cmd/tailscale/web: add support for QNAP

Signed-off-by: Maisem Ali <maisem@tailscale.com>

util/groupmember/groupmember_cgo.go

@@ -0,0 +1,48 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build cgo
+
+package groupmember
+
+import (
+	"os/user"
+	"sync"
+)
+
+func isMemberOfGroup(group, name string) (bool, error) {
+	u, err := user.Lookup(name)
+	if err != nil {
+		return false, err
+	}
+	ugids, err := u.GroupIds()
+	if err != nil {
+		return false, err
+	}
+	gid, err := getGroupID(group)
+	if err != nil {
+		return false, err
+	}
+	for _, ugid := range ugids {
+		if gid == ugid {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+var groupIDCache sync.Map // of string
+
+func getGroupID(groupName string) (string, error) {
+	s, ok := groupIDCache.Load(groupName)
+	if ok {
+		return s.(string), nil
+	}
+	g, err := user.LookupGroup(groupName)
+	if err != nil {
+		return "", err
+	}
+	groupIDCache.Store(groupName, g.Gid)
+	return g.Gid, nil
+}