fix(fake-tcp): when connect()-ing, attempt to get ephemeral port using algorithm similar to Linux (#162)

* fix(fake-tcp): when `connect()`-ing, attempt to get ephemeral port 5 times to reduce the chance
of panicking

Fixes #79

fake-tcp/src/lib.rs

@@ -50,8 +50,10 @@ use rand::prelude::*;
 use std::collections::{HashMap, HashSet};
 use std::fmt;
 use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr};
-use std::sync::atomic::{AtomicU32, Ordering};
-use std::sync::{Arc, RwLock};
+use std::sync::{
+    atomic::{AtomicU32, Ordering},
+    Arc, RwLock,
+};
 use tokio::sync::broadcast;
 use tokio::sync::mpsc;
 use tokio::time;
@@ -401,7 +403,7 @@ impl Stack {
     /// the connection attempt failed.
     pub async fn connect(&mut self, addr: SocketAddr) -> Option<Socket> {
         let mut rng = SmallRng::from_entropy();
-        let local_port: u16 = rng.gen_range(1024..65535);
+        for local_port in rng.gen_range(32768..=60999)..=60999 {
             let local_addr = SocketAddr::new(
                 if addr.is_ipv4() {
                     IpAddr::V4(self.local_ip)
@@ -411,7 +413,20 @@ impl Stack {
                 local_port,
             );
             let tuple = AddrTuple::new(local_addr, addr);
-        let (mut sock, incoming) = Socket::new(
+            let mut sock;
+
+            {
+                let mut tuples = self.shared.tuples.write().unwrap();
+                if tuples.contains_key(&tuple) {
+                    trace!(
+                        "Fake TCP connection to {}, local port number {} already in use, trying another one",
+                        addr, local_port
+                    );
+                    continue;
+                }
+
+                let incoming;
+                (sock, incoming) = Socket::new(
                     self.shared.clone(),
                     self.shared.tun.choose(&mut rng).unwrap().clone(),
                     local_addr,
@@ -420,12 +435,17 @@ impl Stack {
                     State::Idle,
                 );
 
-        {
-            let mut tuples = self.shared.tuples.write().unwrap();
-            assert!(tuples.insert(tuple, incoming.clone()).is_none());
+                assert!(tuples.insert(tuple, incoming).is_none());
             }
 
-        sock.connect().await.map(|_| sock)
+            return sock.connect().await.map(|_| sock);
+        }
+
+        error!(
+            "Fake TCP connection to {} failed, emphemeral port number exhausted",
+            addr
+        );
+        None
     }
 
     async fn reader_task(