fix(studio): a slow thumbnail job keeps a reader that other clips still decode from (#6643)

* fix(studio): a slow thumbnail job keeps a reader that other clips still decode from

A job that timed out judged the reader by its own progress only. A poster job that got
an already-open reader while a sibling was decoding on it, and took more than its
budget to reach its one frame (a slow link, Chrome's six connections held), logged a
timeout and retired a healthy reader, so the next clip re-read the index.

The pooled reader now counts the frames every job decodes from it, and a timed-out job
retires it only when no frame came from it while the job held it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(studio): the reader's frame count says whose frames it counts in its name, not a comment

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

packages/studio/src/player/lib/thumbnailSourcePool.ts

@@ -19,6 +19,7 @@ export interface ThumbnailSource extends OpenedSource {
   release(failed: boolean): void;
   /** The job waited for the reader to open: the open finishing was progress the job saw. */
   waitedForOpen: boolean;
+  progress: { framesFromAnyJob: number };
 }
 
 interface PooledSource {
@@ -28,6 +29,7 @@ interface PooledSource {
   ready: boolean;
   opened: Promise<OpenedSource>;
   users: number;
+  progress: { framesFromAnyJob: number };
   idleTimer: ReturnType<typeof setTimeout> | null;
   disposed: boolean;
 }
@@ -100,6 +102,7 @@ function pooledSource(source: string, version: string | undefined): PooledSource
     ready: false,
     opened: open(source, () => entry),
     users: 0,
+    progress: { framesFromAnyJob: 0 },
     idleTimer: null,
     disposed: false,
   };
@@ -150,7 +153,8 @@ export async function acquireThumbnailSource(
     }
   };
   try {
-    return { ...(await untilAborted(entry.opened, signal)), release, waitedForOpen };
+    const { progress } = entry;
+    return { ...(await untilAborted(entry.opened, signal)), release, waitedForOpen, progress };
   } catch (error) {
     // Timed out still waiting: the open made no progress in the job's whole run.
     release(!isAbortError(error) || timedOut(signal));

packages/studio/src/player/lib/thumbnailVideoDecoder.test.ts

@@ -766,6 +766,32 @@ describe("decodeVideoThumbnail's shared source", () => {
     expect(input.getPrimaryVideoTrack).toHaveBeenCalledTimes(1);
   });
 
+  it("keeps the reader when a clip times out while another clip decodes frames from it", async () => {
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    recordDecodes([]);
+    await decodeVideoThumbnail(clip(0), signal);
+    canvasesAtTimestamps.mockImplementationOnce(async function* () {
+      await new Promise(() => {});
+      yield null;
+    });
+    canvasesAtTimestamps.mockImplementationOnce(async function* (
+      timestamps: AsyncIterable<number>,
+    ) {
+      await timestamps[Symbol.asyncIterator]().next();
+      yield { canvas: document.createElement("canvas") };
+      await new Promise(() => {});
+    });
+    const controller = new AbortController();
+    void decodeVideoThumbnail(clip(6), controller.signal).catch(() => {});
+    void decodeVideoThumbnail(clip(12), new AbortController().signal).catch(() => {});
+    await vi.waitFor(() => expect(URL.createObjectURL).toHaveBeenCalledTimes(4));
+    controller.abort(timedOut());
+
+    expect(warn).not.toHaveBeenCalled();
+    await decodeVideoThumbnail(clip(18), signal);
+    expect(input.getPrimaryVideoTrack).toHaveBeenCalledTimes(1);
+  });
+
   it("fails a job that times out waiting for its source to open, and the next clip opens afresh", async () => {
     const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
     const stalled = untilDisposed();

packages/studio/src/player/lib/thumbnailVideoDecoder.ts

@@ -79,7 +79,6 @@ interface DecodedResources {
   held: HeldFrame[];
   canvases: Set<HTMLCanvasElement | OffscreenCanvas>;
   releasedFrameBytes: number;
-  decodedByJob: number;
 }
 
 interface ThumbnailCanvasSink {
@@ -175,6 +174,7 @@ async function decodeFrames(
   slots: { slot: number; key: string; decodedAt?: number }[],
   signal: AbortSignal,
   resources: DecodedResources,
+  progress: ThumbnailSource["progress"],
 ): Promise<void> {
   let next = 0;
   for await (const wrapped of sink.canvasesAtTimestamps(times)) {
@@ -185,7 +185,7 @@ async function decodeFrames(
     const blob = await canvasToBlob(wrapped.canvas);
     throwIfAborted(signal);
     const frame = shareFrame(target.key, target.decodedAt, URL.createObjectURL(blob), blob.size);
-    resources.decodedByJob += 1;
+    progress.framesFromAnyJob += 1;
     resources.urls[target.slot] = frame.url;
     resources.held.push({ key: target.key, frame });
   }
@@ -232,7 +232,6 @@ export async function decodeVideoThumbnail(
     held: [],
     canvases: new Set(),
     releasedFrameBytes: budgets.sharedFrameCacheBytes,
-    decodedByJob: 0,
   };
   const entry = sourceInfos.get(request.source);
   const known = entry?.version === request.contentVersion ? entry?.info : undefined;
@@ -271,8 +270,9 @@ async function decodeMissingFrames(
     if (timedOut(signal)) reportMediaFetchFailure(timeoutWithoutProgress());
     throw error;
   }
+  const framesBefore = source.progress.framesFromAnyJob;
   const releaseOnAbort = () => {
-    const madeProgress = source.waitedForOpen || resources.decodedByJob > 0;
+    const madeProgress = source.waitedForOpen || source.progress.framesFromAnyJob > framesBefore;
     const failed = timedOut(signal) && !madeProgress;
     if (failed) reportMediaFetchFailure(timeoutWithoutProgress());
     source.release(failed);
@@ -307,7 +307,8 @@ async function decodeMissingFrames(
         fit: request.fit ?? "cover",
         poolSize: 1,
       });
-      await decodeFrames(sink, decodeTimesAtNearbyKeyframes(), slots, signal, resources);
+      const times = decodeTimesAtNearbyKeyframes();
+      await decodeFrames(sink, times, slots, signal, resources, source.progress);
     }
     const result = loadedResult(resources, info.aspect, budgets, timestamps.length);
     source.release(false);