Umbraco-CMS/src/Umbraco.Web/PublishedCache/NuCache/ContentStore.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using CSharpTest.Net.Collections;
using Umbraco.Core;
using Umbraco.Core.Exceptions;
using Umbraco.Core.Logging;
using Umbraco.Core.Models.PublishedContent;
using Umbraco.Core.Scoping;
using Umbraco.Web.PublishedCache.NuCache.Snap;

namespace Umbraco.Web.PublishedCache.NuCache
{
    /// <summary>
    /// Stores content in memory and persists it back to disk
    /// </summary>
    /// <remarks>
    /// <para>
    /// Methods in this class suffixed with the term "Locked" means that those methods can only be called within a WriteLock. A WriteLock
    /// is acquired by the GetScopedWriteLock method. Locks are not allowed to be recursive.
    /// </para>
    /// <para>
    /// This class's logic is based on the <see cref="SnapDictionary{TKey, TValue}"/> class but has been slightly modified to suit these purposes.
    /// </para>
    /// </remarks>
    internal class ContentStore
    {
        // this class is an extended version of SnapDictionary
        // most of the snapshots management code, etc is an exact copy
        // SnapDictionary has unit tests to ensure it all works correctly
        // For locking information, see SnapDictionary

        private readonly IPublishedSnapshotAccessor _publishedSnapshotAccessor;
        private readonly IVariationContextAccessor _variationContextAccessor;
        private readonly ConcurrentDictionary<int, LinkedNode<ContentNode>> _contentNodes;
        private LinkedNode<ContentNode> _root;
        private readonly ConcurrentDictionary<int, LinkedNode<IPublishedContentType>> _contentTypesById;
        private readonly ConcurrentDictionary<string, LinkedNode<IPublishedContentType>> _contentTypesByAlias;
        private readonly ConcurrentDictionary<Guid, int> _xmap;

        private readonly ILogger _logger;
        private BPlusTree<int, ContentNodeKit> _localDb;
        private readonly ConcurrentQueue<GenObj> _genObjs;
        private GenObj _genObj;
        private readonly object _wlocko = new object();
        private readonly object _rlocko = new object();
        private long _liveGen, _floorGen;
        private bool _nextGen, _collectAuto;
        private Task _collectTask;
        private List<KeyValuePair<int, ContentNodeKit>> _wchanges;

        // TODO: collection trigger (ok for now)
        // see SnapDictionary notes
        private const long CollectMinGenDelta = 8;

        #region Ctor

        public ContentStore(
            IPublishedSnapshotAccessor publishedSnapshotAccessor,
            IVariationContextAccessor variationContextAccessor,
            ILogger logger,
            BPlusTree<int, ContentNodeKit> localDb = null)
        {
            _publishedSnapshotAccessor = publishedSnapshotAccessor;
            _variationContextAccessor = variationContextAccessor;
            _logger = logger;
            _localDb = localDb;

            _contentNodes = new ConcurrentDictionary<int, LinkedNode<ContentNode>>();
            _root = new LinkedNode<ContentNode>(new ContentNode(), 0);
            _contentTypesById = new ConcurrentDictionary<int, LinkedNode<IPublishedContentType>>();
            _contentTypesByAlias = new ConcurrentDictionary<string, LinkedNode<IPublishedContentType>>(StringComparer.InvariantCultureIgnoreCase);
            _xmap = new ConcurrentDictionary<Guid, int>();

            _genObjs = new ConcurrentQueue<GenObj>();
            _genObj = null; // no initial gen exists
            _liveGen = _floorGen = 0;
            _nextGen = false; // first time, must create a snapshot
            _collectAuto = true; // collect automatically by default
        }

        #endregion

        #region Locking

        // see notes on SnapDictionary

        private readonly string _instanceId = Guid.NewGuid().ToString("N");

        private class WriteLockInfo
        {
            public bool Taken;
        }

        // a scope contextual that represents a locked writer to the dictionary
        private class ScopedWriteLock : ScopeContextualBase
        {
            private readonly WriteLockInfo _lockinfo = new WriteLockInfo();
            private readonly ContentStore _store;
            private int _released;

            public ScopedWriteLock(ContentStore store, bool scoped)
            {
                _store = store;
                store.Lock(_lockinfo, scoped);
            }

            public override void Release(bool completed)
            {
                if (Interlocked.CompareExchange(ref _released, 1, 0) != 0)
                    return;
                _store.Release(_lockinfo, completed);
            }
        }

        // gets a scope contextual representing a locked writer to the dictionary
        // TODO: GetScopedWriter? should the dict have a ref onto the scope provider?
        public IDisposable GetScopedWriteLock(IScopeProvider scopeProvider)
        {
            return ScopeContextualBase.Get(scopeProvider, _instanceId, scoped => new ScopedWriteLock(this, scoped));
        }

        private void EnsureLocked()
        {
            if (!Monitor.IsEntered(_wlocko))
                throw new InvalidOperationException("Write lock must be acquried.");
        }

        private void Lock(WriteLockInfo lockInfo, bool forceGen = false)
        {
            if (Monitor.IsEntered(_wlocko))
                throw new InvalidOperationException("Recursive locks not allowed");

            Monitor.Enter(_wlocko, ref lockInfo.Taken);

            lock(_rlocko)
            {
                // see SnapDictionary
                try { }
                finally
                {
                    if (_nextGen == false || (forceGen))
                    {
                        // because we are changing things, a new generation
                        // is created, which will trigger a new snapshot
                        if (_nextGen)
                            _genObjs.Enqueue(_genObj = new GenObj(_liveGen));
                        _liveGen += 1;
                        _nextGen = true;
                    }
                }
            }           
        }

        private void Release(WriteLockInfo lockInfo, bool commit = true)
        {
            try
            {
                if (commit == false)
                {
                    lock (_rlocko)
                    {
                        // see SnapDictionary
                        try { }
                        finally
                        {
                            _nextGen = false;
                            _liveGen -= 1;
                        }
                    }

                    Rollback(_contentNodes);
                    RollbackRoot();
                    Rollback(_contentTypesById);
                    Rollback(_contentTypesByAlias);
                }
                else if (_localDb != null && _wchanges != null)
                {
                    foreach (var change in _wchanges)
                    {
                        if (change.Value.IsNull)
                            _localDb.TryRemove(change.Key, out ContentNodeKit unused);
                        else
                            _localDb[change.Key] = change.Value;
                    }
                    _wchanges = null;
                    _localDb.Commit();
                }
            }
            finally
            {
                if (lockInfo.Taken)
                    Monitor.Exit(_wlocko);
            }
        }

        private void RollbackRoot()
        {
            if (_root.Gen <= _liveGen) return;

            if (_root.Next != null)
                _root = _root.Next;
        }

        private void Rollback<TKey, TValue>(ConcurrentDictionary<TKey, LinkedNode<TValue>> dictionary)
            where TValue : class
        {
            foreach (var item in dictionary)
            {
                var link = item.Value;
                if (link.Gen <= _liveGen) continue;

                var key = item.Key;
                if (link.Next == null)
                    dictionary.TryRemove(key, out link);
                else
                    dictionary.TryUpdate(key, link.Next, link);
            }
        }

        #endregion

        #region LocalDb

        public void ReleaseLocalDb()
        {
            var lockInfo = new WriteLockInfo();
            try
            {
                try
                {
                    // Trying to lock could throw exceptions so always make sure to clean up.                    
                    Lock(lockInfo);
                }
                finally
                {
                    try
                    {
                        _localDb?.Dispose();
                    }
                    catch (Exception ex)
                    {
                        /* TBD: May already be throwing so don't throw again */
                        _logger.Error<ContentStore>(ex, "Error trying to release DB");
                    }
                    finally
                    {
                        _localDb = null;
                    }
                }

            }
            catch (Exception ex)
            {
                _logger.Error<ContentStore>(ex, "Error trying to lock");
                throw;
            }
            finally
            {
                Release(lockInfo);
            }
        }

        private void RegisterChange(int id, ContentNodeKit kit)
        {
            if (_wchanges == null) _wchanges = new List<KeyValuePair<int, ContentNodeKit>>();
            _wchanges.Add(new KeyValuePair<int, ContentNodeKit>(id, kit));
        }

        #endregion

        #region Content types

        /// <summary>
        /// Sets data for new content types
        /// </summary>
        /// <param name="types"></param>
        /// <remarks>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public void NewContentTypesLocked(IEnumerable<IPublishedContentType> types)
        {
            EnsureLocked();

            foreach (var type in types)
            {
                SetValueLocked(_contentTypesById, type.Id, type);
                SetValueLocked(_contentTypesByAlias, type.Alias, type);
            }
        }

        /// <summary>
        /// Sets data for updated content types
        /// </summary>
        /// <param name="types"></param>
        /// <remarks>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public void UpdateContentTypesLocked(IEnumerable<IPublishedContentType> types)
        {
            //nothing to do if this is empty, no need to lock/allocate/iterate/etc...
            if (!types.Any()) return;

            EnsureLocked();

            var index = types.ToDictionary(x => x.Id, x => x);

            foreach (var type in index.Values)
            {
                SetValueLocked(_contentTypesById, type.Id, type);
                SetValueLocked(_contentTypesByAlias, type.Alias, type);
            }

            foreach (var link in _contentNodes.Values)
            {
                var node = link.Value;
                if (node == null) continue;
                var contentTypeId = node.ContentType.Id;
                if (index.TryGetValue(contentTypeId, out var contentType) == false) continue;
                SetValueLocked(_contentNodes, node.Id, new ContentNode(node, contentType));
            }
        }

        /// <summary>
        /// Updates/sets data for all content types
        /// </summary>
        /// <param name="types"></param>
        /// <remarks>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public void SetAllContentTypesLocked(IEnumerable<IPublishedContentType> types)
        {
            EnsureLocked();

            // clear all existing content types
            ClearLocked(_contentTypesById);
            ClearLocked(_contentTypesByAlias);

            // set all new content types
            foreach (var type in types)
            {
                SetValueLocked(_contentTypesById, type.Id, type);
                SetValueLocked(_contentTypesByAlias, type.Alias, type);
            }

            // beware! at that point the cache is inconsistent,
            // assuming we are going to SetAll content items!
        }

        /// <summary>
        /// Updates/sets/removes data for content types
        /// </summary>
        /// <param name="removedIds"></param>
        /// <param name="refreshedTypes"></param>
        /// <param name="kits"></param>
        /// <remarks>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public void UpdateContentTypesLocked(IReadOnlyCollection<int> removedIds, IReadOnlyCollection<IPublishedContentType> refreshedTypes, IReadOnlyCollection<ContentNodeKit> kits)
        {
            EnsureLocked();

            var removedIdsA = removedIds ?? Array.Empty<int>();
            var refreshedTypesA = refreshedTypes ?? Array.Empty<IPublishedContentType>();
            var refreshedIdsA = refreshedTypesA.Select(x => x.Id).ToList();
            kits = kits ?? Array.Empty<ContentNodeKit>();

            if (kits.Count == 0 && refreshedIdsA.Count == 0 && removedIdsA.Count == 0)
                return; //exit - there is nothing to do here

            var removedContentTypeNodes = new List<int>();
            var refreshedContentTypeNodes = new List<int>();

            // find all the nodes that are either refreshed or removed,
            // because of their content type being either refreshed or removed
            foreach (var link in _contentNodes.Values)
            {
                var node = link.Value;
                if (node == null) continue;
                var contentTypeId = node.ContentType.Id;
                if (removedIdsA.Contains(contentTypeId)) removedContentTypeNodes.Add(node.Id);
                if (refreshedIdsA.Contains(contentTypeId)) refreshedContentTypeNodes.Add(node.Id);
            }

            // perform deletion of content with removed content type
            // removing content types should have removed their content already
            // but just to be 100% sure, clear again here
            foreach (var node in removedContentTypeNodes)
                ClearBranchLocked(node);

            // perform deletion of removed content types
            foreach (var id in removedIdsA)
            {
                if (_contentTypesById.TryGetValue(id, out var link) == false || link.Value == null)
                    continue;
                SetValueLocked(_contentTypesById, id, null);
                SetValueLocked(_contentTypesByAlias, link.Value.Alias, null);
            }

            // perform update of refreshed content types
            foreach (var type in refreshedTypesA)
            {
                SetValueLocked(_contentTypesById, type.Id, type);
                SetValueLocked(_contentTypesByAlias, type.Alias, type);
            }

            // perform update of content with refreshed content type - from the kits
            // skip missing type, skip missing parents & un-buildable kits - what else could we do?
            // kits are ordered by level, so ParentExists is ok here
            var visited = new List<int>();
            foreach (var kit in kits.Where(x =>
                refreshedIdsA.Contains(x.ContentTypeId) &&
                BuildKit(x, out _)))
            {
                // replacing the node: must preserve the parents
                var node = GetHead(_contentNodes, kit.Node.Id)?.Value;
                if (node != null)
                    kit.Node.FirstChildContentId = node.FirstChildContentId;

                SetValueLocked(_contentNodes, kit.Node.Id, kit.Node);

                visited.Add(kit.Node.Id);
                if (_localDb != null) RegisterChange(kit.Node.Id, kit);
            }

            // all content should have been refreshed - but...
            var orphans = refreshedContentTypeNodes.Except(visited);
            foreach (var id in orphans)
                ClearBranchLocked(id);
        }

        /// <summary>
        /// Updates data types
        /// </summary>
        /// <param name="dataTypeIds"></param>
        /// <param name="getContentType"></param>
        /// <remarks>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public void UpdateDataTypesLocked(IEnumerable<int> dataTypeIds, Func<int, IPublishedContentType> getContentType)
        {
            EnsureLocked();

            var contentTypes = _contentTypesById
                    .Where(kvp =>
                        kvp.Value.Value != null &&
                        kvp.Value.Value.PropertyTypes.Any(p => dataTypeIds.Contains(p.DataType.Id)))
                    .Select(kvp => kvp.Value.Value)
                    .Select(x => getContentType(x.Id))
                    .Where(x => x != null) // poof, gone, very unlikely and probably an anomaly
                    .ToArray();

            var contentTypeIdsA = contentTypes.Select(x => x.Id).ToArray();
            var contentTypeNodes = new Dictionary<int, List<int>>();
            foreach (var id in contentTypeIdsA)
                contentTypeNodes[id] = new List<int>();
            foreach (var link in _contentNodes.Values)
            {
                var node = link.Value;
                if (node != null && contentTypeIdsA.Contains(node.ContentType.Id))
                    contentTypeNodes[node.ContentType.Id].Add(node.Id);
            }

            foreach (var contentType in contentTypes)
            {
                // again, weird situation
                if (contentTypeNodes.ContainsKey(contentType.Id) == false)
                    continue;

                foreach (var id in contentTypeNodes[contentType.Id])
                {
                    _contentNodes.TryGetValue(id, out var link);
                    if (link?.Value == null)
                        continue;
                    var node = new ContentNode(link.Value, contentType);
                    SetValueLocked(_contentNodes, id, node);
                    if (_localDb != null) RegisterChange(id, node.ToKit());
                }
            }
        }

        /// <summary>
        /// Validate the <see cref="ContentNodeKit"/> and try to create a parent <see cref="LinkedNode{ContentNode}"/>
        /// </summary>
        /// <param name="kit"></param>
        /// <param name="parent"></param>
        /// <returns>
        /// Returns false if the parent was not found or if the kit validation failed
        /// </returns>
        private bool BuildKit(ContentNodeKit kit, out LinkedNode<ContentNode> parent)
        {
            // make sure parent exists
            parent = GetParentLink(kit.Node, null);
            if (parent == null)
            {
                _logger.Warn<ContentStore>($"Skip item id={kit.Node.Id}, could not find parent id={kit.Node.ParentContentId}.");
                return false;
            }

            // make sure the kit is valid
            if (kit.DraftData == null && kit.PublishedData == null)
            {
                _logger.Warn<ContentStore>($"Skip item id={kit.Node.Id}, both draft and published data are null.");
                return false;
            }

            // unknown = bad
            if (_contentTypesById.TryGetValue(kit.ContentTypeId, out var link) == false || link.Value == null)
            {
                _logger.Warn<ContentStore>($"Skip item id={kit.Node.Id}, could not find content type id={kit.ContentTypeId}.");
                return false;
            }

            // check whether parent is published
            var canBePublished = ParentPublishedLocked(kit);

            // and use
            kit.Build(link.Value, _publishedSnapshotAccessor, _variationContextAccessor, canBePublished);

            return true;
        }

        #endregion

        #region Set, Clear, Get

        public int Count => _contentNodes.Count;

        /// <summary>
        /// Get the most recent version of the LinkedNode stored in the dictionary for the supplied key
        /// </summary>
        /// <typeparam name="TKey"></typeparam>
        /// <typeparam name="TValue"></typeparam>
        /// <param name="dict"></param>
        /// <param name="key"></param>
        /// <returns></returns>
        private static LinkedNode<TValue> GetHead<TKey, TValue>(ConcurrentDictionary<TKey, LinkedNode<TValue>> dict, TKey key)
            where TValue : class
        {
            dict.TryGetValue(key, out var link); // else null
            return link;
        }

        /// <summary>
        /// Sets the data for a <see cref="ContentNodeKit"/>
        /// </summary>
        /// <param name="kit"></param>
        /// <returns></returns>
        /// <remarks>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public bool SetLocked(ContentNodeKit kit)
        {
            EnsureLocked();

            // ReSharper disable LocalizableElement
            if (kit.IsEmpty)
                throw new ArgumentException("Kit is empty.", nameof(kit));
            if (kit.Node.FirstChildContentId > 0)
                throw new ArgumentException("Kit content cannot have children.", nameof(kit));
            // ReSharper restore LocalizableElement

            _logger.Debug<ContentStore>("Set content ID: {KitNodeId}", kit.Node.Id);

            // get existing
            _contentNodes.TryGetValue(kit.Node.Id, out var link);
            var existing = link?.Value;

            if (!BuildKit(kit, out var parent))
                return false;

            // moving?
            var moving = existing != null && existing.ParentContentId != kit.Node.ParentContentId;

            // manage children
            if (existing != null)
            {
                kit.Node.FirstChildContentId = existing.FirstChildContentId;
                kit.Node.LastChildContentId = existing.LastChildContentId;
            }

            // set
            SetValueLocked(_contentNodes, kit.Node.Id, kit.Node);
            if (_localDb != null) RegisterChange(kit.Node.Id, kit);

            // manage the tree
            if (existing == null)
            {
                // new, add to parent
                AddTreeNodeLocked(kit.Node, parent);
            }
            else if (moving || existing.SortOrder != kit.Node.SortOrder)
            {
                // moved, remove existing from its parent, add content to its parent
                RemoveTreeNodeLocked(existing);
                AddTreeNodeLocked(kit.Node);
            }
            else
            {
                // replacing existing, handle siblings
                kit.Node.NextSiblingContentId = existing.NextSiblingContentId;
                kit.Node.PreviousSiblingContentId = existing.PreviousSiblingContentId;
            }

            _xmap[kit.Node.Uid] = kit.Node.Id;

            return true;
        }

        private void ClearRootLocked()
        {
            if (_root.Gen != _liveGen)
                _root = new LinkedNode<ContentNode>(new ContentNode(), _liveGen, _root);
            else
                _root.Value.FirstChildContentId = -1;
        }

        /// <summary>
        /// Builds all kits on startup using a fast forward only cursor
        /// </summary>
        /// <param name="kits">
        /// All kits sorted by Level + Parent Id + Sort order
        /// </param>
        /// <param name="fromDb">True if the data is coming from the database (not the local cache db)</param>
        /// <returns></returns>
        /// <remarks>
        /// <para>
        /// This requires that the collection is sorted by Level + ParentId + Sort Order. 
        /// This should be used only on a site startup as the first generations.
        /// This CANNOT be used after startup since it bypasses all checks for Generations.
        /// </para>
        /// <para>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </para>        
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public bool SetAllFastSortedLocked(IEnumerable<ContentNodeKit> kits, bool fromDb)
        {
            EnsureLocked();

            var ok = true;

            ClearLocked(_contentNodes);
            ClearRootLocked();

            // The name of the game here is to populate each kit's
            //  FirstChildContentId
            //  LastChildContentId
            //  NextSiblingContentId
            //  PreviousSiblingContentId

            ContentNode previousNode = null;
            ContentNode parent = null;

            foreach (var kit in kits)
            {
                if (!BuildKit(kit, out var parentLink))
                {
                    ok = false;
                    continue; // skip that one
                }

                var thisNode = kit.Node;

                if (parent == null)
                {
                    // first parent
                    parent = parentLink.Value;
                    parent.FirstChildContentId = thisNode.Id; // this node is the first node
                }
                else if (parent.Id != parentLink.Value.Id)
                {
                    // new parent
                    parent = parentLink.Value;
                    parent.FirstChildContentId = thisNode.Id; // this node is the first node
                    previousNode = null; // there is no previous sibling
                }

                _logger.Debug<ContentStore>($"Set {thisNode.Id} with parent {thisNode.ParentContentId}");
                SetValueLocked(_contentNodes, thisNode.Id, thisNode);

                // if we are initializing from the database source ensure the local db is updated
                if (fromDb && _localDb != null) RegisterChange(thisNode.Id, kit);

                // this node is always the last child
                parent.LastChildContentId = thisNode.Id;

                // wire previous node as previous sibling
                if (previousNode != null)
                {
                    previousNode.NextSiblingContentId = thisNode.Id;
                    thisNode.PreviousSiblingContentId = previousNode.Id;
                }

                // this node becomes the previous node
                previousNode = thisNode;

                _xmap[kit.Node.Uid] = kit.Node.Id;
            }

            return ok;
        }

        /// <summary>
        /// Set all data for a collection of <see cref="ContentNodeKit"/>
        /// </summary>
        /// <param name="kits"></param>
        /// <returns></returns>
        /// <remarks>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public bool SetAllLocked(IEnumerable<ContentNodeKit> kits)
        {
            EnsureLocked();

            var ok = true;
            
            ClearLocked(_contentNodes);
            ClearRootLocked();

            // do NOT clear types else they are gone!
            //ClearLocked(_contentTypesById);
            //ClearLocked(_contentTypesByAlias);

            foreach (var kit in kits)
            {
                if (!BuildKit(kit, out var parent))
                {
                    ok = false;
                    continue; // skip that one
                }
                _logger.Debug<ContentStore>($"Set {kit.Node.Id} with parent {kit.Node.ParentContentId}");
                SetValueLocked(_contentNodes, kit.Node.Id, kit.Node);

                if (_localDb != null) RegisterChange(kit.Node.Id, kit);
                AddTreeNodeLocked(kit.Node, parent);

                _xmap[kit.Node.Uid] = kit.Node.Id;
            }

            return ok;
        }

        /// <summary>
        /// Sets data for a branch of <see cref="ContentNodeKit"/>
        /// </summary>
        /// <param name="rootContentId"></param>
        /// <param name="kits"></param>
        /// <returns></returns>
        /// <remarks>
        /// <para>
        /// IMPORTANT kits must be sorted out by LEVEL and by SORT ORDER
        /// </para>
        /// <para>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </para>        
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public bool SetBranchLocked(int rootContentId, IEnumerable<ContentNodeKit> kits)
        {
            EnsureLocked();

            var ok = true;
            
            // get existing
            _contentNodes.TryGetValue(rootContentId, out var link);
            var existing = link?.Value;

            // clear
            if (existing != null)
            {
                //this zero's out the branch (recursively), if we're in a new gen this will add a NULL placeholder for the gen
                ClearBranchLocked(existing);
                //TODO: This removes the current GEN from the tree - do we really want to do that?
                RemoveTreeNodeLocked(existing);
            }

            // now add them all back
            foreach (var kit in kits)
            {
                if (!BuildKit(kit, out var parent))
                {
                    ok = false;
                    continue; // skip that one
                }
                SetValueLocked(_contentNodes, kit.Node.Id, kit.Node);
                if (_localDb != null) RegisterChange(kit.Node.Id, kit);
                AddTreeNodeLocked(kit.Node, parent);

                _xmap[kit.Node.Uid] = kit.Node.Id;
            }

            return ok;
        }

        /// <summary>
        /// Clears data for a given node id
        /// </summary>
        /// <param name="id"></param>
        /// <returns></returns>
        /// <remarks>
        /// This methods MUST be called from within a write lock, normally wrapped within GetScopedWriteLock
        /// otherwise an exception will occur.
        /// </remarks>
        /// <exception cref="InvalidOperationException">
        /// Thrown if this method is not called within a write lock
        /// </exception>
        public bool ClearLocked(int id)
        {
            EnsureLocked();

            // try to find the content
            // if it is not there, nothing to do
            _contentNodes.TryGetValue(id, out var link); // else null
            if (link?.Value == null) return false;

            var content = link.Value;
            _logger.Debug<ContentStore>("Clear content ID: {ContentId}", content.Id);

            // clear the entire branch
            ClearBranchLocked(content);

            // manage the tree
            RemoveTreeNodeLocked(content);

            return true;
        }

        private void ClearBranchLocked(int id)
        {
            _contentNodes.TryGetValue(id, out var link);
            if (link?.Value == null)
                return;
            ClearBranchLocked(link.Value);
        }

        private void ClearBranchLocked(ContentNode content)
        {
            SetValueLocked(_contentNodes, content.Id, null);
            if (_localDb != null) RegisterChange(content.Id, ContentNodeKit.Null);

            _xmap.TryRemove(content.Uid, out _);

            var id = content.FirstChildContentId;
            while (id > 0)
            {
                var link = GetRequiredLinkedNode(id, "child", null);
                ClearBranchLocked(link.Value);
                id = link.Value.NextSiblingContentId;
            }
        }

        /// <summary>
        /// Gets the link node and if it doesn't exist throw a <see cref="PanicException"/>
        /// </summary>
        /// <param name="id"></param>
        /// <param name="description"></param>
        /// <param name="gen">the generation requested, null for the latest stored</param>
        /// <returns></returns>
        private LinkedNode<ContentNode> GetRequiredLinkedNode(int id, string description, long? gen)
        {
            if (_contentNodes.TryGetValue(id, out var link))
            {
                link = GetLinkedNodeGen(link, gen);                
                if (link != null && link.Value != null)
                    return link;
            }   

            throw new PanicException($"failed to get {description} with id={id}");
        }

        /// <summary>
        /// Gets the parent link node, may be null or root if ParentContentId is less than 0
        /// </summary>
        /// <param name="gen">the generation requested, null for the latest stored</param>
        private LinkedNode<ContentNode> GetParentLink(ContentNode content, long? gen)
        {
            if (content.ParentContentId < 0)
            {
                var root = GetLinkedNodeGen(_root, gen);                
                return root;
            }

            if (_contentNodes.TryGetValue(content.ParentContentId, out var link))
                link = GetLinkedNodeGen(link, gen);
                
            return link;
        }

        /// <summary>
        /// Gets the linked parent node and if it doesn't exist throw a <see cref="PanicException"/>
        /// </summary>
        /// <param name="content"></param>
        /// <param name="gen">the generation requested, null for the latest stored</param>
        /// <returns></returns>
        private LinkedNode<ContentNode> GetRequiredParentLink(ContentNode content, long? gen)
        {
            return content.ParentContentId < 0 ? _root : GetRequiredLinkedNode(content.ParentContentId, "parent", gen);
        }

        /// <summary>
        /// Iterates over the LinkedNode's generations to find the correct one
        /// </summary>
        /// <param name="link"></param>
        /// <param name="gen">The generation requested, use null to avoid the lookup</param>
        /// <returns></returns>
        private LinkedNode<TValue> GetLinkedNodeGen<TValue>(LinkedNode<TValue> link, long? gen)
            where TValue : class
        {
            if (!gen.HasValue) return link;

            //find the correct snapshot, find the first that is <= the requested gen
            while (link != null && link.Gen > gen)
            {
                link = link.Next;
            }
            return link;
        }

        /// <summary>
        /// This removes this current node from the tree hiearchy by removing it from it's parent's linked list
        /// </summary>
        /// <param name="content"></param>
        /// <remarks>
        /// This is called within a lock which means a new Gen is being created therefore this will not modify any existing content in a Gen.
        /// </remarks>
        private void RemoveTreeNodeLocked(ContentNode content)
        {
            // NOTE: DO NOT modify `content` here, this would modify data for an existing Gen, all modifications are done to clones
            // which would be targeting the new Gen.

            var parentLink = content.ParentContentId < 0
                ? _root
                : GetRequiredLinkedNode(content.ParentContentId, "parent", null);

            var parent = parentLink.Value;

            // must have children
            if (parent.FirstChildContentId < 0)
                throw new PanicException("no children");

            // if first/last, clone parent, then remove

            if (parent.FirstChildContentId == content.Id || parent.LastChildContentId == content.Id)
                parent = GenCloneLocked(parentLink);

            if (parent.FirstChildContentId == content.Id)
                parent.FirstChildContentId = content.NextSiblingContentId;

            if (parent.LastChildContentId == content.Id)
                parent.LastChildContentId = content.PreviousSiblingContentId;

            // maintain linked list

            if (content.NextSiblingContentId > 0)
            {
                var nextLink = GetRequiredLinkedNode(content.NextSiblingContentId, "next sibling", null);
                var next = GenCloneLocked(nextLink);
                next.PreviousSiblingContentId = content.PreviousSiblingContentId;
            }

            if (content.PreviousSiblingContentId > 0)
            {
                var prevLink = GetRequiredLinkedNode(content.PreviousSiblingContentId, "previous sibling", null);
                var prev = GenCloneLocked(prevLink);
                prev.NextSiblingContentId = content.NextSiblingContentId;
            }
        }

        private bool ParentPublishedLocked(ContentNodeKit kit)
        {
            if (kit.Node.ParentContentId < 0)
                return true;
            var link = GetParentLink(kit.Node, null);
            var node = link?.Value;
            return node != null && node.HasPublished;
        }

        private ContentNode GenCloneLocked(LinkedNode<ContentNode> link)
        {
            var node = link.Value;

            if (node != null && link.Gen != _liveGen)
            {
                node = new ContentNode(link.Value);
                if (link == _root)
                    SetRootLocked(node);
                else
                    SetValueLocked(_contentNodes, node.Id, node);
            }

            return node;
        }

        /// <summary>
        /// Adds a node to the tree structure.
        /// </summary>
        private void AddTreeNodeLocked(ContentNode content, LinkedNode<ContentNode> parentLink = null)
        {
            parentLink = parentLink ?? GetRequiredParentLink(content, null);

            // TODO: This can result in a null value? see https://github.com/umbraco/Umbraco-CMS/issues/7868
            // It seems to be related to having corrupt Paths in the umbracoNode table.
            var parent = parentLink.Value;

            if (parent == null)
                throw new PanicException($"A null Value was returned on the {nameof(parentLink)} LinkedNode with id={content.ParentContentId}, potentially your database paths are corrupted, please see the HealthCheck dashboard and fixup data inconsistencies.");

            // if parent has no children, clone parent + add as first child
            if (parent.FirstChildContentId < 0)
            {
                parent = GenCloneLocked(parentLink);
                parent.FirstChildContentId = content.Id;
                parent.LastChildContentId = content.Id;
                return;
            }

            // get parent's first child
            var childLink = GetRequiredLinkedNode(parent.FirstChildContentId, "first child", null);
            var child = childLink.Value;

            // if first, clone parent + insert as first child
            // NOTE: Don't perform this check if loading from local DB since we know it's already sorted
            if (child.SortOrder > content.SortOrder)
            {
                content.NextSiblingContentId = parent.FirstChildContentId;
                content.PreviousSiblingContentId = -1;

                parent = GenCloneLocked(parentLink);
                parent.FirstChildContentId = content.Id;

                child = GenCloneLocked(childLink);
                child.PreviousSiblingContentId = content.Id;

                return;
            }

            // get parent's last child
            var lastChildLink = GetRequiredLinkedNode(parent.LastChildContentId, "last child", null);
            var lastChild = lastChildLink.Value;

            // if last, clone parent + append as last child
            if (lastChild.SortOrder <= content.SortOrder)
            {
                content.PreviousSiblingContentId = parent.LastChildContentId;
                content.NextSiblingContentId = -1;

                parent = GenCloneLocked(parentLink);
                parent.LastChildContentId = content.Id;

                lastChild = GenCloneLocked(lastChildLink);
                lastChild.NextSiblingContentId = content.Id;

                return;
            }

            // else it's going somewhere in the middle,
            // TODO: There was a note about performance when this occurs and that this only happens when moving and not very often, but that is not true,
            // this also happens anytime a middle node is unpublished or republished (which causes a branch update), i'm unsure if this has perf impacts,
            // i think this used to but it doesn't seem bad anymore that I can see...
            while (child.NextSiblingContentId > 0)
            {
                // get next child
                var nextChildLink = GetRequiredLinkedNode(child.NextSiblingContentId, "next child", null);
                var nextChild = nextChildLink.Value;

                // if here, clone previous + append/insert
                // NOTE: Don't perform this check if loading from local DB since we know it's already sorted
                if (nextChild.SortOrder > content.SortOrder)
                {
                    content.NextSiblingContentId = nextChild.Id;
                    content.PreviousSiblingContentId = nextChild.PreviousSiblingContentId;

                    child = GenCloneLocked(childLink);
                    child.NextSiblingContentId = content.Id;

                    var nnext = GenCloneLocked(nextChildLink);
                    nnext.PreviousSiblingContentId = content.Id;

                    return;
                }

                childLink = nextChildLink;
                child = nextChild;
            }

            // should never get here
            throw new PanicException("No more children.");
        }

        // replaces the root node
        private void SetRootLocked(ContentNode node)
        {
            if (_root.Gen != _liveGen)
            {
                _root = new LinkedNode<ContentNode>(node, _liveGen, _root);
            }
            else
            {
                _root.Value = node;
            }
        }

        // set a node (just the node, not the tree)
        private void SetValueLocked<TKey, TValue>(ConcurrentDictionary<TKey, LinkedNode<TValue>> dict, TKey key, TValue value)
            where TValue : class
        {
            // this is safe only because we're write-locked
            var link = GetHead(dict, key);
            if (link != null)
            {
                // already in the dict
                if (link.Gen != _liveGen)
                {
                    // for an older gen - if value is different then insert a new
                    // link for the new gen, with the new value
                    if (link.Value != value)
                        dict.TryUpdate(key, new LinkedNode<TValue>(value, _liveGen, link), link);
                }
                else
                {
                    // for the live gen - we can fix the live gen - and remove it
                    // if value is null and there's no next gen
                    if (value == null && link.Next == null)
                        dict.TryRemove(key, out link);
                    else
                        link.Value = value;
                }
            }
            else
            {
                dict.TryAdd(key, new LinkedNode<TValue>(value, _liveGen));
            }
        }

        private void ClearLocked<TKey, TValue>(ConcurrentDictionary<TKey, LinkedNode<TValue>> dict)
            where TValue : class
        {
            // this is safe only because we're write-locked
            foreach (var kvp in dict.Where(x => x.Value != null))
            {
                if (kvp.Value.Gen != _liveGen)
                {
                    var link = new LinkedNode<TValue>(null, _liveGen, kvp.Value);
                    dict.TryUpdate(kvp.Key, link, kvp.Value);
                }
                else
                {
                    kvp.Value.Value = null;
                }
            }
        }

        public ContentNode Get(int id, long gen)
        {
            return GetValue(_contentNodes, id, gen);
        }

        public ContentNode Get(Guid uid, long gen)
        {
            return _xmap.TryGetValue(uid, out var id)
                ? GetValue(_contentNodes, id, gen)
                : null;
        }

        public IEnumerable<ContentNode> GetAtRoot(long gen)
        {
            var root = GetLinkedNodeGen(_root, gen);            
            if (root == null)
                yield break;

            var id = root.Value.FirstChildContentId;

            while (id > 0)
            {
                var link = GetRequiredLinkedNode(id, "root", gen);
                yield return link.Value;
                id = link.Value.NextSiblingContentId;
            }
        }

        private TValue GetValue<TKey, TValue>(ConcurrentDictionary<TKey, LinkedNode<TValue>> dict, TKey key, long gen)
            where TValue : class
        {
            // look ma, no lock!
            var link = GetHead(dict, key);
            link = GetLinkedNodeGen(link, gen);
            return link?.Value; // may be null
        }

        public IEnumerable<ContentNode> GetAll(long gen)
        {
            // enumerating on .Values locks the concurrent dictionary,
            // so better get a shallow clone in an array and release
            var links = _contentNodes.Values.ToArray();
            foreach (var l in links)
            {
                var link = GetLinkedNodeGen(l, gen);
                if (link?.Value != null)
                    yield return link.Value;
            }
        }

        public bool IsEmpty(long gen)
        {
            var has = _contentNodes.Any(x =>
            {
                var link = GetLinkedNodeGen(x.Value, gen);
                return link?.Value != null;
            });
            return has == false;
        }

        public IPublishedContentType GetContentType(int id, long gen)
        {
            return GetValue(_contentTypesById, id, gen);
        }

        public IPublishedContentType GetContentType(string alias, long gen)
        {
            return GetValue(_contentTypesByAlias, alias, gen);
        }

        #endregion

        #region Snapshots

        public Snapshot CreateSnapshot()
        {
            lock(_rlocko)
            {
                // if no next generation is required, and we already have one,
                // use it and create a new snapshot
                if (_nextGen == false && _genObj != null)
                    return new Snapshot(this, _genObj.GetGenRef()
#if DEBUG
                        , _logger
#endif
                        );

                // else we need to try to create a new gen ref
                // whether we are wlocked or not, noone can rlock while we do,
                // so _liveGen and _nextGen are safe
                if (Monitor.IsEntered(_wlocko))
                {
                    // write-locked, cannot use latest gen (at least 1) so use previous
                    var snapGen = _nextGen ? _liveGen - 1 : _liveGen;

                    // create a new gen ref unless we already have it
                    if (_genObj == null)
                        _genObjs.Enqueue(_genObj = new GenObj(snapGen));
                    else if (_genObj.Gen != snapGen)
                        throw new PanicException($"The generation {_genObj.Gen} does not equal the snapshot generation {snapGen}");
                }
                else
                {
                    // not write-locked, can use latest gen, create a new gen ref
                    _genObjs.Enqueue(_genObj = new GenObj(_liveGen));
                    _nextGen = false; // this is the ONLY thing that triggers a _liveGen++
                }

                // so...
                // the genRefRef has a weak ref to the genRef, and is queued
                // the snapshot has a ref to the genRef, which has a ref to the genRefRef
                // when the snapshot is disposed, it decreases genRefRef counter
                // so after a while, one of these conditions is going to be true:
                // - the genRefRef counter is zero because all snapshots have properly been disposed
                // - the genRefRef weak ref is dead because all snapshots have been collected
                // in both cases, we will dequeue and collect

                var snapshot = new Snapshot(this, _genObj.GetGenRef()
#if DEBUG
                    , _logger
#endif
                    );

                // reading _floorGen is safe if _collectTask is null
                if (_collectTask == null && _collectAuto && _liveGen - _floorGen > CollectMinGenDelta)
                    CollectAsyncLocked();

                return snapshot;
            }
        }

        public Snapshot LiveSnapshot => new Snapshot(this, _liveGen
#if DEBUG
            , _logger
#endif
        );

        public Task CollectAsync()
        {
            lock (_rlocko)
            {
                return CollectAsyncLocked();
            }
        }

        private Task CollectAsyncLocked()
        {
            if (_collectTask != null)
                return _collectTask;

            // ReSharper disable InconsistentlySynchronizedField
            var task = _collectTask = Task.Run(Collect);
            _collectTask.ContinueWith(_ =>
            {
                lock (_rlocko)
                {
                    _collectTask = null;
                }
            }, TaskContinuationOptions.ExecuteSynchronously);
            // ReSharper restore InconsistentlySynchronizedField

            return task;
        }

        private void Collect()
        {
            // see notes in CreateSnapshot
#if DEBUG
            _logger.Debug<ContentStore>("Collect.");
#endif
            while (_genObjs.TryPeek(out var genObj) && (genObj.Count == 0 || genObj.WeakGenRef.IsAlive == false))
            {
                _genObjs.TryDequeue(out genObj); // cannot fail since TryPeek has succeeded
                _floorGen = genObj.Gen;
#if DEBUG
                //_logger.Debug<ContentStore>("_floorGen=" + _floorGen + ", _liveGen=" + _liveGen);
#endif
            }

            Collect(_contentNodes);
            CollectRoot();
            Collect(_contentTypesById);
            Collect(_contentTypesByAlias);
        }

        private void CollectRoot()
        {
            var link = _root;
            while (link.Next != null && link.Next.Gen > _floorGen)
                link = link.Next;
            link.Next = null;
        }

        private void Collect<TKey, TValue>(ConcurrentDictionary<TKey, LinkedNode<TValue>> dict)
            where TValue : class
        {
            // it is OK to enumerate a concurrent dictionary and it does not lock
            // it - and here it's not an issue if we skip some items, they will be
            // processed next time we collect

            long liveGen;
            lock (_rlocko) // r is good
            {
                liveGen = _liveGen;
                if (_nextGen == false)
                    liveGen += 1;
            }

            foreach (var kvp in dict)
            {
                var link = kvp.Value;

#if DEBUG
                //_logger.Debug<ContentStore>("Collect id:" + kvp.Key + ", gen:" + link.Gen +
                //    ", nxt:" + (link.Next == null ? "null" : "link") +
                //    ", val:" + (link.Value == null ? "null" : "value"));
#endif

                // reasons to collect the head:
                //   gen must be < liveGen (we never collect live gen)
                //   next == null && value == null (we have no data at all)
                //   next != null && value == null BUT gen > floor (noone wants us)
                // not live means .Next and .Value are safe
                if (link.Gen < liveGen && link.Value == null
                    && (link.Next == null || link.Gen <= _floorGen))
                {
                    // not live, null value, no next link = remove that one -- but only if
                    // the dict has not been updated, have to do it via ICollection<> (thanks
                    // Mr Toub) -- and if the dict has been updated there is nothing to collect
                    var idict = dict as ICollection<KeyValuePair<TKey, LinkedNode<TValue>>>;
                    idict.Remove(kvp);
                    continue;
                }

                // in any other case we're not collecting the head, we need to go to Next
                // and if there is no Next, skip
                if (link.Next == null)
                    continue;

                // else go to Next and loop while above floor, and kill everything below
                while (link.Next != null && link.Next.Gen > _floorGen)
                    link = link.Next;
                link.Next = null;
            }
        }

        // TODO: This is never used? Should it be? Maybe move to TestHelper below?
        //public async Task WaitForPendingCollect()
        //{
        //    Task task;
        //    lock (_rlocko)
        //    {
        //        task = _collectTask;
        //    }
        //    if (task != null)
        //        await task;
        //}

        public long GenCount => _genObjs.Count;

        public long SnapCount => _genObjs.Sum(x => x.Count);

        #endregion

        #region Internals/Unit testing

        private TestHelper _unitTesting;

        // note: nothing here is thread-safe
        internal class TestHelper
        {
            private readonly ContentStore _store;

            public TestHelper(ContentStore store)
            {
                _store = store;
            }

            public long LiveGen => _store._liveGen;
            public long FloorGen => _store._floorGen;
            public bool NextGen => _store._nextGen;
            public bool CollectAuto
            {
                get => _store._collectAuto;
                set => _store._collectAuto = value;
            }

            /// <summary>
            /// Return a list of Gen/ContentNode values
            /// </summary>
            /// <param name="id"></param>
            /// <returns></returns>
            public (long gen, ContentNode contentNode)[] GetValues(int id)
            {
                _store._contentNodes.TryGetValue(id, out LinkedNode<ContentNode> link); // else null

                if (link == null)
                    return Array.Empty<(long, ContentNode)>();

                var tuples = new List<(long, ContentNode)>();
                do
                {
                    tuples.Add((link.Gen, link.Value));
                    link = link.Next;
                } while (link != null);
                return tuples.ToArray();
            }
        }

        internal TestHelper Test => _unitTesting ?? (_unitTesting = new TestHelper(this));

        #endregion

        #region Classes

        public class Snapshot : IDisposable
        {
            private readonly ContentStore _store;
            private readonly GenRef _genRef;
            private long _gen;
#if DEBUG
            private readonly ILogger _logger;
#endif

            //private static int _count;
            //private readonly int _thisCount;

            internal Snapshot(ContentStore store, GenRef genRef
#if DEBUG
                    , ILogger logger
#endif
                )
            {
                _store = store;
                _genRef = genRef;
                _gen = genRef.Gen;
                Interlocked.Increment(ref genRef.GenObj.Count);
                //_thisCount = _count++;

#if DEBUG
                _logger = logger;
                _logger.Debug<Snapshot>("Creating snapshot.");
#endif
            }

            internal Snapshot(ContentStore store, long gen
#if DEBUG
                , ILogger logger
#endif
                )
            {
                _store = store;
                _gen = gen;

#if DEBUG
                _logger = logger;
                _logger.Debug<Snapshot>("Creating live.");
#endif
            }

            public ContentNode Get(int id)
            {
                if (_gen < 0)
                    throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
                return _store.Get(id, _gen);
            }

            public ContentNode Get(Guid id)
            {
                if (_gen < 0)
                    throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
                return _store.Get(id, _gen);
            }

            public IEnumerable<ContentNode> GetAtRoot()
            {
                if (_gen < 0)
                    throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
                return _store.GetAtRoot(_gen);
            }

            public IEnumerable<ContentNode> GetAll()
            {
                if (_gen < 0)
                    throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
                return _store.GetAll(_gen);
            }

            public IPublishedContentType GetContentType(int id)
            {
                if (_gen < 0)
                    throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
                return _store.GetContentType(id, _gen);
            }

            public IPublishedContentType GetContentType(string alias)
            {
                if (_gen < 0)
                    throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
                return _store.GetContentType(alias, _gen);
            }

            // this code is here just so you don't try to implement it
            // the only way we can iterate over "all" without locking the entire cache forever
            // is by shallow cloning the cache, which is quite expensive, so we should probably not do it,
            // and implement cache-level indexes
            //public IEnumerable<ContentNode> GetAll()
            //{
            //    if (_gen < 0)
            //        throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
            //    return _store.GetAll(_gen);
            //}

            public bool IsEmpty
            {
                get
                {
                    if (_gen < 0)
                        throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
                    return _store.IsEmpty(_gen);
                }
            }

            public long Gen
            {
                get
                {
                    if (_gen < 0)
                        throw new ObjectDisposedException("snapshot" /*+ " (" + _thisCount + ")"*/);
                    return _gen;
                }
            }

            public void Dispose()
            {
                if (_gen < 0) return;
#if DEBUG
                _logger.Debug<Snapshot>("Dispose snapshot ({Snapshot})", _genRef?.GenObj.Count.ToString() ?? "live");
#endif
                _gen = -1;
                if (_genRef != null)
                    Interlocked.Decrement(ref _genRef.GenObj.Count);
                GC.SuppressFinalize(this);
            }
        }

        #endregion
    }
}