fix(sessions): don't evict loaded images whose waiters haven't taken them

With every cached image in use, a newly loaded entry (refs 0) was the only
evictable one and got revoked before its waiting hook acquired it, leaving
the image stuck on the loading placeholder; two loads finishing together
could also evict each other. Count pending waiters per image and skip
images that still have one when evicting.

src/lib/query/sessions.ts

@@ -292,12 +292,26 @@ interface CachedImage {
 
 const imageCache = new Map<string, CachedImage>();
 const imageLoads = new Map<string, Promise<string>>();
+/** 每张图还在等加载结果、尚未占用的等待方个数；有等待方的图不淘汰 */
+const imageWaiters = new Map<string, number>();
 
-/** 超出上限时按最久未用淘汰没人在用的图并 revoke */
+const startWaiting = (key: string) =>
+  imageWaiters.set(key, (imageWaiters.get(key) ?? 0) + 1);
+
+const stopWaiting = (key: string) => {
+  const left = (imageWaiters.get(key) ?? 0) - 1;
+  if (left > 0) imageWaiters.set(key, left);
+  else imageWaiters.delete(key);
+};
+
+/**
+ * 超出上限时按最久未用淘汰没人在用的图并 revoke。
+ * 加载完、等待方还没来得及占用的图也算在用（几张图同时加载完时会互相淘汰）
+ */
 const evictImages = () => {
   for (const [key, entry] of imageCache) {
     if (imageCache.size <= SESSION_IMAGE_CACHE_SIZE) return;
-    if (entry.refs > 0) continue;
+    if (entry.refs > 0 || imageWaiters.has(key)) continue;
     URL.revokeObjectURL(entry.url);
     imageCache.delete(key);
   }
@@ -347,6 +361,7 @@ export const clearSessionImageCache = () => {
   imageCache.forEach((entry) => URL.revokeObjectURL(entry.url));
   imageCache.clear();
   imageLoads.clear();
+  imageWaiters.clear();
 };
 
 export interface SessionImageResult {
@@ -387,11 +402,16 @@ export const useSessionImage = (
       take();
     } else {
       setState({ url: null, isLoading: true, error: null });
+      startWaiting(key);
       loadImageUrl(key, providerId!, sourcePath!, image!).then(
         () => {
-          if (!cancelled) take();
+          stopWaiting(key);
+          // 已卸载就不占用；不再等它了，补一次淘汰以免超出上限
+          if (cancelled) evictImages();
+          else take();
         },
         (error: unknown) => {
+          stopWaiting(key);
           if (!cancelled) setState({ url: null, isLoading: false, error });
         },
       );

tests/lib/sessionsQuery.test.tsx

@@ -413,6 +413,52 @@ describe("useSessionImage", () => {
     held.unmount();
   });
 
+  it("上限内的图全在用时，新加载的那张也拿得到 URL，不会被立刻淘汰", async () => {
+    const mounted = [];
+    for (let i = 0; i < SESSION_IMAGE_CACHE_SIZE; i += 1) {
+      const hook = renderHook(() =>
+        useSessionImage("codex", "/a", image(`/busy-${i}.png`)),
+      );
+      await waitFor(() => expect(hook.result.current.url).toBeTruthy());
+      mounted.push(hook);
+    }
+    const extra = renderHook(() =>
+      useSessionImage("codex", "/a", image("/extra.png")),
+    );
+    await waitFor(() => expect(extra.result.current.isLoading).toBe(false));
+    expect(extra.result.current.url).toBe(
+      `blob:${SESSION_IMAGE_CACHE_SIZE + 1}`,
+    );
+    expect(revokeObjectURL).not.toHaveBeenCalled();
+    extra.unmount();
+    mounted.forEach((hook) => hook.unmount());
+  });
+
+  it("上限内的图全在用时，两张同时加载完也都拿得到 URL，不会互相淘汰", async () => {
+    const mounted = [];
+    for (let i = 0; i < SESSION_IMAGE_CACHE_SIZE; i += 1) {
+      const hook = renderHook(() =>
+        useSessionImage("codex", "/a", image(`/busy-${i}.png`)),
+      );
+      await waitFor(() => expect(hook.result.current.url).toBeTruthy());
+      mounted.push(hook);
+    }
+    const resolvers: Array<(value: ArrayBuffer) => void> = [];
+    api.getImage.mockImplementation(
+      () => new Promise<ArrayBuffer>((ok) => resolvers.push(ok)),
+    );
+    const x = renderHook(() => useSessionImage("codex", "/a", image("/x.png")));
+    const y = renderHook(() => useSessionImage("codex", "/a", image("/y.png")));
+    await act(async () => {
+      resolvers.forEach((resolve) => resolve(new ArrayBuffer(3)));
+    });
+    await waitFor(() => expect(y.result.current.isLoading).toBe(false));
+    expect(x.result.current.url).toBeTruthy();
+    expect(y.result.current.url).toBeTruthy();
+    expect(revokeObjectURL).not.toHaveBeenCalled();
+    [x, y, ...mounted].forEach((hook) => hook.unmount());
+  });
+
   it("加载中卸载：不再更新状态", async () => {
     let resolve: (value: ArrayBuffer) => void = () => {};
     let reject: (error: Error) => void = () => {};