleep 1
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AIRPLANES_WRITE_STATE_FORCE=1 airplanes_write_state "$TARGET" \
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service=foo \
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state=enabled \
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decided_at=2026-05-18T10:00:30Z
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local mtime_after
292+
mtime_after="$(stat -c %Y "$TARGET")"
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[ "$mtime_after" -gt "$mtime_before" ]
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grep -qx 'decided_at=2026-05-18T10:00:30Z' "$TARGET"
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}
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@test "dedupe: target missing → write proceeds (no dedupe path engaged)" {
298+
[ ! -e "$TARGET" ]
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airplanes_write_state "$TARGET" service=foo state=enabled
300+
[ -f "$TARGET" ]
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grep -qx 'service=foo' "$TARGET"
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}
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@test "dedupe: validation failure short-circuits before dedupe check" {
305+
airplanes_write_state "$TARGET" service=before
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run airplanes_write_state "$TARGET" service=after 'bad-key=value'
307+
[ "$status" -eq 1 ]
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grep -qx 'service=before' "$TARGET"
309+
}

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