journald priority decorator for log4rs

journald_log4rs.rs

/// Implementation of a log-encoder decorator that can prepend
/// journald log priorities to any given `log4rs`-encoder.
#[derive(Debug)]
struct JournaldDecorator<E: Encode>(E);

/// Helper function to map from `log`-crate log levels to journald log
/// priorites. Please see `sd-daemon(3)` for detailed information on
/// these levels.
fn journald_priority(level: &Level) -> &'static str {
    match level {
        Level::Error => "<3>",
        Level::Warn  => "<4>",
        Level::Info  => "<6>",
        // journald & stackdriver do not differentiate between 'Debug'
        // and higher levels, so debug and trace are both mapped to
        // `SD_DEBUG`.
        Level::Debug => "<7>",
        Level::Trace => "<7>",
    }
}

impl <E: Encode> Encode for JournaldDecorator<E> {
    fn encode(&self, w: &mut Write, record: &Record)
              -> Result<(), Box<Error + Sync + Send>> {
        // First write the journald priority, then delegate to the
        // decorated encoder.
        write!(w, "{}", journald_priority(&record.level()))?;
        self.0.encode(w, record)
    }
}