Merge pull request #866 from ElteHupkes/fix-insert-nre-761

Fix to #761, preventing returning a disposed command object

src/SQLite.cs

@@ -1710,21 +1710,20 @@ PreparedSqlLiteInsertCommand GetInsertCommand (TableMapping map, string extra)
 			var key = Tuple.Create (map.MappedType.FullName, extra);
 
 			lock (_insertCommandMap) {
-				_insertCommandMap.TryGetValue (key, out prepCmd);
+				if (_insertCommandMap.TryGetValue (key, out prepCmd)) {
+					return prepCmd;
+				}
 			}
 
-			if (prepCmd == null) {
-				prepCmd = CreateInsertCommand (map, extra);
-				var added = false;
-				lock (_insertCommandMap) {
-					if (!_insertCommandMap.ContainsKey (key)) {
-						_insertCommandMap.Add (key, prepCmd);
-						added = true;
-					}
-				}
-				if (!added) {
+			prepCmd = CreateInsertCommand (map, extra);
+
+			lock (_insertCommandMap) {
+				if (_insertCommandMap.TryGetValue (key, out var existing)) {
 					prepCmd.Dispose ();
+					return existing;
 				}
+
+				_insertCommandMap.Add (key, prepCmd);
 			}
 
 			return prepCmd;

tests/ConcurrencyTest.cs

@@ -187,7 +187,28 @@ public void TestLoad()
             }
         }
 
-
+        /// <summary>
+        /// Test for issue #761. Because the nature of this test is a race condition,
+        /// it is not guaranteed to fail if the issue is present. It does appear to
+        /// fail most of the time, though.
+        /// </summary>
+        [Test]
+        public void TestInsertCommandCreation ()
+        {
+	        using (var dbConnection =
+		        new DbConnection (SQLiteOpenFlags.FullMutex | SQLiteOpenFlags.ReadWrite | SQLiteOpenFlags.Create)) {
+		        var obj1 = new TestObj ();
+		        var obj2 = new TestObj ();
+		        var taskA = Task.Run (() => {
+			        dbConnection.Insert (obj1);
+		        });
+		        var taskB = Task.Run (() => {
+			        dbConnection.Insert (obj2);
+		        });
+
+		        Task.WhenAll (taskA, taskB).Wait ();
+	        }
+        }
     }
 }