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

content/browser/indexed_db/leveldb/leveldb_transaction.h

Index: content/browser/indexed_db/leveldb/leveldb_transaction.h
diff --git a/content/browser/indexed_db/leveldb/leveldb_transaction.h b/content/browser/indexed_db/leveldb/leveldb_transaction.h
index a701a94748a6b02c10dcfdbd4f947084fa0f2a8f..444c4545931c31b9d182114a42ba9457c01096bf 100644
--- a/content/browser/indexed_db/leveldb/leveldb_transaction.h
+++ b/content/browser/indexed_db/leveldb/leveldb_transaction.h
@@ -21,6 +21,7 @@
 namespace content {
 
 class LevelDBTransactionRangeTest;
+class LevelDBTransactionTest;
 class LevelDBWriteBatch;
 
 class CONTENT_EXPORT LevelDBTransaction
@@ -50,6 +51,7 @@ class CONTENT_EXPORT LevelDBTransaction
  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);
@@ -93,6 +95,7 @@ class CONTENT_EXPORT LevelDBTransaction
     leveldb::Status Prev() override;
     base::StringPiece Key() const override;
     base::StringPiece Value() const override;
+
     bool IsDeleted() const;
 
     // Mark the current record as deleted.
@@ -123,6 +126,9 @@ class CONTENT_EXPORT LevelDBTransaction
     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
@@ -133,6 +139,12 @@ class CONTENT_EXPORT LevelDBTransaction
    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();
@@ -141,12 +153,17 @@ class CONTENT_EXPORT LevelDBTransaction
     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;