[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 LevelDBIteratorPoolController;
 
 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 PurgeMemory() override;

[pwnall, 2017/03/24 09:16:03]

I saw the earlier comments regarding these two names. I think it's confusing to
have PurgeMemory / IsEvicted not be consistent with each other. I'd have said
that internal iterator is detached, and comment that detaching is done to save
memory, which can be orthogonal to the caching built on top.

[dmurph, 2017/03/24 23:33:39]

Done.

+  bool IsEvicted() const override;
 
  protected:
-  explicit LevelDBIteratorImpl(std::unique_ptr<leveldb::Iterator> iterator);
+  explicit LevelDBIteratorImpl(std::unique_ptr<leveldb::Iterator> iterator,
+                               LevelDBIteratorPoolController* pool_controller,
+                               const leveldb::Snapshot* snapshot);
 
  private:
+  enum class IteratorState { ACTIVE, EVICTED_AND_VALID, EVICTED_AND_INVALID };
+
   void CheckStatus();
 
+  void RecordUsage();
+  void RestoreDBIteratorIfEvicted();
+
   friend class IndexedDBClassFactory;
   friend class MockBrowserTestIndexedDBClassFactory;
 
   std::unique_ptr<leveldb::Iterator> iterator_;
 
+  // Variables to facilitate memory purging.
+  LevelDBIteratorPoolController* pool_controller_;
+  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_