fix(terrain): end retries early on a long Retry-After; cover the 429 pause

A Retry-After longer than the 10 s retry budget used to sleep the whole
budget out and then give up, so the proxy's 429 pause started 10 s late
and every new-point poll in that window still went upstream and logged.
The retry loop now stops at once in that case.

Tests: the retry loop no longer sleeps for an unreachable Retry-After;
fully cached points, including stale ones, are served during the pause
with no upstream call; a 503 does not pause; a shorter concurrent
Retry-After never cuts the pause short. CURRENT-STATE now says which
requests are still served and which log.

src/data/terrainHeightsProxy.js

@@ -107,7 +107,10 @@ export function terrainRetryAfterMs(value, nowMs = Date.now()) {
 /**
  * Fetch one upstream chunk with jittered exponential backoff. Network errors,
  * 429, and 5xx retry; other HTTP failures and malformed successes fail fast.
- * Retry sleeps and retry attempts share a 10s budget after attempt one.
+ * Retry sleeps and retry attempts share a 10s budget after attempt one. A
+ * Retry-After longer than the budget left ends the retries at once: sleeping
+ * out the budget cannot reach the time the upstream asked for, and it would
+ * delay the caller's own back-off (the proxy's 429 pause) by up to 10 s.
  *
  * Dependencies are injectable for deterministic offline tests.
  * @param {Array<[number, number]>} points
@@ -168,6 +171,7 @@ export async function fetchTerrainChunkWithRetry(
       if (remaining <= 0) break;
 
       const retryAfterMs = terrainRetryAfterMs(error?.retryAfter, now());
+      if (retryAfterMs != null && retryAfterMs > remaining) break;
       const backoffMs = Math.round(
         350 * 2 ** attempt * (0.75 + random() * 0.5),
       );

src/data/terrainHeightsProxy.test.mjs

@@ -91,6 +91,36 @@ test('429 retry honors Retry-After before the next attempt', async () => {
   assert.equal(results[0].id, 'recovered');
 });
 
+test('a Retry-After longer than the retry budget ends the retries without sleeping', async () => {
+  let clock = 3_000_000;
+  let attempts = 0;
+  const sleeps = [];
+  await assert.rejects(
+    fetchTerrainChunkWithRetry([[1, 1]], {
+      now: () => clock,
+      random: () => 0.5,
+      makeSignal: () => undefined,
+      sleep: async (ms) => {
+        sleeps.push(ms);
+        clock += ms;
+      },
+      fetchImpl: async () => {
+        attempts += 1;
+        return {
+          ok: false,
+          status: 429,
+          headers: {
+            get: (name) => (name.toLowerCase() === 'retry-after' ? '30' : null),
+          },
+        };
+      },
+    }),
+    (error) => error.status === 429 && error.retryAfter === '30',
+  );
+  assert.equal(attempts, 1);
+  assert.deepEqual(sleeps, [], 'no sleep that cannot reach the Retry-After');
+});
+
 test('permanently failing upstream exhausts retries and returns 502 without fake results', async () => {
   let clock = 2_000_000;
   let attempts = 0;

src/tooling/environmentProviders.test.mjs

@@ -476,3 +476,91 @@ test('a far-off Retry-After pauses the terrain upstream for at most 10 minutes',
   t.mock.timers.setTime(pausedAt + 10 * 60_000 + 1);
   assert.equal((await request('/?points=3,4')).status, 200, 'pause over');
 });
+
+/** Drive mocked timers until `promise` settles; the retry budget is 10 s. */
+async function settleOnMockedTime(t, promise) {
+  let settled = false;
+  const tracked = promise.finally(() => {
+    settled = true;
+  });
+  for (let step = 0; step < 60 && !settled; step += 1) {
+    await new Promise((resolve) => setImmediate(resolve));
+    t.mock.timers.tick(1_000);
+  }
+  assert.ok(settled, 'the request settles within its retry budget');
+  return tracked;
+}
+
+test('during a terrain 429 pause fully cached points, even stale ones, are still served', async (t) => {
+  isolate(t);
+  t.mock.timers.enable({ apis: ['Date', 'setTimeout'], now: 1_000_000 });
+  let calls = 0;
+  let limited = false;
+  t.mock.method(globalThis, 'fetch', async () => {
+    calls += 1;
+    return limited
+      ? new Response('', { status: 429, headers: { 'retry-after': '300' } })
+      : Response.json({ results: [{ ellipsoid: 100 }] });
+  });
+  const request = install(terrainHeightsProxy());
+  assert.equal((await request('/?points=5,6')).status, 200);
+  // Past the 30-day TTL, so 5,6 is stale and would normally be refreshed.
+  t.mock.timers.setTime(Date.now() + 31 * 24 * 3600_000);
+  limited = true;
+  const first = await settleOnMockedTime(t, request('/?points=1,2'));
+  assert.equal(first.status, 502);
+  assert.equal(calls, 2, 'a Retry-After past the budget is not retried');
+  const stale = await request('/?points=5,6');
+  assert.equal(stale.status, 200);
+  assert.deepEqual(json(stale).results, [{ ellipsoid: 100 }]);
+  assert.equal((await request('/?points=5,6;7,8')).status, 502);
+  assert.equal(calls, 2, 'no upstream call during the pause');
+});
+
+test('a terrain upstream failure other than 429 does not pause later requests', async (t) => {
+  isolate(t);
+  t.mock.timers.enable({ apis: ['Date', 'setTimeout'], now: 1_000_000 });
+  let calls = 0;
+  t.mock.method(globalThis, 'fetch', async () => {
+    calls += 1;
+    return new Response('', { status: 503 });
+  });
+  const request = install(terrainHeightsProxy());
+  const first = await settleOnMockedTime(t, request('/?points=1,2'));
+  assert.equal(first.status, 502);
+  const afterFirst = calls;
+  const second = await settleOnMockedTime(t, request('/?points=3,4'));
+  assert.equal(second.status, 502);
+  assert.ok(calls > afterFirst, 'new points still go upstream after a 503');
+});
+
+test('a shorter concurrent Retry-After never cuts a terrain 429 pause short', async (t) => {
+  isolate(t);
+  t.mock.timers.enable({ apis: ['Date', 'setTimeout'], now: 1_000_000 });
+  let calls = 0;
+  t.mock.method(globalThis, 'fetch', async (raw) => {
+    calls += 1;
+    const points = new URL(raw).searchParams.get('points');
+    if (points.startsWith('0.'))
+      return Response.json({ results: [{ ellipsoid: 1 }] });
+    const retryAfter = points.startsWith('1.') ? '300' : '30';
+    return new Response('', {
+      status: 429,
+      headers: { 'retry-after': retryAfter },
+    });
+  });
+  const request = install(terrainHeightsProxy());
+  // Load the disk cache first so both requests below reach the upstream in
+  // order: the 300 s answer lands first, the 30 s answer after it.
+  assert.equal((await request('/?points=0,0')).status, 200);
+  const [long, short] = await settleOnMockedTime(
+    t,
+    Promise.all([request('/?points=1,2'), request('/?points=3,4')]),
+  );
+  assert.equal(long.status, 502);
+  assert.equal(short.status, 502);
+  const afterBoth = calls;
+  t.mock.timers.setTime(Date.now() + 120_000);
+  assert.equal((await request('/?points=5,6')).status, 502);
+  assert.equal(calls, afterBoth, 'still paused by the 300 s Retry-After');
+});