Fixes: U4-6588 UmbracoSettings Error404 support Guids and Xpath - To achieve this, had to create an Id -> Key and Key -> Id method on EntityService including a cache for it which means updating all relavent cache refreshers to clear this cache when things are removed. Moved the logic for parsing an Umbraco XPath query (with tokens) to a stand-alone testable class (though haven't written tests), move the logic for looking up a not found page to the NotFoundHandlerHelper (instead of legacy 'library'). Fixes the $root query, since I don't think that ever worked. I've tested $root now for both MNTP and for the not found handler and it works.

src/Umbraco.Core/Xml/UmbracoXPathPathSyntaxParser.cs

@@ -0,0 +1,117 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Umbraco.Core.Xml
+{
+    /// <summary>
+    /// This is used to parse our customize Umbraco XPath expressions (i.e. that include special tokens like $site) into 
+    /// a real XPath statement
+    /// </summary>
+    internal class UmbracoXPathPathSyntaxParser
+    {
+        /// <summary>
+        /// Parses custom umbraco xpath expression
+        /// </summary>
+        /// <param name="xpathExpression">The Xpath expression</param>
+        /// <param name="nodeContextId">
+        /// The current node id context of executing the query - null if there is no current node, in which case
+        /// some of the parameters like $current, $parent, $site will be disabled
+        /// </param>
+        /// <param name="getPath">The callback to create the nodeId path, given a node Id</param>
+        /// <param name="publishedContentExists">The callback to return whether a published node exists based on Id</param>
+        /// <returns></returns>
+        public static string ParseXPathQuery(
+            string xpathExpression, 
+            int? nodeContextId, 
+            Func<int, IEnumerable<string>> getPath,
+            Func<int, bool> publishedContentExists)
+        {
+
+            //TODO: This should probably support some of the old syntax and token replacements, currently 
+            // it does not, there is a ticket raised here about it: http://issues.umbraco.org/issue/U4-6364
+            // previous tokens were: "$currentPage", "$ancestorOrSelf", "$parentPage" and I beleive they were 
+            // allowed 'inline', not just at the beginning... whether or not we want to support that is up 
+            // for discussion.
+
+            Mandate.ParameterNotNullOrEmpty(xpathExpression, "xpathExpression");
+            Mandate.ParameterNotNull(getPath, "getPath");
+            Mandate.ParameterNotNull(publishedContentExists, "publishedContentExists");
+
+            //no need to parse it
+            if (xpathExpression.StartsWith("$") == false)
+                return xpathExpression;
+
+            //get nearest published item
+            Func<IEnumerable<string>, int> getClosestPublishedAncestor = (path =>
+            {
+                foreach (var i in path)
+                {
+                    int idAsInt;
+                    if (int.TryParse(i, out idAsInt))
+                    {
+                        var exists = publishedContentExists(int.Parse(i));
+                        if (exists)
+                            return idAsInt;
+                    }
+                }
+                return -1;
+            });
+
+            const string rootXpath = "descendant::*[@id={0}]";
+
+            //parseable items:
+            var vars = new Dictionary<string, Func<string, string>>();
+
+            //These parameters must have a node id context
+            if (nodeContextId.HasValue)
+            {
+                vars.Add("$current", q =>
+                {
+                    var closestPublishedAncestorId = getClosestPublishedAncestor(getPath(nodeContextId.Value));
+                    return q.Replace("$current", string.Format(rootXpath, closestPublishedAncestorId));
+                });
+
+                vars.Add("$parent", q =>
+                {
+                    //remove the first item in the array if its the current node
+                    //this happens when current is published, but we are looking for its parent specifically
+                    var path = getPath(nodeContextId.Value).ToArray();
+                    if (path[0] == nodeContextId.ToString())
+                    {
+                        path = path.Skip(1).ToArray();
+                    }
+
+                    var closestPublishedAncestorId = getClosestPublishedAncestor(path);
+                    return q.Replace("$parent", string.Format(rootXpath, closestPublishedAncestorId));
+                });
+
+
+                vars.Add("$site", q =>
+                {
+                    var closestPublishedAncestorId = getClosestPublishedAncestor(getPath(nodeContextId.Value));
+                    return q.Replace("$site", string.Format(rootXpath, closestPublishedAncestorId) + "/ancestor-or-self::*[@level = 1]");
+                });
+            }
+
+            //TODO: This used to just replace $root with string.Empty BUT, that would never work
+            // the root is always "/root . Need to confirm with Per why this was string.Empty before!
+            vars.Add("$root", q => q.Replace("$root", "/root"));
+
+
+            foreach (var varible in vars)
+            {
+                if (xpathExpression.StartsWith(varible.Key))
+                {
+                    xpathExpression = varible.Value(xpathExpression);
+                    break;
+                }
+            }
+
+            return xpathExpression;
+        }
+
+    }
+}