Fix unspecified support for same host connections

This adds a test and fixes behaivor where a client may want to connect
to a server listening on an unspecified ip (0.0.0.0). This is probably
not the exact correct implementation but this will just route it through
the loopback path.

[Playground
example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=63aad6ad0374fa62e93d83ec813faca8)

Author: LucioFranco

src/host.rs

@@ -488,7 +488,7 @@ pub fn matches(bind: SocketAddr, dst: SocketAddr) -> bool {
 /// Returns true if loopback is supported between two addresses, or
 /// if the IPs are the same (in which case turmoil treats it like loopback)
 pub(crate) fn is_same(src: SocketAddr, dst: SocketAddr) -> bool {
-    dst.ip().is_loopback() || src.ip() == dst.ip()
+    dst.ip().is_loopback() || dst.ip().is_unspecified() || src.ip() == dst.ip()
 }
 
 #[cfg(test)]

src/net/tcp/listener.rs

@@ -92,12 +92,9 @@ impl TcpListener {
             let host = world.current_host_mut();
 
             let mut my_addr = self.local_addr;
-            if origin.ip().is_loopback() {
+            if origin.ip().is_loopback() || origin.ip().is_unspecified() {
                 my_addr.set_ip(origin.ip());
             }
-            if my_addr.ip().is_unspecified() {
-                my_addr.set_ip(host.addr);
-            }
 
             let pair = SocketPair::new(my_addr, origin);
             let rx = host.tcp.new_stream(pair);

src/net/tcp/stream.rs

@@ -67,7 +67,7 @@ impl TcpStream {
 
             let host = world.current_host_mut();
             let mut local_addr = SocketAddr::new(host.addr, host.assign_ephemeral_port());
-            if dst.ip().is_loopback() {
+            if dst.ip().is_loopback() || dst.ip().is_unspecified() {
                 local_addr.set_ip(dst.ip());
             }
 

src/net/udp.rs

@@ -336,7 +336,7 @@ impl UdpSocket {
 
     fn send(&self, world: &mut World, dst: SocketAddr, packet: Datagram) -> Result<()> {
         let mut src = self.local_addr;
-        if dst.ip().is_loopback() {
+        if dst.ip().is_loopback() || dst.ip().is_unspecified() {
             src.set_ip(dst.ip());
         }
         if src.ip().is_unspecified() {

tests/tcp.rs

@@ -1374,3 +1374,42 @@ fn try_write() -> Result {
 
     sim.run()
 }
+
+#[test]
+fn connect_0000() -> Result {
+    init_tracing();
+
+    let mut sim = Builder::new().build();
+    sim.client("client", async move {
+        let listener = TcpListener::bind((Ipv4Addr::UNSPECIFIED, 1234))
+            .await
+            .unwrap();
+
+        tokio::spawn(async move {
+            let (socket, _) = listener.accept().await.unwrap();
+
+            let written = socket.try_write(b"hello!").unwrap();
+            assert_eq!(written, 6);
+        });
+
+        let mut socket = TcpStream::connect((Ipv4Addr::UNSPECIFIED, 1234))
+            .await
+            .unwrap();
+        let mut buf: [u8; 6] = [0; 6];
+        socket.read_exact(&mut buf).await?;
+        assert_eq!(&buf, b"hello!");
+
+        Ok(())
+    });
+
+    sim.run()
+}
+
+fn init_tracing() {
+    use tracing_subscriber::filter::EnvFilter;
+
+    let _ = tracing_subscriber::fmt()
+        .with_env_filter(EnvFilter::from_default_env())
+        .with_test_writer()
+        .try_init();
+}