Enable asynchronous logging

[davidiw]

Have you any plans to enable asynchronous logging via mpsc channels and a std thread? We're exploring that in Libra and I'd love to push that upstream if possible.

[KodrAus]

Hey @davidiw 👋

Since env_logger is exclusively targeted at outputting to an attached stream that needs to be locked anyways I haven't really thought about asynchronous logging. I think the best approach might be to add some methods to env_logger::Logger that let you format a log::Record into something like this:

// Send + Sync
pub struct FormattedRecord {
    value: Vec<u8>,
    output: Stream,
}

impl FormattedRecord {
    pub fn output(&self) -> io::Result<()> {
        // write to the appropriate stream
    }
}

so that you could ask env_logger to format up a record for you into something you could add to a channel and then write to the output stream later.

What do you think?