ref(relay): Make tcp listen backlog configurable (#3899)

The current default from the standard library is 128 and we've seen that
this is not enough.

CHANGELOG.md

@@ -16,6 +16,7 @@
 - Add `client_sample_rate` to spans, pulled from the trace context. ([#3872](https://github.com/getsentry/relay/pull/3872))
 - Collect SDK information in profile chunks. ([#3882](https://github.com/getsentry/relay/pull/3882))
 - Introduce `trim = "disabled"` type attribute to prevent trimming of spans. ([#3877](https://github.com/getsentry/relay/pull/3877))
+- Make the tcp listen backlog configurable and raise the default to 1024. ([#3899](https://github.com/getsentry/relay/pull/3899))
 
 ## 24.7.1
 

relay-config/src/config.rs

@@ -614,6 +614,14 @@ struct Limits {
     ///
     /// By default keep-alive is set to a 5 seconds.
     keepalive_timeout: u64,
+    /// The TCP listen backlog.
+    ///
+    /// Configures the TCP listen backlog for the listening socket of Relay.
+    /// See [`man listen(2)`](https://man7.org/linux/man-pages/man2/listen.2.html)
+    /// for a more detailed description of the listen backlog.
+    ///
+    /// Defaults to `1024`, a value [google has been using for a long time](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=19f92a030ca6d772ab44b22ee6a01378a8cb32d4).
+    pub tcp_listen_backlog: u32,
 }
 
 impl Default for Limits {
@@ -643,6 +651,7 @@ impl Default for Limits {
             query_timeout: 30,
             shutdown_timeout: 10,
             keepalive_timeout: 5,
+            tcp_listen_backlog: 1024,
         }
     }
 }
@@ -2286,6 +2295,11 @@ impl Config {
         Duration::from_secs(self.values.limits.keepalive_timeout)
     }
 
+    /// TCP listen backlog to configure on Relay's listening socket.
+    pub fn tcp_listen_backlog(&self) -> u32 {
+        self.values.limits.tcp_listen_backlog
+    }
+
     /// Returns the number of cores to use for thread pools.
     pub fn cpu_concurrency(&self) -> usize {
         self.values.limits.max_thread_count

relay-server/src/services/server.rs

@@ -1,3 +1,4 @@
+use std::io;
 use std::net::{SocketAddr, TcpListener};
 use std::sync::Arc;
 use std::time::Duration;
@@ -7,6 +8,7 @@ use axum::ServiceExt;
 use axum_server::{AddrIncomingConfig, Handle, HttpConfig};
 use relay_config::Config;
 use relay_system::{Controller, Service, Shutdown};
+use tokio::net::TcpSocket;
 use tower::ServiceBuilder;
 use tower_http::compression::predicate::SizeAbove;
 use tower_http::compression::{CompressionLayer, DefaultPredicate, Predicate};
@@ -124,7 +126,8 @@ impl Service for HttpServer {
             .build();
 
         let handle = Handle::new();
-        match TcpListener::bind(config.listen_addr()) {
+
+        match create_listener(config.listen_addr(), config.tcp_listen_backlog()) {
             Ok(listener) => {
                 listener.set_nonblocking(true).ok();
                 let server = axum_server::from_tcp(listener)
@@ -154,3 +157,13 @@ impl Service for HttpServer {
         });
     }
 }
+
+fn create_listener(addr: SocketAddr, backlog: u32) -> io::Result<TcpListener> {
+    let socket = match addr {
+        SocketAddr::V4(_) => TcpSocket::new_v4(),
+        SocketAddr::V6(_) => TcpSocket::new_v6(),
+    }?;
+    socket.bind(addr)?;
+
+    socket.listen(backlog)?.into_std()
+}