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  return send_file(
    \'ignored', streaming => 1, callbacks => {
...
      },
    },
  );

$ curl -sS -o rcvd https://foo/bar.pdf
curl: (18) transfer closed with outstanding read data remaining
...
<...>
2017-01-12 14:48:58 (6.10 MB/s) - Read error at byte 3316868 (Success.
+).Retrying.
<proceeds to loop endlessly>