Add a new config to set TCP_USER_TIMEOUT

Setting TCP keepalive is not enough to prevent dead TCP connections.

See https://blog.cloudflare.com/when-tcp-sockets-refuse-to-die/

src/client/legacy/connect/http.rs

@@ -78,6 +78,8 @@ struct Config {
     recv_buffer_size: Option<usize>,
     #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
     interface: Option<String>,
+    #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
+    tcp_user_timeout: Option<Duration>,
 }
 
 #[derive(Default, Debug, Clone, Copy)]
@@ -182,6 +184,8 @@ impl<R> HttpConnector<R> {
                 recv_buffer_size: None,
                 #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
                 interface: None,
+                #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
+                tcp_user_timeout: None,
             }),
             resolver,
         }
@@ -324,6 +328,13 @@ impl<R> HttpConnector<R> {
         self
     }
 
+    /// Sets the value of the TCP_USER_TIMEOUT option on the socket.
+    #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
+    #[inline]
+    pub fn set_tcp_user_timeout(&mut self, time: Option<Duration>) {
+        self.config_mut().set_tcp_user_timeout = time;
+    }
+
     // private
 
     fn config_mut(&mut self) -> &mut Config {
@@ -728,6 +739,13 @@ fn connect(
             .map_err(ConnectError::m("tcp bind interface error"))?;
     }
 
+    #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
+    if let Some(tcp_user_timeout) = &config.tcp_user_timeout_config.into_tcpkeepalive() {
+        if let Err(e) = socket.set_tcp_user_timeout(tcp_user_timeout) {
+            warn!("tcp set_tcp_user_timeout error: {}", e);
+        }
+    }
+
     bind_local_address(
         &socket,
         addr,