Replace the hand-made VecDequeue by a FutureUnordered

Cargo.lock

@@ -3212,6 +3212,7 @@ dependencies = [
  "enum-iterator",
  "file-store",
  "flate2",
+ "futures",
  "indexmap",
  "insta",
  "maplit",

crates/index-scheduler/Cargo.toml

@@ -46,10 +46,11 @@ time = { version = "0.3.41", features = [
 tracing = "0.1.41"
 ureq = "2.12.1"
 uuid = { version = "1.17.0", features = ["serde", "v4"] }
-backoff = "0.4.0"
+backoff = { version = "0.4.0", features = ["tokio"] }
 reqwest = { version = "0.12.23", features = ["rustls-tls", "http2"], default-features = false }
 rusty-s3 = "0.8.1"
 tokio = { version = "1.47.1", features = ["full"] }
+futures = "0.3.31"
 
 [dev-dependencies]
 big_s = "1.0.2"

crates/index-scheduler/src/scheduler/process_snapshot_creation.rs

@@ -438,13 +438,13 @@ async fn multipart_stream_to_s3(
     db_name: String,
     reader: std::io::PipeReader,
 ) -> Result<(), Error> {
-    use std::{collections::VecDeque, os::fd::OwnedFd, path::PathBuf};
+    use std::{os::fd::OwnedFd, path::PathBuf};
 
-    use bytes::{Bytes, BytesMut};
-    use reqwest::{Client, Response};
+    use bytes::BytesMut;
+    use futures::stream::{FuturesUnordered, StreamExt};
+    use reqwest::Client;
     use rusty_s3::S3Action as _;
     use rusty_s3::{actions::CreateMultipartUpload, Bucket, BucketError, Credentials, UrlStyle};
-    use tokio::task::JoinHandle;
 
     let reader = OwnedFd::from(reader);
     let reader = tokio::net::unix::pipe::Receiver::from_owned_fd(reader)?;
@@ -482,9 +482,7 @@ async fn multipart_stream_to_s3(
     // We use this bumpalo for etags strings.
     let bump = bumpalo::Bump::new();
     let mut etags = Vec::<&str>::new();
-    let mut in_flight = VecDeque::<(JoinHandle<reqwest::Result<Response>>, Bytes)>::with_capacity(
-        s3_max_in_flight_parts.get(),
-    );
+    let mut in_flight = FuturesUnordered::new();
 
     // Part numbers start at 1 and cannot be larger than 10k
     for part_number in 1u16.. {
@@ -498,8 +496,21 @@ async fn multipart_stream_to_s3(
 
         // Wait for a buffer to be ready if there are in-flight parts that landed
         let mut buffer = if in_flight.len() >= s3_max_in_flight_parts.get() {
-            let (handle, buffer) = in_flight.pop_front().expect("At least one in flight request");
-            let resp = join_and_map_error(handle).await?;
+            let (join_result, buffer): (
+                Result<reqwest::Result<reqwest::Response>, tokio::task::JoinError>,
+                bytes::Bytes,
+            ) = in_flight.next().await.expect("At least one in flight request");
+            // safety: Panic happens if the task (JoinHandle) was aborted, cancelled, or panicked
+            let resp = join_result.unwrap().map_err(Error::S3HttpError)?;
+            let resp = match resp.error_for_status_ref() {
+                Ok(_) => resp,
+                Err(_) => {
+                    return Err(Error::S3Error {
+                        status: resp.status(),
+                        body: resp.text().await.unwrap_or_default(),
+                    })
+                }
+            };
             extract_and_append_etag(&bump, &mut etags, resp.headers())?;
 
             let mut buffer = match buffer.try_into_mut() {
@@ -556,11 +567,24 @@ async fn multipart_stream_to_s3(
                 }
             })
         });
-        in_flight.push_back((task, body));
+
+        // Wrap the task to return both the result and the buffer
+        let task_with_buffer = async move { (task.await, body) };
+        in_flight.push(task_with_buffer);
     }
 
-    for (handle, _buffer) in in_flight {
-        let resp = join_and_map_error(handle).await?;
+    while let Some((join_result, _buffer)) = in_flight.next().await {
+        // safety: Panic happens if the task (JoinHandle) was aborted, cancelled, or panicked
+        let resp = join_result.unwrap().map_err(Error::S3HttpError)?;
+        let resp = match resp.error_for_status_ref() {
+            Ok(_) => resp,
+            Err(_) => {
+                return Err(Error::S3Error {
+                    status: resp.status(),
+                    body: resp.text().await.unwrap_or_default(),
+                })
+            }
+        };
         extract_and_append_etag(&bump, &mut etags, resp.headers())?;
     }
 
@@ -600,22 +624,6 @@ async fn multipart_stream_to_s3(
     }
 }
 
-#[cfg(unix)]
-async fn join_and_map_error(
-    join_handle: tokio::task::JoinHandle<Result<reqwest::Response, reqwest::Error>>,
-) -> Result<reqwest::Response> {
-    // safety: Panic happens if the task (JoinHandle) was aborted, cancelled, or panicked
-    let request = join_handle.await.unwrap();
-    let resp = request.map_err(Error::S3HttpError)?;
-    match resp.error_for_status_ref() {
-        Ok(_) => Ok(resp),
-        Err(_) => Err(Error::S3Error {
-            status: resp.status(),
-            body: resp.text().await.unwrap_or_default(),
-        }),
-    }
-}
-
 #[cfg(unix)]
 fn extract_and_append_etag<'b>(
     bump: &'b bumpalo::Bump,