perf: TTL-based eviction sweep for prefetch cache

packages/vinext/src/shims/navigation.ts

@@ -166,7 +166,7 @@ function withBasePath(p: string): string {
 // ---------------------------------------------------------------------------
 
 /** Maximum number of entries in the RSC prefetch cache. */
-const MAX_PREFETCH_CACHE_SIZE = 50;
+export const MAX_PREFETCH_CACHE_SIZE = 50;
 
 /** TTL for prefetch cache entries in ms (matches Next.js static prefetch TTL). */
 export const PREFETCH_CACHE_TTL = 30_000;
@@ -220,11 +220,28 @@ export function getPrefetchedUrls(): Set<string> {
  */
 export function storePrefetchResponse(rscUrl: string, response: Response): void {
   const cache = getPrefetchCache();
-  // Evict oldest entry if at capacity (Map iterates in insertion order)
+
+  // Sweep expired entries before resorting to FIFO eviction
+  if (cache.size >= MAX_PREFETCH_CACHE_SIZE) {
+    const now = Date.now();
+    const prefetched = getPrefetchedUrls();
+    for (const [key, entry] of cache) {
+      if (now - entry.timestamp >= PREFETCH_CACHE_TTL) {
+        cache.delete(key);
+        prefetched.delete(key);
+      }
+    }
+  }
+
+  // FIFO fallback if still at capacity after sweep
   if (cache.size >= MAX_PREFETCH_CACHE_SIZE) {
     const oldest = cache.keys().next().value;
-    if (oldest !== undefined) cache.delete(oldest);
+    if (oldest !== undefined) {
+      cache.delete(oldest);
+      getPrefetchedUrls().delete(oldest);
+    }
   }
+
   cache.set(rscUrl, { response, timestamp: Date.now() });
 }
 

tests/prefetch-cache.test.ts

@@ -0,0 +1,146 @@
+/**
+ * Prefetch cache eviction tests.
+ *
+ * Verifies that storePrefetchResponse() sweeps expired entries before
+ * falling back to FIFO eviction, preventing expired entries from wasting
+ * cache slots on link-heavy pages.
+ *
+ * The navigation module computes `isServer = typeof window === "undefined"`
+ * at load time, so we must set globalThis.window BEFORE importing it via
+ * vi.resetModules() + dynamic import().
+ */
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+
+type Navigation = typeof import("../packages/vinext/src/shims/navigation.js");
+let storePrefetchResponse: Navigation["storePrefetchResponse"];
+let getPrefetchCache: Navigation["getPrefetchCache"];
+let getPrefetchedUrls: Navigation["getPrefetchedUrls"];
+let MAX_PREFETCH_CACHE_SIZE: Navigation["MAX_PREFETCH_CACHE_SIZE"];
+let PREFETCH_CACHE_TTL: Navigation["PREFETCH_CACHE_TTL"];
+
+beforeEach(async () => {
+  // Set window BEFORE importing so isServer evaluates to false
+  (globalThis as any).window = {
+    __VINEXT_RSC_PREFETCH_CACHE__: new Map(),
+    __VINEXT_RSC_PREFETCHED_URLS__: new Set(),
+    location: { pathname: "/", search: "", hash: "", href: "http://localhost/" },
+    addEventListener: () => {},
+    history: { pushState: () => {}, replaceState: () => {}, state: null },
+    dispatchEvent: () => {},
+  };
+  vi.resetModules();
+  const nav = await import("../packages/vinext/src/shims/navigation.js");
+  storePrefetchResponse = nav.storePrefetchResponse;
+  getPrefetchCache = nav.getPrefetchCache;
+  getPrefetchedUrls = nav.getPrefetchedUrls;
+  MAX_PREFETCH_CACHE_SIZE = nav.MAX_PREFETCH_CACHE_SIZE;
+  PREFETCH_CACHE_TTL = nav.PREFETCH_CACHE_TTL;
+});
+
+afterEach(() => {
+  vi.restoreAllMocks();
+  delete (globalThis as any).window;
+});
+
+/** Helper: fill cache with `count` entries at a given timestamp. */
+function fillCache(count: number, timestamp: number, keyPrefix = "/page-"): void {
+  const cache = getPrefetchCache();
+  const prefetched = getPrefetchedUrls();
+  for (let i = 0; i < count; i++) {
+    const key = `${keyPrefix}${i}.rsc`;
+    cache.set(key, { response: new Response(`body-${i}`), timestamp });
+    prefetched.add(key);
+  }
+}
+
+describe("prefetch cache eviction", () => {
+  it("sweeps all expired entries before FIFO", () => {
+    const now = Date.now();
+    const expired = now - PREFETCH_CACHE_TTL - 1_000; // 31s ago
+
+    fillCache(MAX_PREFETCH_CACHE_SIZE, expired);
+    expect(getPrefetchCache().size).toBe(MAX_PREFETCH_CACHE_SIZE);
+    expect(getPrefetchedUrls().size).toBe(MAX_PREFETCH_CACHE_SIZE);
+
+    vi.spyOn(Date, "now").mockReturnValue(now);
+    storePrefetchResponse("/new.rsc", new Response("new"));
+
+    const cache = getPrefetchCache();
+    expect(cache.size).toBe(1);
+    expect(cache.has("/new.rsc")).toBe(true);
+    // All evicted entries should be removed from prefetched URL set
+    expect(getPrefetchedUrls().size).toBe(0);
+  });
+
+  it("falls back to FIFO when all entries are fresh", () => {
+    const now = Date.now();
+
+    fillCache(MAX_PREFETCH_CACHE_SIZE, now);
+    expect(getPrefetchCache().size).toBe(MAX_PREFETCH_CACHE_SIZE);
+    expect(getPrefetchedUrls().size).toBe(MAX_PREFETCH_CACHE_SIZE);
+
+    vi.spyOn(Date, "now").mockReturnValue(now);
+    storePrefetchResponse("/new.rsc", new Response("new"));
+
+    const cache = getPrefetchCache();
+    // FIFO evicted one, new one added → still at capacity
+    expect(cache.size).toBe(MAX_PREFETCH_CACHE_SIZE);
+    expect(cache.has("/new.rsc")).toBe(true);
+    // First inserted entry should be evicted
+    expect(cache.has("/page-0.rsc")).toBe(false);
+    // Second entry should survive
+    expect(cache.has("/page-1.rsc")).toBe(true);
+    // FIFO-evicted entry should be removed from prefetched URL set
+    expect(getPrefetchedUrls().size).toBe(MAX_PREFETCH_CACHE_SIZE - 1);
+    expect(getPrefetchedUrls().has("/page-0.rsc")).toBe(false);
+  });
+
+  it("sweeps only expired entries when cache has a mix", () => {
+    const now = Date.now();
+    const expired = now - PREFETCH_CACHE_TTL - 1_000;
+
+    const half = Math.floor(MAX_PREFETCH_CACHE_SIZE / 2);
+    const rest = MAX_PREFETCH_CACHE_SIZE - half;
+
+    fillCache(half, expired, "/expired-");
+    fillCache(rest, now, "/fresh-");
+    expect(getPrefetchCache().size).toBe(MAX_PREFETCH_CACHE_SIZE);
+    expect(getPrefetchedUrls().size).toBe(MAX_PREFETCH_CACHE_SIZE);
+
+    vi.spyOn(Date, "now").mockReturnValue(now);
+    storePrefetchResponse("/new.rsc", new Response("new"));
+
+    const cache = getPrefetchCache();
+    // expired swept, fresh kept, 1 new added
+    expect(cache.size).toBe(rest + 1);
+    expect(cache.has("/new.rsc")).toBe(true);
+
+    // All expired entries should be gone
+    for (let i = 0; i < half; i++) {
+      expect(cache.has(`/expired-${i}.rsc`)).toBe(false);
+    }
+    // All fresh entries should survive
+    for (let i = 0; i < rest; i++) {
+      expect(cache.has(`/fresh-${i}.rsc`)).toBe(true);
+    }
+    // Only fresh entries remain in prefetched URL set
+    expect(getPrefetchedUrls().size).toBe(rest);
+  });
+
+  it("does not sweep when cache is below capacity", () => {
+    const now = Date.now();
+    const expired = now - PREFETCH_CACHE_TTL - 1_000;
+
+    const belowCapacity = MAX_PREFETCH_CACHE_SIZE - 1;
+    fillCache(belowCapacity, expired);
+
+    vi.spyOn(Date, "now").mockReturnValue(now);
+    storePrefetchResponse("/new.rsc", new Response("new"));
+
+    const cache = getPrefetchCache();
+    // Below capacity — no eviction, all entries kept + 1 new
+    expect(cache.size).toBe(belowCapacity + 1);
+    // Prefetched URL set unchanged (no eviction triggered)
+    expect(getPrefetchedUrls().size).toBe(belowCapacity);
+  });
+});