fix(terrain): keep a batch's warm points when its own fetches evict them

A warm entry could be evicted by the same call's later fetches and then
reported unresolved. The call now remembers warm hits as it does fetched ones.
Moves the batch LRU test beside the existing cache tests and folds the docs
note into the section's eviction paragraph.

docs/CURRENT-STATE.md

@@ -1192,19 +1192,17 @@ The browser layers and their rendering remain in their existing modules.
 
 ## Terrain height cache bound
 
-Warm batch lookups also promote their cached entries in the least-recently-used eviction order.
-
 The client terrain-height resolver's in-memory cache is bounded at 20 000
 entries (`DEFAULT_MAX_CACHE_ENTRIES`, overridable per instance through
 `createTerrainHeights({ maxCacheEntries })`). Coordinate rounding makes repeated
 visits to one place share a key but does not bound how many distinct places a
 session visits, and the cache previously had no eviction, so a long session
 retained every coordinate it ever resolved. Eviction is least-recently-used:
-a consumer read promotes its entry, so the cell being watched is not dropped for
-having been resolved early. The bound sits far above a city-scale session, so
+a consumer read, including a warm hit in a batch lookup, promotes its entry, so
+the cell being watched is not dropped for having been resolved early. The bound sits far above a city-scale session, so
 normal use never evicts and issues no additional proxy requests. A batch's
-results are assembled from what that call resolved, so eviction during a large
-batch cannot report a just-resolved point as unresolved. Re:Earth and
+results are assembled from what that call resolved or found warm, so eviction
+during a large batch cannot report one of its points as unresolved. Re:Earth and
 geoid-fallback entry semantics, the fallback cooldown and the abort-time flush
 are unchanged.
 

src/services/terrain-warm-batch-lru.test.mjs

@@ -1,24 +0,0 @@
-import test from 'node:test';
-import assert from 'node:assert/strict';
-import { createTerrainHeights } from './terrainHeights.js';
-test('warm batch consumers retain their terrain cells ahead of unused cells', async () => {
-  let calls = 0;
-  const terrain = createTerrainHeights({
-    maxCacheEntries: 2,
-    source: {
-      async getHeights(points) {
-        calls++;
-        return points.map((p) => ({ ellipsoid: 100 + p.lon }));
-      },
-    },
-  });
-  const p = (lon) => ({ lat: 0, lon });
-  await terrain.resolveEllipsoidalGround([p(1)]);
-  await terrain.resolveEllipsoidalGround([p(2)]);
-  await terrain.resolveEllipsoidalGround([p(1)]);
-  assert.equal(calls, 2);
-  await terrain.resolveEllipsoidalGround([p(3)]);
-  assert.equal(terrain.cachedEllipsoidalGround(0, 1), 101);
-  assert.equal(terrain.cachedEllipsoidalGround(0, 2), null);
-  assert.equal(calls, 3);
-});

src/services/terrainHeights.js

@@ -272,28 +272,26 @@ export function createTerrainHeights({
     const uncached = [];
     const seenKeys = new Set();
     const now = Date.now();
+    // What this call resolved or found warm, so the assembly below can report a
+    // point even if a later chunk's writes pushed it past CACHE_MAX_ENTRIES.
+    // Without it a batch big enough to evict its own entries would report
+    // points as unresolved that it had in fact resolved.
+    const resolvedThisCall = new Map();
     for (const item of work) {
       const entry = cacheTouch(item.key);
       const fallbackCooling =
         entry?.source === 'geoid-fallback' &&
         Number.isFinite(entry.retryAt) &&
         now < entry.retryAt;
-      if (
-        (entry && entry.source === 'reearth') ||
-        fallbackCooling ||
-        seenKeys.has(item.key)
-      )
+      if ((entry && entry.source === 'reearth') || fallbackCooling) {
+        resolvedThisCall.set(item.key, entry);
         continue;
+      }
+      if (seenKeys.has(item.key)) continue;
       seenKeys.add(item.key);
       uncached.push(item);
     }
 
-    // What this call resolved, so the assembly below can report a point even
-    // if a later chunk's writes pushed it past CACHE_MAX_ENTRIES. Without it a
-    // batch big enough to evict its own earlier chunks would report points as
-    // unresolved that it had in fact just resolved.
-    const resolvedThisCall = new Map();
-
     // Resolve the network path in sequential <=CHUNK_SIZE chunks. Each chunk's
     // failure is isolated to that chunk's points (geoid fallback), so a single
     // bad chunk doesn't lose results for the rest of a large batch.

src/services/terrainHeights.test.mjs

@@ -114,6 +114,47 @@ test('a batch larger than the cache bound still reports every point it resolved'
   assert.equal(warm.length, 4);
 });
 
+test('warm batch consumers retain their terrain cells ahead of unused cells', async () => {
+  let calls = 0;
+  const terrain = createTerrainHeights({
+    maxCacheEntries: 2,
+    source: {
+      async getHeights(points) {
+        calls++;
+        return points.map((p) => ({ ellipsoid: 100 + p.lon }));
+      },
+    },
+  });
+  const p = (lon) => ({ lat: 0, lon });
+  await terrain.resolveEllipsoidalGround([p(1)]);
+  await terrain.resolveEllipsoidalGround([p(2)]);
+  await terrain.resolveEllipsoidalGround([p(1)]);
+  assert.equal(calls, 2);
+  await terrain.resolveEllipsoidalGround([p(3)]);
+  assert.equal(terrain.cachedEllipsoidalGround(0, 1), 101);
+  assert.equal(terrain.cachedEllipsoidalGround(0, 2), null);
+  assert.equal(calls, 3);
+});
+
+test('a batch still reports its warm points after its own fetches evict them', async () => {
+  const terrain = createTerrainHeights({
+    source: stubSource(),
+    maxCacheEntries: 2,
+  });
+  await terrain.resolveEllipsoidalGround([pointAt(0)]);
+
+  const results = await terrain.resolveEllipsoidalGround([
+    pointAt(0),
+    pointAt(1),
+    pointAt(2),
+  ]);
+
+  results.forEach((result, i) => {
+    assert.equal(result.source, 'reearth', `point ${i} reported resolved`);
+    assert.equal(result.ellipsoid, heightAt(lonAt(i)));
+  });
+});
+
 test('an out-of-range cache bound falls back to the default instead of disabling the cache', async () => {
   for (const maxCacheEntries of [0, -1, Number.NaN, undefined]) {
     const terrain = createTerrainHeights({