Function downloadChunkObjects

packages/aws-lambda/src/handler.ts:445–477

(
  s3: S3Client,
  uris: string[],
  workDir: string,
  format: DistributedFormat,
)

}

async function downloadChunkObjects(
  s3: S3Client,
  uris: string[],
  workDir: string,
  format: DistributedFormat,
): Promise<string[]> {
  const chunksDir = join(workDir, "chunks");
  mkdirSync(chunksDir, { recursive: true });
  // Each chunk is an independent S3 GET (+ untar for png-sequence). Run
  // them in parallel — assemble's wall-clock is otherwise dominated by
  // `Σ chunk-download-ms` instead of `max(chunk-download-ms)`. Preserve
  // the input order by writing into a pre-sized array rather than
  // pushing as each task settles.
  const local: string[] = new Array<string>(uris.length);
  await Promise.all(
    uris.map(async (uri, i) => {
      if (!uri) {
        throw new Error(`[handler] chunk URI at index ${i} is empty`);
      }
      const { key } = parseS3Uri(uri);
      const localPath = join(chunksDir, basename(key));
      await downloadS3ObjectToFile(s3, uri, localPath);
      if (format === "png-sequence") {
        const dirPath = join(chunksDir, `frames-${pad(i)}`);
        await untarDirectory(localPath, dirPath);
        local[i] = dirPath;
      } else {
        local[i] = localPath;
      }
    }),
  );
  return local;
}

// ── Helpers ─────────────────────────────────────────────────────────────────