/*
Technitium DNS Server
Copyright (C) 2020  Shreyas Zare (shreyas@technitium.com)

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

*/

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using TechnitiumLibrary.Net.Dns;
using TechnitiumLibrary.Net.Dns.ResourceRecords;

namespace DnsServerCore.Dns.Zones
{
    class PrimaryZone : AuthZone
    {
        #region variables

        readonly DnsServer _dnsServer;
        readonly bool _internal;

        readonly Timer _notifyTimer;
        const int NOTIFY_TIMER_INTERVAL = 10000;
        readonly List<NameServerAddress> _notifyList = new List<NameServerAddress>();

        const int NOTIFY_TIMEOUT = 10000;
        const int NOTIFY_RETRIES = 5;

        #endregion

        #region constructor

        public PrimaryZone(DnsServer dnsServer, AuthZoneInfo zoneInfo)
            : base(zoneInfo.Name)
        {
            _dnsServer = dnsServer;

            _disabled = zoneInfo.Disabled;

            _notifyTimer = new Timer(NotifyTimerCallback, null, Timeout.Infinite, Timeout.Infinite);
        }

        public PrimaryZone(DnsServer dnsServer, string name, string primaryNameServer, bool @internal)
            : base(name)
        {
            _dnsServer = dnsServer;
            _internal = @internal;

            DnsSOARecord soa = new DnsSOARecord(primaryNameServer, "hostadmin." + name, 1, 14400, 3600, 604800, 900);

            _entries[DnsResourceRecordType.SOA] = new DnsResourceRecord[] { new DnsResourceRecord(_name, DnsResourceRecordType.SOA, DnsClass.IN, soa.Refresh, soa) };
            _entries[DnsResourceRecordType.NS] = new DnsResourceRecord[] { new DnsResourceRecord(_name, DnsResourceRecordType.NS, DnsClass.IN, soa.Refresh, new DnsNSRecord(soa.PrimaryNameServer)) };

            _notifyTimer = new Timer(NotifyTimerCallback, null, Timeout.Infinite, Timeout.Infinite);
        }

        internal PrimaryZone(DnsServer dnsServer, string name, DnsSOARecord soa, DnsNSRecord ns)
            : base(name)
        {
            _dnsServer = dnsServer;
            _internal = true;

            _entries[DnsResourceRecordType.SOA] = new DnsResourceRecord[] { new DnsResourceRecord(_name, DnsResourceRecordType.SOA, DnsClass.IN, soa.Refresh, soa) };
            _entries[DnsResourceRecordType.NS] = new DnsResourceRecord[] { new DnsResourceRecord(_name, DnsResourceRecordType.NS, DnsClass.IN, soa.Refresh, ns) };

            _notifyTimer = new Timer(NotifyTimerCallback, null, Timeout.Infinite, Timeout.Infinite);
        }

        #endregion

        #region IDisposable

        bool _disposed;

        protected override void Dispose(bool disposing)
        {
            if (_disposed)
                return;

            if (disposing)
            {
                if (_notifyTimer != null)
                    _notifyTimer.Dispose();
            }

            _disposed = true;
        }

        #endregion

        #region private

        private async void NotifyTimerCallback(object state)
        {
            try
            {
                IReadOnlyList<NameServerAddress> secondaryNameServers = await GetSecondaryNameServerAddressesAsync(_dnsServer);

                foreach (NameServerAddress secondaryNameServer in secondaryNameServers)
                    NotifyNameServer(secondaryNameServer);
            }
            catch (Exception ex)
            {
                LogManager log = _dnsServer.LogManager;
                if (log != null)
                    log.Write(ex);
            }
        }

        private void NotifyNameServer(NameServerAddress nameServer)
        {
            //use notify list to prevent multiple threads from notifying the same name server
            lock (_notifyList)
            {
                if (_notifyList.Contains(nameServer))
                    return; //already notifying the name server in another thread

                _notifyList.Add(nameServer);
            }

            _ = NotifyNameServerAsync(nameServer);
        }

        private async Task NotifyNameServerAsync(NameServerAddress nameServer)
        {
            try
            {
                DnsClient client = new DnsClient(nameServer);

                client.Timeout = NOTIFY_TIMEOUT;
                client.Retries = NOTIFY_RETRIES;

                DnsDatagram notifyRequest = new DnsDatagram(0, false, DnsOpcode.Notify, true, false, false, false, false, false, DnsResponseCode.NoError, new DnsQuestionRecord[] { new DnsQuestionRecord(_name, DnsResourceRecordType.SOA, DnsClass.IN) }, _entries[DnsResourceRecordType.SOA]);
                DnsDatagram response = await client.ResolveAsync(notifyRequest);

                switch (response.RCODE)
                {
                    case DnsResponseCode.NoError:
                    case DnsResponseCode.NotImplemented:
                        {
                            //transaction complete
                            LogManager log = _dnsServer.LogManager;
                            if (log != null)
                                log.Write("DNS Server successfully notified name server for '" + _name + "' zone changes: " + nameServer.ToString());
                        }
                        break;

                    default:
                        {
                            //transaction failed
                            LogManager log = _dnsServer.LogManager;
                            if (log != null)
                                log.Write("DNS Server received RCODE=" + response.RCODE.ToString() + " from name server for '" + _name + "' zone notification: " + nameServer.ToString());
                        }
                        break;
                }
            }
            catch (Exception ex)
            {
                LogManager log = _dnsServer.LogManager;
                if (log != null)
                {
                    log.Write("DNS Server failed to notify name server for '" + _name + "' zone changes: " + nameServer.ToString());
                    log.Write(ex);
                }
            }
            finally
            {
                lock (_notifyList)
                {
                    _notifyList.Remove(nameServer);
                }
            }
        }

        #endregion

        #region public

        public void IncrementSoaSerial()
        {
            DnsResourceRecord record = _entries[DnsResourceRecordType.SOA][0];
            DnsSOARecord soa = record.RDATA as DnsSOARecord;

            uint serial = soa.Serial;
            if (serial < uint.MaxValue)
                serial++;
            else
                serial = 0;

            DnsResourceRecord newRecord = new DnsResourceRecord(record.Name, record.Type, record.Class, record.TtlValue, new DnsSOARecord(soa.PrimaryNameServer, soa.ResponsiblePerson, serial, soa.Refresh, soa.Retry, soa.Expire, soa.Minimum)) { Tag = record.Tag };
            _entries[DnsResourceRecordType.SOA] = new DnsResourceRecord[] { newRecord };
        }

        public void NotifyNameServers()
        {
            _notifyTimer.Change(NOTIFY_TIMER_INTERVAL, Timeout.Infinite);
        }

        public override void SetRecords(DnsResourceRecordType type, IReadOnlyList<DnsResourceRecord> records)
        {
            switch (type)
            {
                case DnsResourceRecordType.CNAME:
                    throw new InvalidOperationException("Cannot set CNAME record to zone root.");

                case DnsResourceRecordType.SOA:
                    if ((records.Count != 1) || !records[0].Name.Equals(_name, StringComparison.OrdinalIgnoreCase))
                        throw new InvalidOperationException("Invalid SOA record.");

                    //remove any resource record info
                    records[0].Tag = null;
                    break;
            }

            base.SetRecords(type, records);

            IncrementSoaSerial();
            NotifyNameServers();
        }

        public override void AddRecord(DnsResourceRecord record)
        {
            base.AddRecord(record);

            IncrementSoaSerial();
            NotifyNameServers();
        }

        public override bool DeleteRecords(DnsResourceRecordType type)
        {
            if (type == DnsResourceRecordType.SOA)
                throw new InvalidOperationException("Cannot delete SOA record.");

            if (base.DeleteRecords(type))
            {
                IncrementSoaSerial();
                NotifyNameServers();

                return true;
            }

            return false;
        }

        public override bool DeleteRecord(DnsResourceRecordType type, DnsResourceRecordData record)
        {
            if (type == DnsResourceRecordType.SOA)
                throw new InvalidOperationException("Cannot delete SOA record.");

            if (base.DeleteRecord(type, record))
            {
                IncrementSoaSerial();
                NotifyNameServers();

                return true;
            }

            return false;
        }

        #endregion

        #region properties

        public bool Internal
        { get { return _internal; } }

        public override bool Disabled
        {
            get { return _disabled; }
            set
            {
                if (_disabled != value)
                {
                    _disabled = value;

                    if (_disabled)
                        _notifyTimer.Change(Timeout.Infinite, Timeout.Infinite);
                    else
                        NotifyNameServers();
                }
            }
        }

        #endregion
    }
}