🤝 world_planes: read the shared OpenSky cache (quota-collision fix)

Reader side of the OpenSky shared-cache v1 contract (mirrored in GODSIGH
SPEC2.md §4). Path ~/.cache/godverse/opensky-states.json (env OPENSKY_CACHE_FILE),
content the raw global /states/all body, freshness = mtime within 120 s.

Each poll cycle now checks the cache first: on a fresh hit it reads the global
body, filters the states to our bbox client-side (indices 5=lon, 6=lat), and
skips the HTTP call entirely (logs "cache hit"). On 429 or a network failure it
falls back to a stale cache rather than nothing. This worker only ever polls a
bbox, so it never WRITES the cache (never partial data into a global-contract
file) — it is a pure reader, and degrades to its old ≥450 s bbox fetch when no
writer is around (i.e. prod). Result: godstrument + GODSIGH running together
spend ~one poller's quota instead of colliding and 429-ing each other out.

Verified: unit tests for read_cache freshness/stale/missing + bbox filtering,
and an integration run where a fresh cache is served (count filtered to bbox,
zero HTTP).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

workers/world_planes.py

@@ -5,6 +5,7 @@ and mean groundspeed (intensity) — the sky's traffic becomes a slow chord.
 """
 import argparse
 import json
+import os
 import sys
 import time
 import urllib.error
@@ -23,6 +24,57 @@ POLL_OK = 450.0     # normal poll interval (s)
 POLL_429 = 600.0    # backoff poll interval after HTTP 429
 REEMIT = 10.0       # re-emit last values every 10s
 
+# --- OpenSky shared cache v1 (reader side) --------------------------------
+# Path: ~/.cache/godverse/opensky-states.json, override via OPENSKY_CACHE_FILE.
+# Content: the raw, unmodified OpenSky /states/all body (global, no bbox), written
+# atomically by WHOEVER fetched it (GODSIGH's dev proxy is the writer). Freshness =
+# file mtime. We treat the cache as fresh within 120 s; on a fresh hit we skip our
+# own HTTP entirely and just filter the global states to our bbox, so a godstrument
+# + GODSIGH running together spend ~one poller's quota instead of two. This worker
+# only ever polls a bbox, so it never WRITES the cache (never partial data into a
+# global-contract file) — it is a pure reader, and degrades to its old bbox fetch
+# when no writer is around (i.e. prod).
+CACHE_FILE = os.environ.get("OPENSKY_CACHE_FILE") or \
+    os.path.expanduser("~/.cache/godverse/opensky-states.json")
+CACHE_FRESH = 120.0     # a cache within this many seconds of its mtime is "fresh"
+
+
+def read_cache(max_age):
+    """Return (data, age_seconds) from the shared cache, or None. If max_age is not
+    None, returns None when the cache is older than that (i.e. not fresh)."""
+    try:
+        mtime = os.path.getmtime(CACHE_FILE)
+    except OSError:
+        return None
+    age = time.time() - mtime
+    if max_age is not None and age > max_age:
+        return None
+    try:
+        with open(CACHE_FILE, "r", encoding="utf-8") as f:
+            return json.load(f), age
+    except (OSError, json.JSONDecodeError, ValueError):
+        return None
+
+
+def filter_states_bbox(data, bbox):
+    """Filter a global states payload to our bbox client-side (the cache is global).
+    OpenSky state vectors: index 5 = longitude, index 6 = latitude."""
+    s, w, n, e = bbox
+    states = (data or {}).get("states") or []
+    out = []
+    for row in states:
+        if not row or len(row) < 7:
+            continue
+        lon, lat = row[5], row[6]
+        if lon is None or lat is None:
+            continue
+        try:
+            if s <= float(lat) <= n and w <= float(lon) <= e:
+                out.append(row)
+        except (TypeError, ValueError):
+            continue
+    return {"states": out}
+
 
 def fetch(bbox, timeout=25):
     """Fetch OpenSky states for the bbox. Returns parsed JSON dict.
@@ -95,25 +147,42 @@ def main():
     while True:
         now = time.monotonic()
         if now >= next_poll:
-            try:
-                data = fetch(args.bbox)
-                last = reduce_states(data)
+            cached = read_cache(CACHE_FRESH)          # a fresh global cache -> skip HTTP entirely
+            if cached is not None:
+                data, age = cached
+                last = reduce_states(filter_states_bbox(data, args.bbox))
                 if poll != POLL_OK:
                     print(f"[{NAME}] recovered; back to {POLL_OK:.0f}s poll")
                 poll = POLL_OK
-            except urllib.error.HTTPError as ex:
-                if ex.code == 429:
-                    retry_after = ex.headers.get("X-Rate-Limit-Retry-After-Seconds")
-                    try:
-                        poll = max(POLL_429, float(retry_after))
-                    except (TypeError, ValueError):
-                        poll = POLL_429
-                    print(f"[{NAME}] HTTP 429 rate limited; backing off to {poll:.0f}s")
-                else:
-                    print(f"[{NAME}] HTTP {ex.code}; retrying in {poll:.0f}s")
-            except (urllib.error.URLError, TimeoutError, OSError,
-                    json.JSONDecodeError, ValueError) as ex:
-                print(f"[{NAME}] fetch error: {ex}; retrying in {poll:.0f}s")
+                print(f"[{NAME}] cache hit ({age:.0f}s old) -> {int(last[0])} aircraft in bbox")
+            else:
+                try:
+                    data = fetch(args.bbox)
+                    last = reduce_states(data)
+                    if poll != POLL_OK:
+                        print(f"[{NAME}] recovered; back to {POLL_OK:.0f}s poll")
+                    poll = POLL_OK
+                except urllib.error.HTTPError as ex:
+                    if ex.code == 429:
+                        retry_after = ex.headers.get("X-Rate-Limit-Retry-After-Seconds")
+                        try:
+                            poll = max(POLL_429, float(retry_after))
+                        except (TypeError, ValueError):
+                            poll = POLL_429
+                        print(f"[{NAME}] HTTP 429 rate limited; backing off to {poll:.0f}s")
+                    else:
+                        print(f"[{NAME}] HTTP {ex.code}; retrying in {poll:.0f}s")
+                    _stale = read_cache(None)         # 429 -> a stale cache beats nothing
+                    if _stale is not None:
+                        last = reduce_states(filter_states_bbox(_stale[0], args.bbox))
+                        print(f"[{NAME}] serving stale cache ({_stale[1]:.0f}s old)")
+                except (urllib.error.URLError, TimeoutError, OSError,
+                        json.JSONDecodeError, ValueError) as ex:
+                    print(f"[{NAME}] fetch error: {ex}; retrying in {poll:.0f}s")
+                    _stale = read_cache(None)         # network down -> fall back to a stale cache
+                    if _stale is not None:
+                        last = reduce_states(filter_states_bbox(_stale[0], args.bbox))
+                        print(f"[{NAME}] serving stale cache ({_stale[1]:.0f}s old)")
             next_poll = time.monotonic() + poll
 
         # Emit (or re-emit) the last known values.