Function parse_engine_spec 

pub fn parse_engine_spec(
    raw: &str,
    headers: &[(String, String)],
) -> Result<EngineSpec>

Classify a raw --server value (the reference’s parseEngineSpec): an http(s):// value is an HTTP endpoint (verbatim; headers ride along); anything else is split on whitespace and spawned. Errors: an empty value, or headers for a command (they have nowhere to go).