hyperframes/packages/producer/src/services/distributed/renderChunk.ts

/**
 * Activity B of the distributed render pipeline.
 *
 * `renderChunk(planDir, chunkIndex, outputChunkPath)` validates the planDir
 * against the worker's environment, captures the chunk's frame range, and
 * encodes a single closed-GOP video chunk (or, for png-sequence, a directory
 * of PNGs). The output is byte-identical across retries on the same worker
 * and PSNR-equivalent across workers — that contract is what makes Temporal
 * activity retries safe.
 *
 * Pure function over local paths. No networking. Spins up its own headless
 * Chrome + file server scoped to the chunk; tears them down before
 * returning. The caller is responsible for moving `outputChunkPath` to its
 * orchestration-level storage (S3 / GCS / EFS / …).
 *
 * Hard contracts:
 *   - The worker re-applies `meta/encoder.json.runtimeEnv` into
 *     `process.env` BEFORE the file server starts so the served HTML's
 *     `RENDER_MODE_SCRIPT` sees the same env it would have seen on the
 *     controller.
 *   - Browser is launched with `browserGpuMode: "software"` and verified
 *     against `chrome://gpu` via `assertSwiftShader` — a non-SwiftShader
 *     backend trips a non-retryable `BROWSER_GPU_NOT_SOFTWARE`.
 *   - The file server serves with the seeded-random shim
 *     (`buildVirtualTimeShim({ seedRandomFromFrame: true })`) so any
 *     composition that uses `Math.random` / `crypto.getRandomValues`
 *     produces byte-identical pixels per `(planDir, chunkIndex)`.
 *   - No `lastFrameCache` priming: every frame seeks fresh DOM so the
 *     cache is never read, and priming would deadlock the compositor.
 *   - The chunk's encode runs with `lockGopForChunkConcat: true` and
 *     `gopSize === framesInChunk` so concat-copy at assemble time is safe.
 *
 * Every determinism toggle above is opt-in — only this primitive enables them.
 * In-process renders (`executeRenderJob`) leave them off.
 */

import { randomBytes } from "node:crypto";
import { existsSync, mkdirSync, readFileSync, readdirSync, rmSync, writeFileSync } from "node:fs";
import { extname, join } from "node:path";
import {
  assertSwiftShader,
  type BeforeCaptureHook,
  BROWSER_GPU_NOT_SOFTWARE,
  calculateOptimalWorkers,
  type CaptureOptions,
  type CaptureSession,
  closeCaptureSession,
  createCaptureSession,
  createFrameLookupTable,
  createVideoFrameInjector,
  type EngineConfig,
  type ExtractedFrames,
  getEncoderPreset,
  initializeSession,
  readWebGlVendorInfoFromCanvas,
  resolveConfig,
} from "@hyperframes/engine";
import { defaultLogger } from "../../logger.js";
import { runEncodeStage } from "../render/stages/encodeStage.js";
import { runCaptureStage } from "../render/stages/captureStage.js";
import {
  type ChunkSliceJson,
  type LockedRenderConfig,
  recomputePlanHashFromPlanDir,
} from "../render/stages/freezePlan.js";
import { sha256Hex } from "../render/stages/planHash.js";
import { applyRuntimeEnvSnapshot } from "../render/runtimeEnvSnapshot.js";
import {
  buildVirtualTimeShim,
  closeFileServerSafely,
  createFileServer,
  type FileServerHandle,
} from "../fileServer.js";
import {
  buildSyntheticRenderJob,
  type DistributedFormat,
  PLAN_VIDEOS_META_RELATIVE_PATH,
  type PlanVideosJson,
  readFfmpegVersion,
} from "./shared.js";

/**
 * Non-retryable error codes raised when the planDir is structurally
 * malformed, semantically out of range, or fingerprints differently from
 * what the controller wrote. Each is distinct so adapter retry policies
 * can route them independently — e.g. `MISSING_PLAN_ARTIFACT` may point
 * to a partial S3 download that a retry could heal, while
 * `PLAN_HASH_MISMATCH` strictly indicates cross-version drift that
 * retries won't fix.
 */
export const FFMPEG_VERSION_MISMATCH = "FFMPEG_VERSION_MISMATCH";
export const PLAN_HASH_MISMATCH = "PLAN_HASH_MISMATCH";
export const MISSING_PLAN_ARTIFACT = "MISSING_PLAN_ARTIFACT";
export const CHUNK_INDEX_OUT_OF_RANGE = "CHUNK_INDEX_OUT_OF_RANGE";
export const MISSING_RUNTIME_ENV_SNAPSHOT = "MISSING_RUNTIME_ENV_SNAPSHOT";
const LEGACY_DISTRIBUTED_VP9_CPU_USED = 2;

export type RenderChunkValidationCode =
  | typeof FFMPEG_VERSION_MISMATCH
  | typeof PLAN_HASH_MISMATCH
  | typeof MISSING_PLAN_ARTIFACT
  | typeof CHUNK_INDEX_OUT_OF_RANGE
  | typeof MISSING_RUNTIME_ENV_SNAPSHOT
  | typeof BROWSER_GPU_NOT_SOFTWARE;

/**
 * Typed non-retryable error raised by `renderChunk` when the planDir is
 * malformed or the worker's runtime doesn't match the planDir's
 * controller-side fingerprint. Workflow adapters key retry policies off
 * `code` — most of these failures will not heal on retry.
 */
export class RenderChunkValidationError extends Error {
  readonly code: RenderChunkValidationCode;
  constructor(code: RenderChunkValidationCode, message: string) {
    super(message);
    this.name = "RenderChunkValidationError";
    this.code = code;
  }
}

/**
 * Result of {@link renderChunk}. The `sha256` field is the byte hash of the
 * primary output (the mp4/mov file, or, for png-sequence, the sorted-frame
 * fingerprint). Retries on the same `(planDir, chunkIndex)` MUST produce
 * the same `sha256` — that contract is the byte-identical-retry axis.
 */
export interface ChunkResult {
  /** Absolute path the encoded chunk was written to (file or directory). */
  outputPath: string;
  /** `"file"` for mp4/mov; `"frame-dir"` for png-sequence. */
  outputKind: "file" | "frame-dir";
  framesEncoded: number;
  sha256: string;
  durationMs: number;
  /**
   * Stage wall-clock split of `durationMs`, for separating per-chunk fixed
   * overhead from frame-proportional work in fleet cost models:
   *
   * - `planHashMs` — full planDir content-hash recomputation (validation).
   * - `sessionBootMs` — sequential-branch Chrome boot + SwiftShader assert +
   *   composition warmup. Stays 0 when `workers > 1` (each parallel worker
   *   boots inside the capture stage instead).
   * - `captureStageMs` — the capture stage call; includes per-worker session
   *   boots in the parallel branch.
   * - `encodeStageMs` — the encode stage call (single ffmpeg invocation, or
   *   the frame-dir arrangement for png-sequence).
   *
   * The remainder of `durationMs` is validation + file-server setup + output
   * hashing + cleanup.
   */
  planHashMs: number;
  sessionBootMs: number;
  captureStageMs: number;
  encodeStageMs: number;
  /** Capture workers used for this chunk (`calculateOptimalWorkers` result). */
  workers: number;
  /**
   * Path to a sidecar JSON containing per-chunk perf counters. Adapters
   * upload this alongside the chunk so per-chunk regressions are
   * inspectable without the workflow having to carry the payload.
   */
  perfPath: string;
}

/**
 * Rebuild the engine's in-memory `ExtractedFrames[]` from the on-disk
 * planDir layout. `<planDir>/video-frames/<videoId>/` holds the numbered
 * frame files plan() extracted; this lists each dir and rebuilds the
 * 1-based `framePaths` Map that `FrameLookupTable` / `videoFrameInjector`
 * both index against.
 */
function rebuildExtractedFramesFromPlanDir(
  planDir: string,
  videos: PlanVideosJson["extracted"],
): ExtractedFrames[] {
  const result: ExtractedFrames[] = [];
  for (const v of videos) {
    const outputDir = join(planDir, "video-frames", v.videoId);
    if (!existsSync(outputDir)) {
      throw new Error(
        `[renderChunk] planDir missing extracted video frames for ${JSON.stringify(v.videoId)}: ` +
          `${outputDir} not present. plan() should have written frames here; the planDir is malformed.`,
      );
    }
    // framePattern looks like `frame_%05d.jpg`; sprintf isn't available at
    // runtime so list-and-sort the directory. Sorted-by-name matches
    // sorted-by-frame-index because the extractor writes zero-padded
    // monotonic indices.
    const ext = (extname(v.framePattern) || ".jpg").toLowerCase();
    const frames = readdirSync(outputDir)
      .filter((name) => name.toLowerCase().endsWith(ext))
      .sort();
    const framePaths = new Map<number, string>();
    for (let i = 0; i < frames.length; i++) {
      const frameName = frames[i];
      if (!frameName) continue;
      // FrameLookupTable indexes frames 1-based.
      framePaths.set(i + 1, join(outputDir, frameName));
    }
    result.push({
      videoId: v.videoId,
      srcPath: v.srcPath,
      outputDir,
      framePattern: v.framePattern,
      fps: v.fps,
      totalFrames: v.totalFrames,
      metadata: v.metadata,
      framePaths,
      // The chunk worker doesn't own the planDir's video-frames/ directory
      // (the controller does — adapters that fan out chunks across machines
      // share the planDir as read-only). Mark ownership as false so the
      // injector's eventual cleanup doesn't rm bytes another worker may
      // still be reading.
      ownedByLookup: false,
    });
  }
  return result;
}

/** Plan-time JSON manifest written by `freezePlan`. */
interface PlanJson {
  planHash: string;
  producerVersion: string;
  ffmpegVersion: string;
  fontSnapshotSha: string;
  dimensions: {
    fpsNum: number;
    fpsDen: number;
    width: number;
    height: number;
    format: DistributedFormat;
  };
  chunkCount: number;
  totalFrames: number;
  duration: number;
  hasAudio: boolean;
}

/**
 * Re-export the runtime-env apply helper so adapters that import only
 * this subpath can prime `process.env` before instantiating their own
 * file server. Returns a `{ restore }` handle — adapters that fan out
 * multiple chunks per process MUST call `restore()` between chunks.
 */
export { applyRuntimeEnvSnapshot } from "../render/runtimeEnvSnapshot.js";

// `readWebGlVendorInfoFromCanvas` lives in `@hyperframes/engine` (it's
// used both here and by `parallelCoordinator.executeWorkerTask`). Re-exported
// from this subpath so downstream consumers that already import it from
// `@hyperframes/producer/distributed` keep working.
export { readWebGlVendorInfoFromCanvas } from "@hyperframes/engine";

/**
 * Compute a deterministic SHA-256 fingerprint for the chunk's output.
 *
 *   - file output (mp4/mov): straight hash of the file bytes.
 *   - frame-dir (png-sequence): hash the sorted list of `(name, sha256)`
 *     pairs. Avoids the cost of streaming every frame's contents through
 *     a single sha context while still detecting any byte-level drift in
 *     any individual frame.
 *
 * The fingerprint flows into the `ChunkResult.sha256` which adapters
 * compare across retries to enforce the byte-identical-retry contract.
 */
function hashChunkOutput(outputPath: string, kind: "file" | "frame-dir"): string {
  if (kind === "file") return sha256Hex(readFileSync(outputPath));
  const entries = readdirSync(outputPath)
    .filter((name) => /\.(png|jpg|jpeg)$/i.test(name))
    .sort();
  // Hash the sorted (name, perFileSha) list. Encoded as null-separated
  // utf-8 to keep concatenation unambiguous if a frame name ever contains
  // an unusual character.
  const lines = entries.map(
    (name) => `${name}\0${sha256Hex(readFileSync(join(outputPath, name)))}`,
  );
  return sha256Hex(lines.join("\0"));
}

/**
 * Apply the planDir's locked-encoder choice on top of an
 * `EncoderPreset` from `getEncoderPreset`. `getEncoderPreset` returns
 * h265 only on the HDR branch, but distributed mode is SDR-only — for
 * an `libx265-software` planDir we still need to flip the preset's
 * codec to h265 so `runEncodeStage` invokes libx265. Exported so a
 * unit test can pin the override independently of the heavyweight
 * Docker fixture: a refactor that moves the override (e.g. into
 * `getEncoderPreset` itself) shouldn't be able to silently regress
 * the contract without a fast-test signal.
 */
export function resolvePresetForLockedEncoder<
  P extends { codec: "h264" | "h265" | "vp9" | "prores" },
>(basePreset: P, lockedEncoder: LockedRenderConfig["encoder"]): P {
  if (lockedEncoder === "libx265-software") {
    return { ...basePreset, codec: "h265" as const };
  }
  return basePreset;
}

export function resolveLockedVp9CpuUsed(
  lockedEncoder: Pick<LockedRenderConfig, "encoder" | "vp9CpuUsed">,
): number | undefined {
  if (lockedEncoder.encoder !== "libvpx-vp9-software") return undefined;
  // Pre-vp9CpuUsed WebM planDirs used the old closed-GOP literal. Keep replay
  // bytes stable for those plans while new planDirs carry their resolved value.
  return lockedEncoder.vp9CpuUsed ?? LEGACY_DISTRIBUTED_VP9_CPU_USED;
}

/**
 * Activity B: render a single chunk of the planDir. The `outputChunkPath`
 * argument is a file for mp4/mov outputs and a directory for png-sequence
 * outputs — the caller picks the right shape based on `meta/encoder.json`.
 * `renderChunk` enforces the same choice via `outputKind` on the result.
 */
export async function renderChunk(
  planDir: string,
  chunkIndex: number,
  outputChunkPath: string,
): Promise<ChunkResult> {
  const start = Date.now();
  const log = defaultLogger;

  // ── Read + validate the plan ──
  const planJsonPath = join(planDir, "plan.json");
  const encoderJsonPath = join(planDir, "meta", "encoder.json");
  const chunksJsonPath = join(planDir, "meta", "chunks.json");
  for (const required of [planJsonPath, encoderJsonPath, chunksJsonPath]) {
    if (!existsSync(required)) {
      throw new RenderChunkValidationError(
        MISSING_PLAN_ARTIFACT,
        `[renderChunk] planDir is missing required artifact: ${required}`,
      );
    }
  }
  const plan = JSON.parse(readFileSync(planJsonPath, "utf-8")) as PlanJson;
  const encoder = JSON.parse(readFileSync(encoderJsonPath, "utf-8")) as LockedRenderConfig;
  const chunks = JSON.parse(readFileSync(chunksJsonPath, "utf-8")) as ChunkSliceJson[];

  // `meta/videos.json` only exists when the composition has `<video>`
  // elements; absence means no injector is needed.
  const videosJsonPath = join(planDir, PLAN_VIDEOS_META_RELATIVE_PATH);
  let planVideos: PlanVideosJson | null = null;
  if (existsSync(videosJsonPath)) {
    try {
      planVideos = JSON.parse(readFileSync(videosJsonPath, "utf-8")) as PlanVideosJson;
    } catch (err) {
      throw new RenderChunkValidationError(
        MISSING_PLAN_ARTIFACT,
        `[renderChunk] failed to parse ${videosJsonPath}: ${err instanceof Error ? err.message : String(err)}`,
      );
    }
  }

  if (chunkIndex < 0 || chunkIndex >= chunks.length) {
    throw new RenderChunkValidationError(
      CHUNK_INDEX_OUT_OF_RANGE,
      `[renderChunk] chunkIndex ${chunkIndex} is out of range [0, ${chunks.length})`,
    );
  }
  // The bounds check above guarantees this hits, but TS doesn't narrow
  // the indexed access — re-check explicitly.
  const slice = chunks[chunkIndex];
  if (slice === undefined) {
    throw new RenderChunkValidationError(CHUNK_INDEX_OUT_OF_RANGE, "[renderChunk] missing slice");
  }
  const framesInChunk = slice.endFrame - slice.startFrame;
  if (framesInChunk <= 0) {
    throw new RenderChunkValidationError(
      CHUNK_INDEX_OUT_OF_RANGE,
      `[renderChunk] chunk ${chunkIndex} has non-positive frame count: ${framesInChunk}`,
    );
  }

  const compiledDir = join(planDir, "compiled");
  if (!existsSync(compiledDir)) {
    throw new RenderChunkValidationError(
      MISSING_PLAN_ARTIFACT,
      `[renderChunk] planDir missing compiled/ directory: ${compiledDir}`,
    );
  }

  // ── Cross-version sanity ──
  const ffmpegVersion = await readFfmpegVersion();
  if (ffmpegVersion !== plan.ffmpegVersion) {
    throw new RenderChunkValidationError(
      FFMPEG_VERSION_MISMATCH,
      `[renderChunk] ffmpeg version on this worker does not match planDir. ` +
        `planDir: ${JSON.stringify(plan.ffmpegVersion)}; worker: ${JSON.stringify(ffmpegVersion)}. ` +
        `Distributed retries require byte-identical ffmpeg builds across workers. ` +
        `Re-plan from a worker matching this version, or run all renders on an image with the planDir's ffmpeg.`,
    );
  }
  if (encoder.browserGpuMode !== "software") {
    throw new RenderChunkValidationError(
      BROWSER_GPU_NOT_SOFTWARE,
      `[renderChunk] planDir requires browserGpuMode=software, got ${JSON.stringify(encoder.browserGpuMode)}.`,
    );
  }

  // Re-derive `planHash` from the on-disk bytes and compare to the value
  // the controller wrote into `plan.json`. Catches corrupted artifacts
  // (truncated meta files, partial S3 downloads, manual tampering) before
  // the chunk renders. Distinct from the other validation paths above
  // because `MISSING_PLAN_ARTIFACT` etc. are structural; this is purely
  // content-fingerprint drift.
  const planHashStarted = Date.now();
  const recomputedPlanHash = recomputePlanHashFromPlanDir(planDir);
  const planHashMs = Date.now() - planHashStarted;
  if (recomputedPlanHash !== plan.planHash) {
    throw new RenderChunkValidationError(
      PLAN_HASH_MISMATCH,
      `[renderChunk] planDir content fingerprint does not match plan.json.planHash. ` +
        `plan.json: ${plan.planHash}; recomputed: ${recomputedPlanHash}. ` +
        `Likely a corrupted artifact (partial S3 download, manual tampering) or a planDir ` +
        `produced by an incompatible producer version. Re-plan and re-fan-out.`,
    );
  }

  // Distinct from the silent `?? {}` fallback we used before: missing
  // `runtimeEnv` means the planDir was produced by a controller that
  // forgot to snapshot, and the chunk's pixels would diverge silently.
  // Surface it as a typed validation error so the workflow can re-plan.
  if (!encoder.runtimeEnv || typeof encoder.runtimeEnv !== "object") {
    throw new RenderChunkValidationError(
      MISSING_RUNTIME_ENV_SNAPSHOT,
      "[renderChunk] planDir is missing meta/encoder.json.runtimeEnv snapshot. " +
        "Re-plan with the current producer.",
    );
  }

  // Apply the controller's runtime-env snapshot. Must happen BEFORE the
  // file server is created — RENDER_MODE_SCRIPT bakes env vars into
  // served HTML at module load. The `restore()` handle is invoked in
  // `finally` so multi-chunk workers (Cloud Run Jobs, Temporal activity
  // worker) don't leak chunk N's env into chunk N+1.
  const envRestore = applyRuntimeEnvSnapshot(encoder.runtimeEnv);

  try {
    // Synthesize a RenderJob the existing stages can consume. The chunk's
    // duration is its own frame count over fps — not the plan's full
    // duration — so the stages see this chunk as a self-contained render.
    const job = buildSyntheticRenderJob({
      fps: { num: plan.dimensions.fpsNum, den: plan.dimensions.fpsDen },
      quality: encoder.quality,
      format: plan.dimensions.format,
      crf: encoder.crf,
      bitrate: encoder.bitrate,
      hdrMode: "force-sdr",
      entryFile: "index.html",
    });
    job.totalFrames = framesInChunk;
    job.duration = (framesInChunk * plan.dimensions.fpsDen) / plan.dimensions.fpsNum;

    const cfg: EngineConfig = {
      ...resolveConfig(),
      browserGpuMode: "software",
      forceScreenshot: encoder.forceScreenshot,
    };

    // Build the BeforeCaptureHook that injects pre-extracted video frames
    // into the page once per chunk and reuse — `runCaptureStage` may
    // invoke `createRenderVideoFrameInjector` multiple times, and
    // re-listing `planDir/video-frames/` each call would be wasteful.
    // Compositions with no video elements produce `null`, matching the
    // in-process renderer's skip path.
    const videoInjector: BeforeCaptureHook | null =
      planVideos && planVideos.extracted.length > 0
        ? createVideoFrameInjector(
            createFrameLookupTable(
              planVideos.videos,
              rebuildExtractedFramesFromPlanDir(planDir, planVideos.extracted),
            ),
          )
        : null;

    // ── Per-chunk work + frames directories ──
    // Suffix workDir with pid + random bytes so concurrent invocations on
    // the SAME `(planDir, chunkIndex)` (e.g. a scheduler that double-fires
    // due to heartbeat skew) don't race on the same tmp tree. The output
    // path itself is still the caller's contract — concurrent writers to
    // `outputChunkPath` produce undefined bytes, but we don't make it worse
    // by also deleting their workDirs out from under them.
    const workDir = `${outputChunkPath}.work.${process.pid}.${randomBytes(4).toString("hex")}`;
    mkdirSync(workDir, { recursive: true });
    const framesDir = join(workDir, "captured-frames");
    mkdirSync(framesDir, { recursive: true });

    // ── File server with the seeded-random shim ──
    // `Math.random` / `crypto.getRandomValues` are seeded from virtual
    // time so retries are pixel-identical. Only distributed renders flip this.
    const fileServer: FileServerHandle = await createFileServer({
      projectDir: compiledDir,
      compiledDir,
      port: 0,
      preHeadScripts: [buildVirtualTimeShim({ seedRandomFromFrame: true })],
    });

    const captureOptions: CaptureOptions = {
      width: plan.dimensions.width,
      height: plan.dimensions.height,
      fps: { num: plan.dimensions.fpsNum, den: plan.dimensions.fpsDen },
      format: plan.dimensions.format === "mp4" ? "jpeg" : "png",
      quality: plan.dimensions.format === "mp4" ? 80 : undefined,
      deviceScaleFactor: encoder.deviceScaleFactor,
      // Re-inject the controller's snapshotted variables so the chunk's
      // first capture sees the same `window.__hfVariables` the in-process
      // renderer would have seen. Optional — compositions that don't
      // declare `data-composition-variables` leave this undefined and the
      // engine skips the `evaluateOnNewDocument` injection.
      variables: encoder.variables,
      // lock the BeginFrame warmup loop to a fixed iteration count so
      // `beginFrameTimeTicks` is host-independent. Only chunks ever set this.
      lockWarmupTicks: true,
    };

    // Resolve worker count up-front so we can decide whether to bother
    // pre-warming a probe session at all. The parallel branch
    // (chunkWorkerCount > 1) closes the probe immediately and creates fresh
    // per-worker sessions; `executeWorkerTask` runs `assertSwiftShader`
    // on worker 0 only (gated on `cfg.browserGpuMode === "software"`), so
    // the safety contract holds without the eager pre-probe and without
    // every worker concurrently navigating to the GL probe page. See
    // `heygen-com/hyperframes#955` for the worst-case wall regression that
    // motivated gating the probe to worker 0.
    //
    // Capture-cost calibration based on shader transitions / renderModeHints
    // is not threaded through to chunks yet; the in-process renderer's
    // `resolveRenderWorkerCount` wraps this with that reduction, but
    // `PlanJson` doesn't carry the compiled hints needed to call it
    // directly. The existing adaptive-retry path reduces workers if
    // compositor contention surfaces as CDP timeouts.
    const chunkWorkerCount = calculateOptimalWorkers(framesInChunk, undefined, cfg);

    // ── Browser + warmup ──
    let session: CaptureSession | null = null;
    let outputKind: "file" | "frame-dir";
    let framesEncoded = 0;
    // Stage wall-clock split for the cost model: per-chunk fixed overhead
    // (boot, warmup, hash, IO) vs frame-proportional work. Consumed from the
    // perf sidecar / ChunkResult by the orchestration's telemetry.
    let sessionBootMs = 0;
    let captureStageMs = 0;
    let encodeStageMs = 0;
    try {
      if (chunkWorkerCount === 1) {
        // Sequential branch reuses the probe session for the actual capture.
        // SwiftShader assertion runs BEFORE initializeSession (which
        // navigates to the composition); on failure we tear down without
        // ever touching the composition URL. We pass
        // `readWebGlVendorInfoFromCanvas` rather than letting
        // `assertSwiftShader` use its default `chrome://gpu` reader —
        // `chrome-headless-shell` serves chrome:// pages as empty documents,
        // which would trip a false-negative even when the GL backend is in
        // fact SwiftShader. The canvas + WEBGL_debug_renderer_info probe
        // works on any page (we navigate to about:blank inside the helper).
        const bootStarted = Date.now();
        session = await createCaptureSession(fileServer.url, framesDir, captureOptions, null, cfg);
        await assertSwiftShader(session.page, readWebGlVendorInfoFromCanvas);
        await initializeSession(session);
        sessionBootMs = Date.now() - bootStarted;
        // `discardWarmupCapture` is intentionally NOT called: every frame
        // seeks fresh DOM, so `lastFrameCache` is never read; priming it
        // would deadlock Chrome's compositor by issuing a second beginFrame
        // at a `frameTimeTicks` it had just advanced to.
      }
      // chunkWorkerCount > 1: skip the probe entirely. Each parallel worker
      // creates its own session and runs `assertSwiftShader` before its
      // first frame.

      // In the parallel branch (chunkWorkerCount > 1) this stage also boots
      // one Chrome session per worker, so captureStageMs includes those
      // boots; sessionBootMs stays 0 there.
      const captureStarted = Date.now();
      await runCaptureStage({
        fileServer,
        workDir,
        framesDir,
        job,
        totalFrames: framesInChunk,
        cfg,
        forceScreenshot: encoder.forceScreenshot,
        log,
        workerCount: chunkWorkerCount,
        probeSession: session,
        needsAlpha: plan.dimensions.format !== "mp4",
        captureAttempts: [],
        // Distributed chunks run on Linux (beginframe) where dedup never arms;
        // a throwaway sink satisfies the type without per-chunk dedup reporting.
        dedupPerfs: [],
        buildCaptureOptions: () => captureOptions,
        createRenderVideoFrameInjector: () => videoInjector,
        abortSignal: undefined,
        assertNotAborted: () => {},
        frameRange: { startFrame: slice.startFrame, endFrame: slice.endFrame },
      });
      // captureStage closes the session it consumed.
      captureStageMs = Date.now() - captureStarted;
      session = null;
      framesEncoded = framesInChunk;

      // ── Encode the chunk ──
      const isPngSequence = plan.dimensions.format === "png-sequence";
      outputKind = isPngSequence ? "frame-dir" : "file";
      // For mp4 / mov / webm we use the standard preset machinery; the
      // locked encoder values come from `meta/encoder.json` and the
      // `lockGopForChunkConcat` toggle is the only Phase-2 flag that flips
      // on at this site. png-sequence has no encoder, but `runEncodeStage`
      // still reads `preset.quality` for bookkeeping (it never reaches
      // ffmpeg on the pngseq branch). Fall back to the mp4 preset shape —
      // same trick `renderOrchestrator` plays.
      const presetFormat: "mp4" | "mov" | "webm" = isPngSequence
        ? "mp4"
        : (plan.dimensions.format as "mp4" | "mov" | "webm");
      const basePreset = getEncoderPreset(job.config.quality, presetFormat, undefined);
      const preset = resolvePresetForLockedEncoder(basePreset, encoder.encoder);
      const effectiveQuality = encoder.crf ?? preset.quality;
      const effectiveBitrate = encoder.crf != null ? undefined : encoder.bitrate;
      // For non-pngseq, encodeStage writes to `outputPath` when `isPngSequence`
      // is false. `videoOnlyPath` is the encoder's direct output (no mux —
      // mux happens in assemble()).
      const videoOnlyPath = outputChunkPath;
      if (isPngSequence) {
        if (!existsSync(outputChunkPath)) mkdirSync(outputChunkPath, { recursive: true });
      } else {
        const outDir = join(outputChunkPath, "..");
        if (!existsSync(outDir)) mkdirSync(outDir, { recursive: true });
      }

      const encodeStarted = Date.now();
      await runEncodeStage({
        job,
        log,
        outputPath: outputChunkPath,
        framesDir,
        videoOnlyPath,
        width: plan.dimensions.width * encoder.deviceScaleFactor,
        height: plan.dimensions.height * encoder.deviceScaleFactor,
        needsAlpha: plan.dimensions.format !== "mp4",
        // Each chunk produces video only — audio is muxed once at assemble
        // time. Suppressing `hasAudio` skips the png-sequence audio sidecar
        // AND the mp4 audio mux.
        hasAudio: false,
        isPngSequence,
        // `DistributedFormat` has no "gif" member — distributed chunks are
        // always video segments (gif renders in-process only).
        isGif: false,
        preset,
        effectiveQuality,
        effectiveBitrate,
        engineConfig: {
          ffmpegEncodeTimeout: cfg.ffmpegEncodeTimeout,
          vp9CpuUsed: resolveLockedVp9CpuUsed(encoder) ?? cfg.vp9CpuUsed,
        },
        // Distributed chunks emit a single ffmpeg call per chunk; the
        // in-process per-chunk-within-chunk path would re-split our
        // already-chunked work.
        enableChunkedEncode: false,
        chunkedEncodeSize: framesInChunk,
        abortSignal: undefined,
        assertNotAborted: () => {},
        // GOP === framesInChunk + force-keyframe at frame 0 → the chunk's
        // first frame is an IDR keyframe and concat-copy at assemble time
        // round-trips losslessly.
        lockGopForChunkConcat: !isPngSequence,
        gopSize: framesInChunk,
      });
      encodeStageMs = Date.now() - encodeStarted;
    } finally {
      // Cleanest path: captureStage closed the session for us. The defensive
      // close handles error paths where we threw before delegating.
      if (session) {
        try {
          await closeCaptureSession(session);
        } catch (err) {
          log.warn("[renderChunk] error closing capture session in finally", {
            error: err instanceof Error ? err.message : String(err),
          });
        }
      }
      closeFileServerSafely(fileServer, "renderChunk", log);
      // Leave the temp work dir on failure (helps debugging); remove it on
      // success below.
    }

    // ── Hash the output + write the perf sidecar ──
    const sha256 = hashChunkOutput(outputChunkPath, outputKind);
    const durationMs = Date.now() - start;
    const perfPath = `${outputChunkPath}.perf.json`;
    const perfPayload = {
      planHash: plan.planHash,
      chunkIndex,
      startFrame: slice.startFrame,
      endFrame: slice.endFrame,
      framesEncoded,
      durationMs,
      planHashMs,
      sessionBootMs,
      captureStageMs,
      encodeStageMs,
      workers: chunkWorkerCount,
      sha256,
      outputKind,
      producerVersion: plan.producerVersion,
      ffmpegVersion,
    };
    writeFileSync(perfPath, `${JSON.stringify(perfPayload, null, 2)}\n`, "utf-8");

    // Clean up only after the hash + perf sidecar landed. Any failure above
    // leaves the framesDir in place for inspection.
    try {
      rmSync(workDir, { recursive: true, force: true, maxRetries: 3, retryDelay: 100 });
    } catch (err) {
      log.warn("[renderChunk] failed to remove work dir", {
        workDir,
        error: err instanceof Error ? err.message : String(err),
      });
    }

    return {
      outputPath: outputChunkPath,
      outputKind,
      framesEncoded,
      sha256,
      durationMs,
      planHashMs,
      sessionBootMs,
      captureStageMs,
      encodeStageMs,
      workers: chunkWorkerCount,
      perfPath,
    };
  } finally {
    // Restore the controller's runtime env even on the error path so the
    // next chunk on the same process boots from a clean env.
    envRestore.restore();
  }
}