Use a GUID when creating the temp path (#4645)

* Use a GUID when creating the temp path
* Update where the GUID is in the temp path
* Remove braces
* Use random file name instead of GUID
* Switch to concurrent dictionary
* TryAdd updates

Note: method implemented is `Path.GetRandomFileName()` instead of a GUID as implied in the PR title.

Author: jwood803

src/Microsoft.ML.Core/Data/Repository.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
@@ -93,7 +94,7 @@ public void Dispose()
         // more than once.
         // REVIEW: Should we garbage collect to some degree? Currently we don't delete any
         // of these temp files until the repository is disposed.
-        protected readonly Dictionary<string, string> PathMap;
+        protected readonly ConcurrentDictionary<string, string> PathMap;
 
         /// <summary>
         /// Exception context.
@@ -107,7 +108,7 @@ internal Repository(bool needDir, IExceptionContext ectx)
             Contracts.AssertValueOrNull(ectx);
             _ectx = ectx;
 
-            PathMap = new Dictionary<string, string>();
+            PathMap = new ConcurrentDictionary<string, string>();
             _open = new List<Entry>();
             if (needDir)
                 DirTemp = GetShortTempDir();
@@ -256,7 +257,7 @@ protected void GetPath(out string pathEnt, out string pathTemp, string dir, stri
             string root = Path.GetFullPath(DirTemp ?? @"x:\dummy");
             string entityPath = Path.Combine(root, dir ?? "", name);
             entityPath = Path.GetFullPath(entityPath);
-            string tempPath = Path.Combine(root, PathMap.Count.ToString());
+            string tempPath = Path.Combine(root, Path.GetRandomFileName());
             tempPath = Path.GetFullPath(tempPath);
 
             string parent = Path.GetDirectoryName(entityPath);
@@ -333,10 +334,8 @@ public Entry CreateEntry(string dir, string name)
             string pathEnt;
             string pathTemp;
             GetPath(out pathEnt, out pathTemp, dir, name, true);
-            if (PathMap.ContainsKey(pathEnt))
+            if (!PathMap.TryAdd(pathEnt, pathTemp))
                 throw ExceptionContext.ExceptParam(nameof(name), "Duplicate entry: '{0}'", pathEnt);
-            else
-                PathMap.Add(pathEnt, pathTemp);
 
             Stream stream;
             if (pathTemp != null)
@@ -525,7 +524,9 @@ public Entry OpenEntryOrNull(string dir, string name)
                     // Extract to a temporary file.
                     Directory.CreateDirectory(Path.GetDirectoryName(pathTemp));
                     entry.ExtractToFile(pathTemp);
-                    PathMap.Add(pathLower, pathTemp);
+                    if (!PathMap.TryAdd(pathLower, pathTemp))
+                        throw ExceptionContext.ExceptParam(nameof(name), "Duplicate entry: '{0}'", pathLower);
+
                     stream = new FileStream(pathTemp, FileMode.Open, FileAccess.Read);
                 }
                 else