[Bug] Fix MemorySnapshot bug (#3953)

* Fix MemorySnapshot bug

* new ut

Author: shargon

src/Neo/Persistence/Providers/MemorySnapshot.cs

@@ -31,6 +31,8 @@ internal class MemorySnapshot : IStoreSnapshot
 
         public IStore Store { get; }
 
+        internal int WriteBatchLength => _writeBatch.Count;
+
         internal MemorySnapshot(MemoryStore store, ConcurrentDictionary<byte[], byte[]> innerData)
         {
             Store = store;
@@ -46,6 +48,8 @@ public void Commit()
                     _innerData.TryRemove(pair.Key, out _);
                 else
                     _innerData[pair.Key] = pair.Value;
+
+            _writeBatch.Clear();
         }
 
         public void Delete(byte[] key)

tests/Neo.UnitTests/Persistence/UT_MemorySnapshot.cs

@@ -34,6 +34,51 @@ public void CleanUp()
             _memoryStore.Reset();
         }
 
+        [TestMethod]
+        public void TestDobleCommit()
+        {
+            var key1 = new byte[] { 0x05, 0x02 };
+            var value1 = new byte[] { 0x06, 0x04 };
+
+            var snapshot = (MemorySnapshot)_memoryStore.GetSnapshot();
+            Assert.AreEqual(0, snapshot.WriteBatchLength);
+
+            snapshot.Put(key1, value1);
+            Assert.AreEqual(1, snapshot.WriteBatchLength);
+
+            snapshot.Delete(key1);
+            Assert.AreEqual(1, snapshot.WriteBatchLength);
+            snapshot.Commit();
+
+            Assert.AreEqual(0, snapshot.WriteBatchLength);
+        }
+
+        [TestMethod]
+        public void TestDobleCommitTwo()
+        {
+            var key1 = new byte[] { 0x51, 0x02 };
+            var value1 = new byte[] { 0x06, 0x04 };
+
+            var snapshotA = (MemorySnapshot)_memoryStore.GetSnapshot();
+            var snapshotB = (MemorySnapshot)_memoryStore.GetSnapshot();
+
+            Assert.IsFalse(_memoryStore.Contains(key1));
+            snapshotA.Put(key1, value1);
+            snapshotA.Commit();
+            Assert.IsTrue(_memoryStore.Contains(key1));
+
+            snapshotB.Delete(key1);
+            snapshotB.Commit();
+            Assert.IsFalse(_memoryStore.Contains(key1));
+
+            snapshotA.Put(key1, value1);
+            snapshotA.Commit();
+            Assert.IsTrue(_memoryStore.Contains(key1));
+
+            snapshotB.Commit(); // Already committed
+            Assert.IsTrue(_memoryStore.Contains(key1)); // It fails before #3953
+        }
+
         [TestMethod]
         public void SingleSnapshotTest()
         {