internal/net: enforce timeout for probing stack

On Windows host with StarLink network, ctrld hangs on startup for ~30s
before continue running. This dues to IPv6 is configured but no external
IPv6 can be reached. When probing stack, ctrld is dialing using ipv6
without any timeout set, so the dialing timeout is enforced by OS.

This commit adds a timeout for probing dialer, so we ensure the probing
process will fail fast.

internal/net/net.go

@@ -28,6 +28,13 @@ var Dialer = &net.Dialer{
 	},
 }
 
+const probeStackTimeout = 2 * time.Second
+
+var probeStackDialer = &net.Dialer{
+	Resolver: Dialer.Resolver,
+	Timeout:  probeStackTimeout,
+}
+
 var (
 	stackOnce          atomic.Pointer[sync.Once]
 	ipv4Enabled        bool
@@ -41,12 +48,12 @@ func init() {
 }
 
 func supportIPv4() bool {
-	_, err := Dialer.Dial("tcp4", net.JoinHostPort(controldIPv4Test, "80"))
+	_, err := probeStackDialer.Dial("tcp4", net.JoinHostPort(controldIPv4Test, "80"))
 	return err == nil
 }
 
 func supportIPv6(ctx context.Context) bool {
-	_, err := Dialer.DialContext(ctx, "tcp6", net.JoinHostPort(controldIPv6Test, "80"))
+	_, err := probeStackDialer.DialContext(ctx, "tcp6", net.JoinHostPort(controldIPv6Test, "80"))
 	return err == nil
 }
 
@@ -61,7 +68,7 @@ func supportListenIPv6Local() bool {
 func probeStack() {
 	b := backoff.NewBackoff("probeStack", func(format string, args ...any) {}, time.Minute)
 	for {
-		if _, err := Dialer.Dial("udp", bootstrapDNS); err == nil {
+		if _, err := probeStackDialer.Dial("udp", bootstrapDNS); err == nil {
 			hasNetworkUp = true
 			break
 		} else {

internal/net/net_test.go

@@ -0,0 +1,24 @@
+package net
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+func TestProbeStackTimeout(t *testing.T) {
+	done := make(chan struct{})
+	started := make(chan struct{})
+	go func() {
+		defer close(done)
+		close(started)
+		supportIPv6(context.Background())
+	}()
+
+	<-started
+	select {
+	case <-time.After(probeStackTimeout + time.Second):
+		t.Error("probeStack timeout is not enforce")
+	case <-done:
+	}
+}