fix(state-writer): skip atomic rename when content unchanged#94
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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.
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.
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Daemons that run under
Restart=alwayswith a sleep-then-exit-0 disabled branch (dump978-fa'sno_hardwarecycle,airplanes-mlat'smlat_enabled_falsecycle) re-classify on every restart and callairplanes_write_statewith the same decision but a freshdecided_attimestamp. The atomic rename was firing about 1200 times a day on a typical no-978-dongle feeder — invisible on its own, but enough to wake any inotify consumer watching the file (e.g. a systemd.pathunit).The writer now compares the proposed content against the current file with
decided_at=lines stripped from both sides and skips the rename when they match. On-diskdecided_atstays at the time of the last material decision rather than being touched on every wrapper restart, which is the more meaningful value for status consumers.AIRPLANES_WRITE_STATE_FORCE=1opts out and restores the always-rename behaviour for any caller that needs it.No API change. Every current call-site keeps the same arguments; the dedupe is invisible to them. Mode 0644 is still enforced on rename and the validation-failure paths still short-circuit before the dedupe check.