From b3804361d287875ac4e9c046043c962d936a2f94 Mon Sep 17 00:00:00 2001
From: Rob Ede <robjtede@icloud.com>
Date: Mon, 17 Jul 2023 03:48:45 +0100
Subject: [PATCH] add log message for MPTCP bind failure

---
 actix-server/src/builder.rs |  2 ++
 actix-server/src/socket.rs  | 13 +++++++------
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/actix-server/src/builder.rs b/actix-server/src/builder.rs
index 8ccd691e..9869fe52 100644
--- a/actix-server/src/builder.rs
+++ b/actix-server/src/builder.rs
@@ -13,6 +13,8 @@ use crate::{
 };
 
 /// Multipath TCP (MPTCP) preference.
+///
+/// Also see [`ServerBuilder::mptcp()`].
 #[derive(Debug, Clone)]
 pub enum MpTcp {
     /// MPTCP will not be used when binding sockets.
diff --git a/actix-server/src/socket.rs b/actix-server/src/socket.rs
index 7bbef962..63fbd525 100644
--- a/actix-server/src/socket.rs
+++ b/actix-server/src/socket.rs
@@ -240,13 +240,14 @@ pub(crate) fn create_mio_tcp_listener(
 
     let socket = match Socket::new(Domain::for_address(addr), Type::STREAM, Some(protocol)) {
         Ok(sock) => sock,
-        Err(err) => {
-            if matches!(mptcp, MpTcp::TcpFallback) {
-                Socket::new(Domain::for_address(addr), Type::STREAM, Some(Protocol::TCP))?
-            } else {
-                return Err(err);
-            }
+
+        Err(err) if matches!(mptcp, MpTcp::TcpFallback) => {
+            tracing::warn!("binding socket as MPTCP failed: {err}");
+            tracing::warn!("falling back to TCP");
+            Socket::new(Domain::for_address(addr), Type::STREAM, Some(Protocol::TCP))?
         }
+
+        Err(err) => return Err(err),
     };
 
     socket.set_reuse_address(true)?;