fix it so that disposing the RocksDb instance twice does not throw Ac…

…cessViolationException and crash the process

RocksDbSharp/RocksDb.cs

@@ -10,6 +10,8 @@ namespace RocksDbSharp
 {
     public class RocksDb : IDisposable
     {
+        bool disposed;
+
         internal static ReadOptions DefaultReadOptions { get; } = new ReadOptions();
         internal static OptionsHandle DefaultOptions { get; } = new DbOptions();
         internal static WriteOptions DefaultWriteOptions { get; } = new WriteOptions();
@@ -31,12 +33,21 @@ private RocksDb(IntPtr handle, dynamic optionsReferences, dynamic cfOptionsRefs,
 
         public void Dispose()
         {
-            if (columnFamilies != null)
+            if (disposed) return;
+            try
+            {
+                if (columnFamilies != null)
+                {
+                    foreach (var cfh in columnFamilies.Values)
+                        cfh.Dispose();
+                }
+
+                Native.Instance.rocksdb_close(Handle);
+            }
+            finally
             {
-                foreach (var cfh in columnFamilies.Values)
-                    cfh.Dispose();
+                disposed = true;
             }
-            Native.Instance.rocksdb_close(Handle);
         }
 
         public static RocksDb Open(OptionsHandle options, string path)

tests/RocksDbSharpTest/LifestyleTest.cs

@@ -0,0 +1,44 @@
+using System;
+using System.IO;
+using RocksDbSharp;
+using Xunit;
+// ReSharper disable RedundantArgumentDefaultValue
+// ReSharper disable ArgumentsStyleLiteral
+
+namespace RocksDbSharpTest
+{
+    public class LifestyleTest
+    {
+        [Fact]
+        public void DoubleDisposableDoesNotThrow()
+        {
+            var testdir = Path.Combine(Path.GetTempPath(), "lifestyle_test");
+            var testdb = Path.Combine(testdir, "main");
+            var path = Environment.ExpandEnvironmentVariables(testdb);
+
+            if (Directory.Exists(testdir))
+            {
+                Directory.Delete(testdir, recursive: true);
+            }
+
+            Directory.CreateDirectory(testdir);
+
+            var options = new DbOptions().SetCreateIfMissing(true).EnableStatistics();
+
+            var db = RocksDb.Open(options, path);
+
+            db.Dispose();
+
+            // throws AccessViolationException, which on my machine crashed the process so hard that XUnit coulnd't cope...
+            //
+            db.Dispose();
+            //
+            // got this in Event Viewer though:
+            //
+            //  Application: dotnet.exe
+            //  CoreCLR Version: 4.6.28619.1
+            //  Description: The process was terminated due to an internal error in the .NET Runtime at IP 00007FFF39BC5AA3 (00007FFF39A20000) with exit code c0000005.
+            //
+        }
+    }
+}