net/dns: do not wait for the interface registry key to appear if the windowsManager is being closed

The WinTun adapter may have been removed by the time we're closing
the dns.windowsManager, and its associated interface registry key might
also have been deleted. We shouldn't use winutil.OpenKeyWait and wait
for the interface key to appear when performing a cleanup as a part of
the windowsManager shutdown.

Updates #11222

Signed-off-by: Nick Khyl <nickk@tailscale.com>

net/dns/manager_windows.go

@@ -14,6 +14,7 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"sync"
 	"syscall"
 	"time"
 
@@ -38,6 +39,9 @@ type windowsManager struct {
 	guid       string
 	nrptDB     *nrptRuleDatabase
 	wslManager *wslManager
+
+	mu      sync.Mutex
+	closing bool
 }
 
 func NewOSConfigurator(logf logger.Logf, interfaceName string) (OSConfigurator, error) {
@@ -64,14 +68,37 @@ func NewOSConfigurator(logf logger.Logf, interfaceName string) (OSConfigurator,
 }
 
 func (m *windowsManager) openInterfaceKey(pfx winutil.RegistryPathPrefix) (registry.Key, error) {
+	var key registry.Key
+	var err error
 	path := pfx.WithSuffix(m.guid)
-	key, err := winutil.OpenKeyWait(registry.LOCAL_MACHINE, path, registry.SET_VALUE)
+
+	m.mu.Lock()
+	closing := m.closing
+	m.mu.Unlock()
+	if closing {
+		// Do not wait for the interface key to appear if the manager is being closed.
+		// If it's being closed due to the removal of the wintun adapter,
+		// the key would already be gone by now and will not reappear until tailscaled is restarted.
+		key, err = registry.OpenKey(registry.LOCAL_MACHINE, string(path), registry.SET_VALUE)
+	} else {
+		key, err = winutil.OpenKeyWait(registry.LOCAL_MACHINE, path, registry.SET_VALUE)
+	}
 	if err != nil {
 		return 0, fmt.Errorf("opening %s: %w", path, err)
 	}
 	return key, nil
 }
 
+func (m *windowsManager) muteKeyNotFoundIfClosing(err error) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if !m.closing || (err != windows.ERROR_FILE_NOT_FOUND && err != windows.ERROR_PATH_NOT_FOUND) {
+		return err
+	}
+
+	return nil
+}
+
 func delValue(key registry.Key, name string) error {
 	if err := key.DeleteValue(name); err != nil && err != registry.ErrNotExist {
 		return err
@@ -205,7 +232,7 @@ func (m *windowsManager) setPrimaryDNS(resolvers []netip.Addr, domains []dnsname
 
 	key4, err := m.openInterfaceKey(winutil.IPv4TCPIPInterfacePrefix)
 	if err != nil {
-		return err
+		return m.muteKeyNotFoundIfClosing(err)
 	}
 	defer key4.Close()
 
@@ -227,7 +254,7 @@ func (m *windowsManager) setPrimaryDNS(resolvers []netip.Addr, domains []dnsname
 
 	key6, err := m.openInterfaceKey(winutil.IPv6TCPIPInterfacePrefix)
 	if err != nil {
-		return err
+		return m.muteKeyNotFoundIfClosing(err)
 	}
 	defer key6.Close()
 
@@ -387,6 +414,14 @@ func (m *windowsManager) SupportsSplitDNS() bool {
 }
 
 func (m *windowsManager) Close() error {
+	m.mu.Lock()
+	if m.closing {
+		m.mu.Unlock()
+		return nil
+	}
+	m.closing = true
+	m.mu.Unlock()
+
 	err := m.SetDNS(OSConfig{})
 	if m.nrptDB != nil {
 		m.nrptDB.Close()
@@ -407,7 +442,7 @@ func (m *windowsManager) disableDynamicUpdates() error {
 	for _, prefix := range prefixen {
 		k, err := m.openInterfaceKey(prefix)
 		if err != nil {
-			return err
+			return m.muteKeyNotFoundIfClosing(err)
 		}
 		defer k.Close()
 
@@ -426,7 +461,7 @@ func (m *windowsManager) disableDynamicUpdates() error {
 func (m *windowsManager) setSingleDWORD(prefix winutil.RegistryPathPrefix, value string, data uint32) error {
 	k, err := m.openInterfaceKey(prefix)
 	if err != nil {
-		return err
+		return m.muteKeyNotFoundIfClosing(err)
 	}
 	defer k.Close()
 	return k.SetDWordValue(value, data)