example/linux: Add vsock testing

This commit adds a VZ_VSOCK_PORT to indicate a virtio-vsock port to
connect to after the VM has finished booting.
The example code tries to call ConnectToPort every 10 seconds, and when
the connection succeeds, it sends 'hello VM' to that connection.
`nc --listen --vsock $VZ_VSOCK_PORT` needs to be run on the VM for the
connection test to work.

example/linux/main.go

@@ -1,11 +1,14 @@
 package main
 
 import (
+	"fmt"
 	l "log"
 	"os"
 	"os/signal"
+	"strconv"
 	"strings"
 	"syscall"
+	"time"
 
 	"github.com/Code-Hex/vz/v2"
 	"github.com/pkg/term/termios"
@@ -135,6 +138,42 @@ func main() {
 			errCh <- err
 		}
 	})
+	vsockCh := make(chan *vz.VirtioSocketConnection, 1)
+	go func() {
+		vsockPortStr := os.Getenv("VZ_VSOCK_PORT")
+		if vsockPortStr == "" {
+			return
+		}
+		vsockPort, err := strconv.Atoi(vsockPortStr)
+		if err != nil {
+			log.Fatalf("Could not convert %s to int", vsockPortStr)
+		}
+		vsockConnected := func(conn *vz.VirtioSocketConnection, err error) {
+			if err != nil {
+				log.Println("vsock connection error:", err)
+				return
+			}
+			log.Println("vsock connection succeeded")
+			vsockCh <- conn
+			/*
+				// Running conn.Write from the callback passed to ConnectToPort works
+				if _, err := conn.Write([]byte("hello VM")); err != nil {
+					log.Println("sending 'hello VM' failed:", err)
+					return
+				}
+			*/
+			// `nc --listen --vsock $vsockPort` in the VM closes when this function returns.
+			// This indicates `conn` was closed.
+		}
+		for {
+			time.Sleep(10 * time.Second)
+			for _, vsock := range vm.SocketDevices() {
+				// Wait until `nc --listen --vsock $vsockPort` is running inside the VM
+				fmt.Printf("Trying to connect to vsock port %d", vsockPort)
+				vsock.ConnectToPort(uint32(vsockPort), vsockConnected)
+			}
+		}
+	}()
 
 	for {
 		select {
@@ -153,6 +192,14 @@ func main() {
 				log.Println("stopped successfully")
 				return
 			}
+		case conn := <-vsockCh:
+			// fails with `sending 'hello VM' failed: write : bad
+			// file descriptor` if VirtioSocketConnection.dup() is
+			// not used
+			if _, err := conn.Write([]byte("hello VM")); err != nil {
+				log.Println("sending 'hello VM' failed:", err)
+				return
+			}
 		case err := <-errCh:
 			log.Println("in start:", err)
 		}