fix(traffic): name the proxy's own 502/503 accurately (#665)

502 (every mirror unreachable) and 503 (the local limiter, before any mirror
is asked) come from our proxy, so they no longer read as a mirror refusing
the query. Credit the fix in CHANGELOG and note the mapping in CURRENT-STATE.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -5,7 +5,9 @@
   `Overpass refused the road query (HTTP 406)` instead of a general
   "Road data temporarily unavailable", so a reader is not sent to check a
   TomTom key when the public OpenStreetMap mirrors are the side that failed.
-  Failures the layer cannot classify keep the general line.
+  The proxy's own 502 (every mirror unreachable) and 503 (local limiter busy)
+  read `Overpass mirrors unreachable` and `Overpass temporarily unavailable`.
+  Failures the layer cannot classify keep the general line (daikaginza, #665).
 
 - Bound the client terrain-height cache at 20 000 entries with least-recently-used eviction, so a long session no longer retains every coordinate it ever resolved. Consumer reads promote their entry and a batch still reports every point it resolved (Pedro Lobato, #594).
 

docs/CURRENT-STATE.md

@@ -3483,7 +3483,7 @@ silently demoting every later lookup for the session.
 - HTTP refusals such as 406 now rotate alongside the existing network, rate-limit, and runtime-error cases. A refusal from one mirror no longer prevents reaching healthy alternatives or persists under the seven-day road/month-long boundary cache TTLs. Concurrent identical queries share one mirror sequence; if it fails, both the initiating and joined callers can use the same last-good data.
 - A refusal every mirror agrees on is still reported with the first mirror's status and body, so a genuinely malformed query says what upstream said — but only after every mirror has had the chance to answer it. `fetchOverpassPayload` takes injectable endpoints and fetch so the rotation is tested without a live mirror (`src/overpassProxy.test.mjs`).
 - Every mirror is asked with a User-Agent that names the application, its version and the project address (`OVERPASS_USER_AGENT` in `server/providers/overpass/constants.js`), which is what the OSM API usage policy asks for. Mirrors may refuse a client they cannot identify; such a refusal is never treated as data and costs the fan-out that mirror, so the header is what keeps the list at full strength.
-- When the fan-out has nothing left to offer, the traffic row says which upstream declined and how: `Overpass rate-limited`, `Overpass timed out`, or `Overpass refused the road query (HTTP 406)`. Street Traffic draws on two upstreams — OpenStreetMap for geometry, TomTom for flow — so a row that only said "road data unavailable" sent readers to check a TomTom key that was never the problem (#661). `roadRequestError` in `src/layers/traffic/source.js` owns the mapping and tags the error with its status. A refusal whose code is unreadable — the layer takes its source by injection, so an adapter may not supply one — reports `Overpass temporarily unavailable` rather than printing `HTTP undefined`, and anything the layer did not classify at all (a dropped connection, a malformed snapshot) still reads `Road data temporarily unavailable`, because a platform exception is not a sentence to put on a panel.
+- When the fan-out has nothing left to offer, the traffic row says which upstream declined and how: `Overpass rate-limited`, `Overpass timed out`, or `Overpass refused the road query (HTTP 406)`; the proxy's own 502 (every mirror unreachable) and 503 (local limiter busy) read `Overpass mirrors unreachable` and `Overpass temporarily unavailable`. Street Traffic draws on two upstreams — OpenStreetMap for geometry, TomTom for flow — so a row that only said "road data unavailable" sent readers to check a TomTom key that was never the problem (#661). `roadRequestError` in `src/layers/traffic/source.js` owns the mapping and tags the error with its status. A refusal whose code is unreadable — the layer takes its source by injection, so an adapter may not supply one — reports `Overpass temporarily unavailable` rather than printing `HTTP undefined`, and anything the layer did not classify at all (a dropped connection, a malformed snapshot) still reads `Road data temporarily unavailable`, because a platform exception is not a sentence to put on a panel.
 
 ### Share-link v2 layer state (August 2026)
 

src/layers/traffic/source.js

@@ -60,13 +60,20 @@ export function roadRequestError(status) {
       ? 'Overpass rate-limited'
       : code === 504
         ? 'Overpass timed out'
-        : // A source is injected, so a caller's adapter may hand back a
-          // refusal with no status on it. "HTTP undefined" on a panel row is
-          // worse than not naming a number, so an unreadable code falls back
-          // to what `alpr` says when it cannot be more specific either.
-          code === null
-          ? 'Overpass temporarily unavailable'
-          : `Overpass refused the road query (HTTP ${code})`;
+        : // Our own proxy answers these, not a mirror: 502 when every mirror
+          // failed at the network level, 503 from its local concurrency
+          // limiter before any mirror was asked. Neither is a refusal.
+          code === 502
+          ? 'Overpass mirrors unreachable'
+          : code === 503
+            ? 'Overpass temporarily unavailable'
+            : // A source is injected, so a caller's adapter may hand back a
+              // refusal with no status on it. "HTTP undefined" on a panel row is
+              // worse than not naming a number, so an unreadable code falls back
+              // to what `alpr` says when it cannot be more specific either.
+              code === null
+              ? 'Overpass temporarily unavailable'
+              : `Overpass refused the road query (HTTP ${code})`;
   return new RoadRequestError(message, { status: code });
 }
 

src/layers/traffic/source.test.mjs

@@ -207,6 +207,15 @@ test('an Overpass refusal is named by status, and keeps the status beside the wo
     'Overpass refused the road query (HTTP 500)',
   );
 
+  // 502 and 503 come from the local proxy (mirrors unreachable, local
+  // limiter busy), so they must not read as a mirror refusing the query.
+  assert.equal(roadRequestError(502).message, 'Overpass mirrors unreachable');
+  assert.equal(
+    roadRequestError(503).message,
+    'Overpass temporarily unavailable',
+  );
+  assert.equal(roadRequestError(503).status, 503);
+
   // An injected source may refuse without a readable code; a row must never
   // print "HTTP undefined" at a reader.
   for (const missing of [undefined, null, NaN, 'four-oh-six']) {