wgengine/router/dns: use OpenKeyWait to set DNS configuration.

Fixes tailscale/corp#839.

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/tailscale/depaware.txt

@@ -30,7 +30,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
    W 💣 github.com/tailscale/wireguard-go/tun/wintun/iphlpapi        from github.com/tailscale/wireguard-go/tun/wintun
    W 💣 github.com/tailscale/wireguard-go/tun/wintun/namespaceapi    from github.com/tailscale/wireguard-go/tun/wintun
    W 💣 github.com/tailscale/wireguard-go/tun/wintun/nci             from github.com/tailscale/wireguard-go/tun/wintun
-   W 💣 github.com/tailscale/wireguard-go/tun/wintun/registry        from github.com/tailscale/wireguard-go/tun/wintun
+   W 💣 github.com/tailscale/wireguard-go/tun/wintun/registry        from github.com/tailscale/wireguard-go/tun/wintun+
    W 💣 github.com/tailscale/wireguard-go/tun/wintun/setupapi        from github.com/tailscale/wireguard-go/tun/wintun
         github.com/tailscale/wireguard-go/wgcfg                      from github.com/tailscale/wireguard-go/conn+
         github.com/tcnksm/go-httpstat                                from tailscale.com/net/netcheck

cmd/tailscaled/depaware.txt

@@ -33,7 +33,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
    W 💣 github.com/tailscale/wireguard-go/tun/wintun/iphlpapi        from github.com/tailscale/wireguard-go/tun/wintun
    W 💣 github.com/tailscale/wireguard-go/tun/wintun/namespaceapi    from github.com/tailscale/wireguard-go/tun/wintun
    W 💣 github.com/tailscale/wireguard-go/tun/wintun/nci             from github.com/tailscale/wireguard-go/tun/wintun
-   W 💣 github.com/tailscale/wireguard-go/tun/wintun/registry        from github.com/tailscale/wireguard-go/tun/wintun
+   W 💣 github.com/tailscale/wireguard-go/tun/wintun/registry        from github.com/tailscale/wireguard-go/tun/wintun+
    W 💣 github.com/tailscale/wireguard-go/tun/wintun/setupapi        from github.com/tailscale/wireguard-go/tun/wintun
         github.com/tailscale/wireguard-go/wgcfg                      from github.com/tailscale/wireguard-go/conn+
         github.com/tcnksm/go-httpstat                                from tailscale.com/net/netcheck

wgengine/router/dns/manager_windows.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/tailscale/wireguard-go/tun"
+	wgregistry "github.com/tailscale/wireguard-go/tun/wintun/registry"
 	"golang.org/x/sys/windows/registry"
 	"tailscale.com/types/logger"
 )
@@ -34,8 +35,14 @@ func newManager(mconfig ManagerConfig) managerImpl {
 	}
 }
 
+// keyOpenTimeout is how long we wait for a registry key to
+// appear. For some reason, registry keys tied to ephemeral interfaces
+// can take a long while to appear after interface creation, and we
+// can end up racing with that.
+const keyOpenTimeout = time.Minute
+
 func setRegistryString(path, name, value string) error {
-	key, err := registry.OpenKey(registry.LOCAL_MACHINE, path, registry.SET_VALUE)
+	key, err := wgregistry.OpenKeyWait(registry.LOCAL_MACHINE, path, registry.SET_VALUE, keyOpenTimeout)
 	if err != nil {
 		return fmt.Errorf("opening %s: %w", path, err)
 	}