refactor(connlib): don't recreate the tun device for Apple and Windows (#4260)

This is done to fix a bug where the file descriptor is unregistered from
the reactor after the new `Tun` struct is created if the old one is
dropped after.

rust/connlib/tunnel/src/client.rs

@@ -212,7 +212,7 @@ where
 
         let callbacks = self.callbacks.clone();
 
-        self.io.device_mut().initialize(
+        self.io.device_mut().set_config(
             &config,
             // We can just sort in here because sentinel ips are created in order
             dns_mapping.left_values().copied().sorted().collect(),

rust/connlib/tunnel/src/device_channel.rs

@@ -63,8 +63,8 @@ impl Device {
         }
     }
 
-    #[cfg(target_family = "unix")]
-    pub(crate) fn initialize(
+    #[cfg(any(target_os = "android", target_os = "linux"))]
+    pub(crate) fn set_config(
         &mut self,
         config: &Interface,
         dns_config: Vec<IpAddr>,
@@ -83,8 +83,34 @@ impl Device {
         Ok(())
     }
 
+    #[cfg(any(target_os = "ios", target_os = "macos"))]
+    pub(crate) fn set_config(
+        &mut self,
+        config: &Interface,
+        dns_config: Vec<IpAddr>,
+        callbacks: &impl Callbacks,
+    ) -> Result<(), ConnlibError> {
+        // For macos the filedescriptor is the same throughout its lifetime.
+        // If we reinitialzie tun, we might drop the old tun after the new one is created
+        // this unregisters the file descriptor with the reactor so we never wake up
+        // in case an event is triggered.
+        if self.tun.is_none() {
+            self.tun = Some(Tun::new()?);
+        }
+
+        self.mtu = ioctl::interface_mtu_by_name(self.tun.as_ref().unwrap().name())?;
+
+        callbacks.on_set_interface_config(config.ipv4, config.ipv6, dns_config);
+
+        if let Some(waker) = self.waker.take() {
+            waker.wake();
+        }
+
+        Ok(())
+    }
+
     #[cfg(target_family = "windows")]
-    pub(crate) fn initialize(
+    pub(crate) fn set_config(
         &mut self,
         config: &Interface,
         dns_config: Vec<IpAddr>,

rust/connlib/tunnel/src/device_channel/tun_darwin.rs

@@ -1,12 +1,11 @@
 use crate::device_channel::{ipv4, ipv6};
-use connlib_shared::{messages::Interface as InterfaceConfig, Callbacks, Error, Result};
+use connlib_shared::{Callbacks, Error, Result};
 use ip_network::IpNetwork;
 use libc::{
     ctl_info, fcntl, getpeername, getsockopt, ioctl, iovec, msghdr, recvmsg, sendmsg, sockaddr_ctl,
     socklen_t, AF_INET, AF_INET6, AF_SYSTEM, CTLIOCGINFO, F_GETFL, F_SETFL, IF_NAMESIZE,
     O_NONBLOCK, SYSPROTO_CONTROL, UTUN_OPT_IFNAME,
 };
-use std::net::IpAddr;
 use std::task::{Context, Poll};
 use std::{
     collections::HashSet,
@@ -70,11 +69,7 @@ impl Tun {
         }
     }
 
-    pub fn new(
-        config: &InterfaceConfig,
-        dns_config: Vec<IpAddr>,
-        callbacks: &impl Callbacks,
-    ) -> Result<Self> {
+    pub fn new() -> Result<Self> {
         let mut info = ctl_info {
             ctl_id: 0,
             ctl_name: [0; 96],
@@ -131,8 +126,6 @@ impl Tun {
             }
 
             if addr.sc_id == info.ctl_id {
-                callbacks.on_set_interface_config(config.ipv4, config.ipv6, dns_config);
-
                 set_non_blocking(fd)?;
 
                 return Ok(Self {

rust/connlib/tunnel/src/gateway.rs

@@ -50,7 +50,7 @@ where
         let callbacks = self.callbacks.clone();
         self.io
             .device_mut()
-            .initialize(config, vec![], &callbacks)?;
+            .set_config(config, vec![], &callbacks)?;
         self.io.device_mut().set_routes(
             HashSet::from([PEERS_IPV4.parse().unwrap(), PEERS_IPV6.parse().unwrap()]),
             &callbacks,