fix: use LRU eviction for cron schedule cache

Fixes #39679

src/cron/schedule.test.ts

@@ -4,7 +4,9 @@ import {
   clearCronScheduleCacheForTest,
   computeNextRunAtMs,
   computePreviousRunAtMs,
+  getCronScheduleCacheMaxForTest,
   getCronScheduleCacheSizeForTest,
+  hasCronInCacheForTest,
 } from "./schedule.js";
 
 describe("cron schedule", () => {
@@ -144,6 +146,39 @@ describe("cron schedule", () => {
     expect(getCronScheduleCacheSizeForTest()).toBe(2);
   });
 
+  it("promotes accessed entries to avoid premature LRU eviction", () => {
+    const nowMs = Date.parse("2026-03-01T00:00:00.000Z");
+    const cacheMax = getCronScheduleCacheMaxForTest();
+
+    // Fill cache to capacity with unique expressions.
+    // i=0 → "0 0 * * *", i=1 → "1 0 * * *", ..., i=511 → "31 8 * * *"
+    for (let i = 0; i < cacheMax; i++) {
+      computeNextRunAtMs(
+        { kind: "cron", expr: `${i % 60} ${Math.floor(i / 60)} * * *`, tz: "UTC" },
+        nowMs,
+      );
+    }
+    expect(getCronScheduleCacheSizeForTest()).toBe(cacheMax);
+
+    // Entry #0 ("0 0 * * *") is the oldest by insertion order.
+    // Access it so LRU promotes it (delete + re-insert at end of Map).
+    computeNextRunAtMs({ kind: "cron", expr: "0 0 * * *", tz: "UTC" }, nowMs);
+
+    // Entry #1 ("1 0 * * *") is now the least-recently-used.
+    // Insert a new entry to trigger one eviction.
+    computeNextRunAtMs({ kind: "cron", expr: "0 0 1 1 *", tz: "UTC" }, nowMs);
+    expect(getCronScheduleCacheSizeForTest()).toBe(cacheMax);
+
+    // Under LRU: entry #0 survived (was promoted), entry #1 was evicted.
+    // Under FIFO: entry #0 would be evicted instead — this assertion would fail.
+    expect(hasCronInCacheForTest("0 0 * * *", "UTC")).toBe(true);
+    expect(hasCronInCacheForTest("1 0 * * *", "UTC")).toBe(false);
+
+    // The new entry and a non-evicted middle entry should both be present.
+    expect(hasCronInCacheForTest("0 0 1 1 *", "UTC")).toBe(true);
+    expect(hasCronInCacheForTest("2 0 * * *", "UTC")).toBe(true);
+  });
+
   describe("cron with specific seconds (6-field pattern)", () => {
     // Pattern: fire at exactly second 0 of minute 0 of hour 12 every day
     const dailyNoon = { kind: "cron" as const, expr: "0 0 12 * * *", tz: "UTC" };

src/cron/schedule.ts

@@ -18,6 +18,9 @@ function resolveCachedCron(expr: string, timezone: string): Cron {
   const key = `${timezone}\u0000${expr}`;
   const cached = cronEvalCache.get(key);
   if (cached) {
+    // Move to end of Map iteration order for LRU eviction
+    cronEvalCache.delete(key);
+    cronEvalCache.set(key, cached);
     return cached;
   }
   if (cronEvalCache.size >= CRON_EVAL_CACHE_MAX) {
@@ -169,3 +172,11 @@ export function clearCronScheduleCacheForTest(): void {
 export function getCronScheduleCacheSizeForTest(): number {
   return cronEvalCache.size;
 }
+
+export function getCronScheduleCacheMaxForTest(): number {
+  return CRON_EVAL_CACHE_MAX;
+}
+
+export function hasCronInCacheForTest(expr: string, tz: string): boolean {
+  return cronEvalCache.has(`${tz}\u0000${expr}`);
+}