ETag middleware for use with Axum. It'd be nice to write a proper implementation as a Tower service.

etag_middleware.rs

async fn etag_middleware<B: std::fmt::Debug>(
    req: Request<B>,
    next: Next<B>,
) -> Result<Response, StatusCode> {
    let if_none_match_header = req.headers().get(header::IF_NONE_MATCH).cloned();
    let path = req.uri().path().to_owned();
    let res = next.run(req).await;
    let (mut parts, mut body) = res.into_parts();

    let bytes = {
        let mut body_bytes = vec![];

        while let Some(inner) = body.data().await {
            let bytes = inner.unwrap();
            body_bytes.put(bytes);
        }

        body_bytes
    };

    match bytes.len() == 0 {
        true => {
            trace!(path, "response without body, skipping etag");
            Ok(parts.into_response())
        }
        false => match if_none_match_header {
            None => {
                let etag = EntityTag::from_data(&bytes);

                parts.headers.insert(
                    header::ETAG,
                    HeaderValue::from_str(&etag.to_string()).unwrap(),
                );

                trace!(path, %etag, "no if-none-match header");

                Ok((parts, bytes).into_response())
            }
            Some(if_none_match) => {
                let if_none_match_etag = if_none_match.to_str().unwrap().parse::<EntityTag>();
                match if_none_match_etag {
                    Err(ref err) => {
                        error!("{} - {:?}", err, &if_none_match_etag);
                        let etag = EntityTag::from_data(&bytes);
                        parts.headers.insert(
                            header::ETAG,
                            HeaderValue::from_str(&etag.to_string()).unwrap(),
                        );
                        Ok((parts, bytes).into_response())
                    }
                    Ok(if_none_match_etag) => {
                        let etag = EntityTag::from_data(&bytes);

                        parts.headers.insert(
                            header::ETAG,
                            HeaderValue::from_str(&etag.to_string()).unwrap(),
                        );

                        let some_match = etag.strong_eq(&if_none_match_etag);

                        trace!(
                            path,
                            %etag,
                            some_match,
                            "if-none-match" = %if_none_match_etag
                        );

                        if some_match {
                            Ok((StatusCode::NOT_MODIFIED, parts).into_response())
                        } else {
                            Ok((parts, bytes).into_response())
                        }
                    }
                }
            }
        },
    }
}

On a second though, this is probably as good as it gets. For really large responses like files this should probably be solved by having metadata separate from the content anyways. For queries that are costly to compute it might make sense to use some sort of caching, but that would be the responsibility of the next step 😅

I made a few modifications for my own purposes: https://gist.github.com/sdasda7777/3acaa1a7bee2692a939526252b567119