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

content/browser/indexed_db/leveldb/leveldb_transaction.h

 // Copyright (c) 2013 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_TRANSACTION_H_
 #define CONTENT_BROWSER_INDEXED_DB_LEVELDB_LEVELDB_TRANSACTION_H_
 
 #include <map>
 #include <memory>
 #include <set>
 #include <string>
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/strings/string_piece.h"
 #include "content/browser/indexed_db/leveldb/leveldb_comparator.h"
 #include "content/browser/indexed_db/leveldb/leveldb_database.h"
 #include "content/browser/indexed_db/leveldb/leveldb_iterator.h"
 
 namespace content {
 
 class LevelDBTransactionRangeTest;
+class LevelDBTransactionTest;
 class LevelDBWriteBatch;
 
 class CONTENT_EXPORT LevelDBTransaction
     : public base::RefCounted<LevelDBTransaction> {
  public:
   void Put(const base::StringPiece& key, std::string* value);
 
   void Remove(const base::StringPiece& key);
 
   leveldb::Status RemoveRange(const base::StringPiece& begin,
                               const base::StringPiece& end,
                               bool upper_open);
 
   virtual leveldb::Status Get(const base::StringPiece& key,
                               std::string* value,
                               bool* found);
   virtual leveldb::Status Commit();
   void Rollback();
 
   std::unique_ptr<LevelDBIterator> CreateIterator();
 
  protected:
   virtual ~LevelDBTransaction();
   explicit LevelDBTransaction(LevelDBDatabase* db);
   friend class IndexedDBClassFactory;
 
  private:
   friend class base::RefCounted<LevelDBTransaction>;
   friend class content::LevelDBTransactionRangeTest;
+  friend class content::LevelDBTransactionTest;
   FRIEND_TEST_ALL_PREFIXES(LevelDBTransactionTest, GetAndPut);
   FRIEND_TEST_ALL_PREFIXES(LevelDBTransactionTest, Commit);
   FRIEND_TEST_ALL_PREFIXES(LevelDBTransactionTest, Iterator);
 
   struct Record {
     Record();
     ~Record();
     std::string key;
     std::string value;
     bool deleted = false;
@@ skipping 23 matching lines @@
     static std::unique_ptr<DataIterator> Create(
         LevelDBTransaction* transaction);
     ~DataIterator() override;
     bool IsValid() const override;
     leveldb::Status SeekToLast() override;
     leveldb::Status Seek(const base::StringPiece& slice) override;
     leveldb::Status Next() override;
     leveldb::Status Prev() override;
     base::StringPiece Key() const override;
     base::StringPiece Value() const override;
+
     bool IsDeleted() const;
 
     // Mark the current record as deleted.
     void Delete();
 
    private:
     explicit DataIterator(LevelDBTransaction* transaction);
     DataType* data_;
     DataType::iterator iterator_;
 
@@ skipping 10 matching lines @@
     static std::unique_ptr<TransactionIterator> Create(
         scoped_refptr<LevelDBTransaction> transaction);
 
     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;
+    bool IsMemoryPurged() const override;

[jsbell, 2017/03/22 23:55:32]

I think I'd keep this called 'IsEvicted' to be consistent. (Sorry for waffling.
I still like PurgeMemory!)

[dmurph, 2017/03/23 20:56:48]

Done.

+
     void DataChanged();
 
     // Mark the current record as deleted. If an existing record
     // is present in the uncommitted data this will convert it to
     // a deletion record, otherwise it will insert a new one.
     void Delete();
 
    private:
     enum Direction { FORWARD, REVERSE };
 
+    enum class DBIteratorState {
+      ACTIVE,
+      EVICTED_AND_VALID,
+      EVICTED_AND_INVALID
+    };
+
     explicit TransactionIterator(scoped_refptr<LevelDBTransaction> transaction);
     void HandleConflictsAndDeletes();
     void SetCurrentIteratorToSmallestKey();
     void SetCurrentIteratorToLargestKey();
     void RefreshDataIterator() const;
     bool DataIteratorIsLower() const;
     bool DataIteratorIsHigher() const;
 
+    void LoadDBIteratorIfEvicted();

[jsbell, 2017/03/22 23:55:32]

Maybe 'RestoreDBIteratorIfEvicted' ? 

[dmurph, 2017/03/23 20:56:48]

Done.

+
     scoped_refptr<LevelDBTransaction> transaction_;
     const LevelDBComparator* comparator_;
     mutable std::unique_ptr<DataIterator> data_iterator_;
     std::unique_ptr<LevelDBIterator> db_iterator_;
+    DBIteratorState db_iterator_state_ = DBIteratorState::ACTIVE;
     LevelDBIterator* current_ = nullptr;
 
+    std::string db_key_before_eviction_;
+
     Direction direction_ = FORWARD;
     mutable bool data_changed_ = false;
 
     DISALLOW_COPY_AND_ASSIGN(TransactionIterator);
   };
 
   void Set(const base::StringPiece& key, std::string* value, bool deleted);
   void RegisterIterator(TransactionIterator* iterator);
   void UnregisterIterator(TransactionIterator* iterator);
   void NotifyIterators();
@@ skipping 29 matching lines @@
   LevelDBDatabase* db_;
   std::unique_ptr<LevelDBWriteBatch> write_batch_;
   bool finished_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(LevelDBDirectTransaction);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_INDEXED_DB_LEVELDB_LEVELDB_TRANSACTION_H_