Merge pull request #2481 from oleksii-datsiuk/odatsiuk/prevent-ensure…

…-clear-cache-and-db-corruption

Prevent using disposed snapshots

LiteDB/Engine/Services/SnapShot.cs

@@ -33,6 +33,8 @@ internal class Snapshot : IDisposable
         // local page cache - contains only pages about this collection (but do not contains CollectionPage - use this.CollectionPage)
         private readonly Dictionary<uint, BasePage> _localPages = new Dictionary<uint, BasePage>();
 
+        private bool _disposed;
+
         // expose
         public LockMode Mode => _mode;
         public string CollectionName => _collectionName;
@@ -89,6 +91,8 @@ public Snapshot(
         /// </summary>
         public IEnumerable<BasePage> GetWritablePages(bool dirty, bool includeCollectionPage)
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
+
             // if snapshot is read only, just exit
             if (_mode == LockMode.Read) yield break;
 
@@ -110,6 +114,8 @@ public IEnumerable<BasePage> GetWritablePages(bool dirty, bool includeCollection
         /// </summary>
         public void Clear()
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
+
             // release pages only if snapshot are read only
             if (_mode == LockMode.Read)
             {
@@ -128,9 +134,16 @@ public void Clear()
         /// </summary>
         public void Dispose()
         {
+            if (_disposed)
+            {
+                return;
+            }
+
             // release all data/index pages
             this.Clear();
 
+            _disposed = true;
+
             // release collection page (in read mode)
             if (_mode == LockMode.Read && _collectionPage != null)
             {
@@ -160,6 +173,7 @@ public T GetPage<T>(uint pageID)
         public T GetPage<T>(uint pageID, out FileOrigin origin, out long position, out int walVersion)
             where T : BasePage
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
             ENSURE(pageID <= _header.LastPageID, "request page must be less or equals lastest page in data file");
 
             // check for header page (return header single instance)
@@ -259,6 +273,8 @@ private T ReadPage<T>(uint pageID, out FileOrigin origin, out long position, out
         /// </summary>
         public DataPage GetFreeDataPage(int bytesLength)
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
+
             var length = bytesLength + BasePage.SLOT_SIZE; // add +4 bytes for footer slot
 
             // get minimum slot to check for free page. Returns -1 if need NewPage
@@ -292,6 +308,8 @@ public DataPage GetFreeDataPage(int bytesLength)
         /// </summary>
         public IndexPage GetFreeIndexPage(int bytesLength, ref uint freeIndexPageList)
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
+
             IndexPage page;
 
             // if there is not page in list pages, create new page
@@ -318,6 +336,7 @@ public IndexPage GetFreeIndexPage(int bytesLength, ref uint freeIndexPageList)
         public T NewPage<T>()
             where T : BasePage
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
             ENSURE(_collectionPage == null, typeof(T) == typeof(CollectionPage), "if no collection page defined yet, must be first request");
             ENSURE(typeof(T) == typeof(CollectionPage), _collectionPage == null, "there is no new collection page if page already exists");
 
@@ -392,6 +411,8 @@ public T NewPage<T>()
         /// </summary>
         public void AddOrRemoveFreeDataList(DataPage page)
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
+
             var newSlot = DataPage.FreeIndexSlot(page.FreeBytes);
             var initialSlot = page.PageListSlot;
 
@@ -423,6 +444,8 @@ public void AddOrRemoveFreeDataList(DataPage page)
         /// </summary>
         public void AddOrRemoveFreeIndexList(IndexPage page, ref uint startPageID)
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
+
             var newSlot = IndexPage.FreeIndexSlot(page.FreeBytes);
             var isOnList = page.PageListSlot == 0;
             var mustKeep = newSlot == 0;
@@ -567,6 +590,8 @@ private void DeletePage<T>(T page)
         /// </summary>
         public void DropCollection(Action safePoint)
         {
+            ENSURE(!_disposed, "the snapshot is disposed");
+
             var indexer = new IndexService(this, _header.Pragmas.Collation, _disk.MAX_ITEMS_COUNT);
 
             // CollectionPage will be last deleted page (there is no NextPageID from CollectionPage)