[RFC PATCH proxmox-backup 1/2] api: optimize content streaming reader for file download

[Christian Ebner <c.ebner@proxmox.com>]

The current implementation using FramedRead and BytesCodec reads the
file in 8K chunks only. FramedRead starts out with a default buffer
capacity of 8K [0], the reader filling only up to this capacity [1],
which BytesCodec::decode() consumes completely [2]. Therefore, the
next read will recreate the same 8K buffer again because
BytesMut::reserve() [3] will fill to the initial capacity again.

This has additional overhead since tokio's async reader uses the
blocking thread pool for each read and is especially problematic if
the blocking pool is heavily utilized, e.g. by concurrent operations
such as chunk inserts. Latter also run on the blocking pool since
commit 23400016 ("api: do not block tokio worker threads during chunk
inserts").

Use the ReaderStream with tokio's default maximum buffer size for
blocking I/O instead [4].

[0] https://docs.rs/tokio-util/0.7.10/src/tokio_util/codec/framed_impl.rs.html#53
[1] https://docs.rs/tokio-util/0.7.10/src/tokio_util/codec/framed_impl.rs.html#221
[2] https://docs.rs/tokio-util/0.7.10/src/tokio_util/codec/bytes_codec.rs.html#58
[3] https://docs.rs/tokio-util/0.7.10/src/tokio_util/codec/framed_impl.rs.html#221
[4] https://docs.rs/tokio/1.54.1/src/tokio/io/blocking.rs.html#29

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
---
 src/api2/helpers.rs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/api2/helpers.rs b/src/api2/helpers.rs
index aa582a53c..7d1fb8e35 100644
--- a/src/api2/helpers.rs
+++ b/src/api2/helpers.rs
@@ -1,12 +1,14 @@
 use std::path::PathBuf;
 
 use anyhow::Error;
-use futures::stream::TryStreamExt;
 use hyper::{Response, StatusCode, header};
+use tokio_util::io::ReaderStream;
 
 use proxmox_http::Body;
 use proxmox_router::http_bail;
 
+const DOWNLOAD_BUFFER_SIZE: usize = 2 * 1024 * 1024;
+
 pub async fn create_download_response(path: PathBuf) -> Result<Response<Body>, Error> {
     let file = match tokio::fs::File::open(path.clone()).await {
         Ok(file) => file,
@@ -16,9 +18,7 @@ pub async fn create_download_response(path: PathBuf) -> Result<Response<Body>, E
         Err(err) => http_bail!(BAD_REQUEST, "open file {:?} failed: {}", path, err),
     };
 
-    let payload = tokio_util::codec::FramedRead::new(file, tokio_util::codec::BytesCodec::new())
-        .map_ok(|bytes| bytes.freeze());
-
+    let payload = ReaderStream::with_capacity(file, DOWNLOAD_BUFFER_SIZE);
     let body = Body::wrap_stream(payload);
 
     // fixme: set other headers ?
-- 
2.47.3