[mort96]

No clue if this is the reason, but exceptions are actually really fast. With them (or longjmp-based error handling), the "happy path" gets to not even thing about errors. With error returns (be it Go's error interface, Rust's Result type, or C's "this function returns a negative number on error" pattern), you need a branch after every call which might fail, which does have a measurable (if usually small) impact on performance. Given pigz's entire raison d'etre is performance, it wouldn't surprise me if this impacted the choice of error handling style.