Ah, just realized this could be a problem if len(line) == len(needle). I will change len(line) >= len(needle) to len(line) > len(needle), because in my use case I only use lines with a value as well as a key.

Fixed. FWIW, the current master also crashes in this case a line with no value. e.g.

echo helloworld > test.tsv
sortdb -db-file=test.tsv &
curl localhost:8080/get?key=helloworld