fix: don't exit TUN thread on errors (#7354)

I noticed that in case there is an error when reading from the TUN
device, we currently exit that thread and we don't have a mechanism at
the moment to restart it. Discarding the thread also means we can no
longer send new instances of `Tun` into it.

Instead of exiting the thread, we now just log the error and continue.
In case the error was caused by the FD being closed, we discard the
instance of `Tun` and wait for a new one.

rust/connlib/tunnel/src/device_channel.rs

@@ -41,6 +41,8 @@ impl Device {
         let n = std::task::ready!(tun.poll_read(ip_packet.buf(), cx))?;
 
         if n == 0 {
+            self.tun = None;
+
             return Poll::Ready(Err(io::Error::new(
                 io::ErrorKind::UnexpectedEof,
                 "device is closed",

rust/connlib/tunnel/src/io.rs

@@ -1,6 +1,6 @@
 use crate::{device_channel::Device, dns, sockets::Sockets};
 use domain::base::Message;
-use firezone_logging::{err_with_sources, std_dyn_err, telemetry_event, telemetry_span};
+use firezone_logging::{err_with_sources, telemetry_event, telemetry_span};
 use futures::{
     future::{self, Either},
     stream, Stream, StreamExt,
@@ -379,10 +379,9 @@ async fn tun_send_recv(
             }
             Either::Right((Err(e), _)) => {
                 tracing::debug!(
-                    error = std_dyn_err(&e),
-                    "Failed to read packet from TUN device"
+                    "Failed to read packet from TUN device: {}",
+                    err_with_sources(&e)
                 );
-                return;
             }
         };
     }