proxmox-backup-proxy: stop accept() loop on daemon shutdown

On reload the old process hands over to the new process but needs to
keep running until all its worker tasks are finished to avoid
breaking a in-progress action like a xterm.js web shell or a backup
creation/restore.

During that wait time the receiving channel was already closed, but
the TCP sockt accept listener was still left active by mistake.

That paired with the `SO_REUSEPORT` being set on the underlying
socket, made the kernel choose either the old or new process for new
incoming connections, both still listened for them after all and
reuse-port + multiple processes is often used as load-balancer
mechanism.

As the old proxy accepted connections but didn't process them anymore
one could observer sporadic connection failures on any API call, well
any new connection to the proxy, depending on which process got the
it assigned.

The fix is to stop accepting new connections one we shutdown, so poll
the shutdown_future too during accept and just exit the accept-loop
on shutdown.

Note: This part of the code, nor other parts that could influence it,
wasn't changed at all in recent times, so it's still unresolved for
why it pops up only now.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Co-authored-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
 [ T: add more (root cause) info and reword a bit ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/bin/proxmox-backup-proxy.rs

@@ -394,14 +394,18 @@ async fn accept_connection(
     sender: tokio::sync::mpsc::Sender<ClientStreamResult>,
 ) {
     let accept_counter = Arc::new(());
+    let mut shutdown_future = proxmox_rest_server::shutdown_future().fuse();
 
     loop {
-        let (sock, peer) = match listener.accept().await {
+        let (sock, peer) = select! {
+            res = listener.accept().fuse() => match res {
                 Ok(conn) => conn,
                 Err(err) => {
                     eprintln!("error accepting tcp connection: {}", err);
                     continue;
                 }
+            },
+            _ =  shutdown_future => break,
         };
 
         sock.set_nodelay(true).unwrap();