Make the EfMap mapping cache thread-safe

src/EntityFramework.MappingAPI/EfMap.cs

@@ -1,6 +1,6 @@
 using EntityFramework.MappingAPI.Mappings;
 using System;
-using System.Collections.Generic;
+using System.Collections.Concurrent;
 using System.Data.Entity;
 using System.Data.Entity.Infrastructure;
 
@@ -12,9 +12,14 @@ namespace EntityFramework.MappingAPI
     internal class EfMap
     {
         /// <summary>
-        /// 
+        /// Mappings are cached per model for the lifetime of the process. A concurrent dictionary is
+        /// required, not just convenient: BulkInsert is reachable from several threads at once (any
+        /// app that bulk inserts from concurrent jobs or requests), and a plain Dictionary being
+        /// written on one thread while another reads it can throw or return the wrong entry. The
+        /// caller then sees an unrelated failure - via MappedDataReader, which catches per entity
+        /// type, that surfaces as the misleading "No table mappings provided."
         /// </summary>
-        private static readonly Dictionary<string, DbMapping> Mappings = new Dictionary<string, DbMapping>();
+        private static readonly ConcurrentDictionary<string, DbMapping> Mappings = new ConcurrentDictionary<string, DbMapping>();
 
         /// <summary>
         /// 
@@ -60,14 +65,11 @@ public static DbMapping Get(DbContext context)
                 cacheKey = iDbModelCacheKeyProvider.CacheKey;
             }
 
-            DbMapping mapping;
-            if (Mappings.TryGetValue(cacheKey, out mapping))
-                return mapping;
-
-            mapping = new DbMapping(context);
-
-            Mappings[cacheKey] = mapping;
-            return mapping;
+            // GetOrAdd rather than TryGetValue-then-assign: two threads racing here used to corrupt
+            // the cache. They may still both build a DbMapping on first use and one result is
+            // discarded, which is wasteful but correct - and it keeps a failed build from being
+            // cached, which a Lazy would not.
+            return Mappings.GetOrAdd(cacheKey, _ => new DbMapping(context));
         }
     }
 }