fix #1342 - avoid race condition in exception processing

docs/ReleaseNotes.md

@@ -17,6 +17,7 @@
 - add: support for `HSTRLEN` (#1241 via eitanhs)
 - add: `GeoRadiusResult` is now mockable (#1175 via firenero)
 - fix: various documentation fixes (#1162, #1135, #1203, #1240, #1245, #1159, #1311, #1339)
+- fix: rare race-condition around exception data (#1342)
 
 ## 2.0.600
 

src/StackExchange.Redis/ServerEndPoint.cs

@@ -81,15 +81,16 @@ internal Exception LastException
         {
             get
             {
-                var tmp1 = interactive;
-                var tmp2 = subscription;
+                var snapshot = interactive;
+                var subEx = subscription?.LastException;
+                var subExData = subEx?.Data;
 
                 //check if subscription endpoint has a better lastexception
-                if (tmp2?.LastException != null && tmp2.LastException.Data.Contains("Redis-FailureType") && !tmp2.LastException.Data["Redis-FailureType"].ToString().Equals(nameof(ConnectionFailureType.UnableToConnect)))
+                if (subExData != null && subExData.Contains("Redis-FailureType") && subExData["Redis-FailureType"]?.ToString() != nameof(ConnectionFailureType.UnableToConnect))
                 {
-                    return tmp2.LastException;
+                    return subEx;
                 }
-                return tmp1?.LastException;
+                return snapshot?.LastException;
             }
         }