响应解压

    fn call(&mut self, mut req: Request<ReqBody>) -> Self::Future {
        if let header::Entry::Vacant(entry) = req.headers_mut().entry(ACCEPT_ENCODING) {
            if let Some(accept) = self.accept.to_header_value() {
                entry.insert(accept);
            }
        }

        ResponseFuture {
            inner: self.inner.call(req),
            accept: self.accept,
        }
    }

                entry.remove();
                parts.headers.remove(header::CONTENT_LENGTH);

    fn apply(input: Self::Input, _quality: CompressionLevel) -> Self::Output {
        GzipDecoder::new(input)
    }
    // ...
    fn apply(input: Self::Input, _quality: CompressionLevel) -> Self::Output {
        let mut decoder = ZstdDecoder::new(input);
        decoder.multiple_members(true);
        decoder
    }

tower-http = { version = "=0.7.1", features = ["compression-gzip", "compression-zstd", "decompression-full"] }
tower = { version = "=0.5.3", features = ["util"] }
tokio = { version = "=1.54.1", features = ["macros", "rt"] }
http = "1"
http-body = "1"
http-body-util = "0.1"
bytes = "1"
futures = "0.3"

type Body = UnsyncBoxBody<Bytes, BoxError>;

// 借 CompressionLayer 造压缩字节；谓词恒真，免得被 32 字节下限挡住
async fn compress(enc: &str, text: &'static str) -> Bytes {
  let svc = CompressionLayer::new().compress_when(|_, _, _: &_, _: &_| true)
    .layer(service_fn(move |_: Request<Full<Bytes>>| async move { Ok::<_, Infallible>(Response::new(Full::<Bytes>::from(text))) }));
  let req = Request::get("/").header("accept-encoding", enc).body(Full::default()).unwrap();
  svc.oneshot(req).await.unwrap().into_body().collect().await.unwrap().to_bytes()
}

fn frames(v: Vec<Frame<Bytes>>) -> Body {
  StreamBody::new(stream::iter(v.into_iter().map(Ok::<_, BoxError>))).boxed_unsync()
}

// run(label, content_encoding, body)：上游回这个头和这些帧（另加 content-length: 999），
// 经 DecompressionLayer 后 collect，打印剩下的两个头、字节和 trailers，或者错误

#[tokio::main(flavor = "current_thread")]
async fn main() {
  // call 时：默认、no_br()、请求已带 accept-encoding: identity 三种，让上游回显它看到的头
  // ...
  let g1 = compress("gzip", "hello ").await;
  let g2 = compress("gzip", "world").await;
  let z1 = compress("zstd", "hello ").await;
  let z2 = compress("zstd", "world").await;
  let cat = |a: &Bytes, b: &Bytes| Bytes::from([a.as_ref(), b.as_ref()].concat());

  run("gzip one member", "gzip", frames(vec![Frame::data(g1.clone())])).await;
  run("gzip two members, one frame", "gzip", frames(vec![Frame::data(cat(&g1, &g2))])).await;
  run("gzip two members, two frames", "gzip", frames(vec![Frame::data(g1.clone()), Frame::data(g2.clone())])).await;
  run("zstd two frames, one frame", "zstd", frames(vec![Frame::data(cat(&z1, &z2))])).await;
  run("gzip + empty frame + trailers", "gzip", {
    let mut t = HeaderMap::new();
    t.insert("x-checksum", "abc".parse().unwrap());
    frames(vec![Frame::data(g1.clone()), Frame::data(Bytes::new()), Frame::trailers(t)])
  }).await;
  run("empty body", "gzip", frames(vec![])).await;
  run("gzip, br", "gzip, br", frames(vec![Frame::data(g1.clone())])).await;
}

default  upstream saw accept-encoding Some("zstd,gzip,deflate,br")
no_br    upstream saw accept-encoding Some("zstd,gzip,deflate")
given    upstream saw accept-encoding Some("identity")
gzip one member                ce=None cl=None -> b"hello " trailers=None
gzip two members, one frame    ce=None cl=None -> b"hello " trailers=None
gzip two members, two frames   ce=None cl=None -> error: there are extra bytes after body has been decompressed
zstd two frames, one frame     ce=None cl=None -> b"hello world" trailers=None
gzip + empty frame + trailers  ce=None cl=None -> b"hello " trailers=Some({"x-checksum": "abc"})
empty body                     ce=None cl=None -> b"" trailers=None
gzip, br                       ce=Some("gzip, br") cl=Some("999") -> b"\x1f\x8b\x08\0\0\0\0\0\0\xff\xcbH\xcd\xc9\xc9W\0\0\xf6\xf9\x81\xed\x06\0\0\0" trailers=None

gzip + empty frame + trailers  ce=None cl=None -> b"hello " trailers=None