Report runtime status so Cockpit can show what ACARSCOT is hearing

[ampledata]

The worker already counted received messages, positionless messages and emitted events. Those numbers went nowhere except a log line every hundredth message, so the only way to answer "is this thing working" was to read the journal.

This writes pytak's StatusWriter surface (snstac/pytak#107) to /run/acarscot/status.json, which the Cockpit plugin renders as a decode feed, rate, trend and health. Systemd already gives us RuntimeDirectory=acarscot, so no packaging change is needed.

Every message is recorded, not just the plotted ones

This matters more for ACARS than for any other gateway we run: 1829 of 1841 messages in a measured San Francisco capture carried no position. A feed showing only plotted aircraft would sit empty on a receiver working perfectly — which an operator reads as a fault.

The placed flag distinguishes "heard it, couldn't place it" from "put it on the map" without hiding either.

Startup write + 5s heartbeat

• Without the startup write, a quiet band leaves the UI reporting "no status from this gateway" — indistinguishable from a gateway that failed to start — for minutes.
• Without the heartbeat, silence on the band renders as a wedged service, because the UI judges freshness by whether the file keeps changing.

Graceful, visible degradation

A pytak without StatusWriter (anything before 7.4.0) degrades to a no-op rather than failing to import. Taking the gateway down over its telemetry helper would be exactly backwards.

The degradation is visible: with nothing writing the file, the plugin says so explicitly rather than rendering an empty feed as though the band were quiet. The requirement floor stays at pytak >= 7.3.0 for that reason.

Tests

Environment	Result
pytak with StatusWriter	43 passed
pytak 7.3.14 (installed)	36 passed, 7 skipped

Both deployment states covered. Two things worth recording about the tests:

• I first wrote these as bare async def tests. pytest-asyncio isn't installed, so pytest skipped all four while still counting them in the pass total — four tests that couldn't fail. They now drive the coroutine with asyncio.run().
• test_rate_limiting_means_the_file_can_lag_briefly pins the deliberate 1s write rate limit, so nobody later "fixes" the lag into a per-message write and makes a busy gateway spend its time serialising JSON.

🤖 Generated with Claude Code

https://claude.ai/code/session_0197da7dhvcPoHxYKamrYqyM