[IndexedDB] Pool and evict leveldb iterators, to save memory

content/browser/indexed_db/leveldb/leveldb_iterator_impl.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_INDEXED_DB_LEVELDB_LEVELDB_ITERATOR_IMPL_H_
 #define CONTENT_BROWSER_INDEXED_DB_LEVELDB_LEVELDB_ITERATOR_IMPL_H_
 
 #include <memory>
+#include <string>
 
 #include "base/macros.h"
 #include "content/browser/indexed_db/leveldb/leveldb_iterator.h"
 #include "content/common/content_export.h"
 #include "third_party/leveldatabase/src/include/leveldb/iterator.h"
 
+namespace leveldb {
+class Snapshot;
+}
+
 namespace content {
+class LevelDBDatabase;
 
 class CONTENT_EXPORT LevelDBIteratorImpl : public content::LevelDBIterator {
  public:
   ~LevelDBIteratorImpl() override;
   bool IsValid() const override;
   leveldb::Status SeekToLast() override;
   leveldb::Status Seek(const base::StringPiece& target) override;
   leveldb::Status Next() override;
   leveldb::Status Prev() override;
   base::StringPiece Key() const override;
   base::StringPiece Value() const override;
+  void Detach() override;
+  bool IsDetached() const override;
 
  protected:
-  explicit LevelDBIteratorImpl(std::unique_ptr<leveldb::Iterator> iterator);
+  explicit LevelDBIteratorImpl(std::unique_ptr<leveldb::Iterator> iterator,
+                               LevelDBDatabase* db,
+                               const leveldb::Snapshot* snapshot);
 
  private:
-  void CheckStatus();
+  enum class IteratorState { ACTIVE, EVICTED_AND_VALID, EVICTED_AND_INVALID };
+
+  leveldb::Status CheckStatus();
+
+  // Notifies the database of iterator usage and recreates iterator if needed.
+  void WillUseDBIterator();
 
   friend class IndexedDBClassFactory;
   friend class MockBrowserTestIndexedDBClassFactory;
 
   std::unique_ptr<leveldb::Iterator> iterator_;
 
+  // State used to facilitate memory purging.
+  LevelDBDatabase* db_;
+  IteratorState iterator_state_ = IteratorState::ACTIVE;
+  std::string key_before_eviction_;
+  const leveldb::Snapshot* snapshot_;
+
   DISALLOW_COPY_AND_ASSIGN(LevelDBIteratorImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_INDEXED_DB_LEVELDB_LEVELDB_ITERATOR_IMPL_H_