fake UDP association broken since 5.1.9

[terrytw]

It's exactly what was discussed in #2571, which was fixed in 5.1.7 by 84ac0c4.

After upgrading to 5.1.9, the same problem occurs again. It could possibly be broken in 5.1.8, but there is no way to know since proxy mode had that critical bug in 5.1.8.

Update: I tried VPN mode in 5.1.8, and tested the connectivity, it was most likely broken already in 5.1.8. I have tried every version afterwards, the result is the same.

[madeye]

@zonyitoo It should be a regression of shadowsocks-rust.

With this patch, I can get udp_bind_addr working in tcp-only mode:

diff --git a/crates/shadowsocks-service/src/local/socks/server/mod.rs b/crates/shadowsocks-service/src/local/socks/server/mod.rs
index d3308fd..da5d676 100644
--- a/crates/shadowsocks-service/src/local/socks/server/mod.rs
+++ b/crates/shadowsocks-service/src/local/socks/server/mod.rs
@@ -113,14 +113,10 @@ impl Socks {

         info!("shadowsocks socks TCP listening on {}", listener.local_addr()?);

-        // If UDP is enabled, SOCK5 UDP_ASSOCIATE command will let client to send requests to this address
-        let udp_bind_addr = if self.mode.enable_udp() {
-            let udp_bind_addr = self.udp_bind_addr.as_ref().unwrap_or(client_config);
-            let udp_bind_addr = Arc::new(udp_bind_addr.clone());
-            Some(udp_bind_addr)
-        } else {
-            None
-        };
+        // If UDP bind addr is specified, SOCK5 UDP_ASSOCIATE command
+        // will let client to send requests to this address
+        let udp_bind_addr = self.udp_bind_addr.as_ref().unwrap_or(client_config);
+        let udp_bind_addr = Some(Arc::new(udp_bind_addr.clone()));

         loop {
             let (stream, peer_addr) = match listener.accept().await {

[zonyitoo]

Sincerely sorry for the mess.