docs: def CheckBlobsExist(self, blob_ids):
      """Check if blobs for the given digests already exist."""
      res = {blob_id: False for blob_id in blob_ids}
  
      urns = {self._BlobUrn(blob_id): blob_id for blob_id in blob_ids}
  
      existing = aff4.FACTORY.MultiOpen(
          urns, aff4_type=aff4.AFF4MemoryStreamBase, mode="r")
  
      for blob in existing:
        res[urns[blob.urn]] = True
  
      return res