fix(producer): start segmented render workers together so slow media waits overlap (#5573)

* fix(producer): start segmented render workers together so slow media waits overlap

Each worker's page load used to wait for the previous worker's media,
fonts and Tailwind to be ready, so a slow readiness wait was paid once
per worker. Open every worker session at once, as non-segmented
parallel capture already does.

* fix(producer): trim the worker-opening comment

* fix(producer): name the worker-opening helper instead of commenting it

packages/producer/src/services/render/stages/captureSegmentedStage.test.ts

@@ -700,6 +700,36 @@ describe("runCaptureSegmentedStage", () => {
     expect(createdSessionConfigs).toEqual([expect.objectContaining({ enableBrowserPool: false })]);
   });
 
+  it("opens every worker's session together, so readiness waits overlap", async () => {
+    let opening = 0;
+    let mostOpeningAtOnce = 0;
+    let id = 0;
+    const result = await runCaptureSegmentedStage({
+      ...fakeStageInput({ totalFrames: 9 }),
+      probeSession: null,
+      segmentFrames: 3,
+      segmentDir: join(fixtureRoot, "concurrent-open"),
+      workerCount: 3,
+      sessionFactory: {
+        create: async () => {
+          mostOpeningAtOnce = Math.max(mostOpeningAtOnce, ++opening);
+          await new Promise((resolve) => setImmediate(resolve));
+          opening--;
+          return fakeSession(id++);
+        },
+      },
+      deps: {
+        spawnEncoder: mock(async () => okEncoder()),
+        captureFrame: mock(async () => ({ buffer: Buffer.alloc(1) })),
+        concat: mock(async () => ({ success: true as const })),
+        closeSession: mock(async () => {}),
+        removeFile: () => {},
+      },
+    });
+    expect(result.success).toBe(true);
+    expect(mostOpeningAtOnce).toBe(3);
+  });
+
   it("closes a fresh session whose initialization fails", async () => {
     closeCaptureSession.mockClear();
     failInitializeSession = true;

packages/producer/src/services/render/stages/captureSegmentedStage.ts

@@ -408,6 +408,18 @@ async function createSegmentWorker(run: SegmentRun, id: number): Promise<Segment
   };
 }
 
+async function openSegmentWorkersTogether(
+  run: SegmentRun,
+  workersToCloseLater: SegmentWorker[],
+): Promise<void> {
+  const opened = await Promise.allSettled(
+    Array.from({ length: run.workerCount }, (_, id) => createSegmentWorker(run, id)),
+  );
+  for (const o of opened) if (o.status === "fulfilled") workersToCloseLater.push(o.value);
+  const failure = opened.find((o) => o.status === "rejected");
+  if (failure && failure.status === "rejected") throw failure.reason;
+}
+
 /** Close this worker's browser and open a fresh one at a segment boundary. */
 async function recycleWorkerSession(
   run: SegmentRun,
@@ -585,7 +597,7 @@ export async function runCaptureSegmentedStage(
 
   try {
     assertNotAborted();
-    for (let id = 0; id < run.workerCount; id++) workers.push(await createSegmentWorker(run, id));
+    await openSegmentWorkersTogether(run, workers);
     // allSettled, not all: closing a session out from under a worker that is
     // still capturing orphans its ffmpeg and races the CDP connection, so
     // every worker has to stop before the finally runs. The first error is