all: rework bootstrap IP discovering

At startup, ctrld gathers bootstrap IP information and use this
bootstrap IP for connecting to upstream. However, in case the network
stack changed, for example, dues to VPN connection, ctrld will still use
this old (maybe invalid) bootstrap IP for the current network stack.

This commit rework the discovering process, and re-initializing the
bootstrap IP if connecting to upstream failed.

internal/net/net.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"net"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"tailscale.com/logtail/backoff"
@@ -28,13 +29,17 @@ var Dialer = &net.Dialer{
 }
 
 var (
-	stackOnce          sync.Once
+	stackOnce          atomic.Pointer[sync.Once]
 	ipv4Enabled        bool
 	ipv6Enabled        bool
 	canListenIPv6Local bool
 	hasNetworkUp       bool
 )
 
+func init() {
+	stackOnce.Store(new(sync.Once))
+}
+
 func probeStack() {
 	b := backoff.NewBackoff("probeStack", func(format string, args ...any) {}, time.Minute)
 	for {
@@ -57,23 +62,27 @@ func probeStack() {
 	}
 }
 
+func Reset() {
+	stackOnce.Store(new(sync.Once))
+}
+
 func Up() bool {
-	stackOnce.Do(probeStack)
+	stackOnce.Load().Do(probeStack)
 	return hasNetworkUp
 }
 
 func SupportsIPv4() bool {
-	stackOnce.Do(probeStack)
+	stackOnce.Load().Do(probeStack)
 	return ipv4Enabled
 }
 
 func SupportsIPv6() bool {
-	stackOnce.Do(probeStack)
+	stackOnce.Load().Do(probeStack)
 	return ipv6Enabled
 }
 
 func SupportsIPv6ListenLocal() bool {
-	stackOnce.Do(probeStack)
+	stackOnce.Load().Do(probeStack)
 	return canListenIPv6Local
 }