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

content/browser/indexed_db/leveldb/leveldb_transaction.cc

Index: content/browser/indexed_db/leveldb/leveldb_transaction.cc
diff --git a/content/browser/indexed_db/leveldb/leveldb_transaction.cc b/content/browser/indexed_db/leveldb/leveldb_transaction.cc
index 1722549384aeec3bb98b4a0d8afc4ac75507d778..feb6ec3be73358c958c31e6ee2e485ff4366dd40 100644
--- a/content/browser/indexed_db/leveldb/leveldb_transaction.cc
+++ b/content/browser/indexed_db/leveldb/leveldb_transaction.cc
@@ -41,7 +41,11 @@ LevelDBTransaction::LevelDBTransaction(LevelDBDatabase* db)
 LevelDBTransaction::Record::Record() {}
 LevelDBTransaction::Record::~Record() {}
 
-LevelDBTransaction::~LevelDBTransaction() {}
+LevelDBTransaction::~LevelDBTransaction() {
+  for (TransactionIterator* iter : iterators_) {
+    db_->NotifyIteratorDestroyed(iter);
+  }
+}
 
 void LevelDBTransaction::Set(const StringPiece& key,
                              std::string* value,
@@ -248,6 +252,7 @@ bool LevelDBTransaction::TransactionIterator::IsValid() const {
 }
 
 leveldb::Status LevelDBTransaction::TransactionIterator::SeekToLast() {
+  LoadDBIteratorIfEvicted();
   leveldb::Status s = data_iterator_->SeekToLast();
   DCHECK(s.ok());
   s = db_iterator_->SeekToLast();
@@ -262,6 +267,7 @@ leveldb::Status LevelDBTransaction::TransactionIterator::SeekToLast() {
 
 leveldb::Status LevelDBTransaction::TransactionIterator::Seek(
     const StringPiece& target) {
+  LoadDBIteratorIfEvicted();
   leveldb::Status s = data_iterator_->Seek(target);
   DCHECK(s.ok());
   s = db_iterator_->Seek(target);
@@ -275,6 +281,7 @@ leveldb::Status LevelDBTransaction::TransactionIterator::Seek(
 }
 
 leveldb::Status LevelDBTransaction::TransactionIterator::Next() {
+  LoadDBIteratorIfEvicted();
   DCHECK(IsValid());
   if (data_changed_)
     RefreshDataIterator();
@@ -311,6 +318,7 @@ leveldb::Status LevelDBTransaction::TransactionIterator::Next() {
 }
 
 leveldb::Status LevelDBTransaction::TransactionIterator::Prev() {
+  LoadDBIteratorIfEvicted();
   DCHECK(IsValid());
   leveldb::Status s;
   if (data_changed_)
@@ -354,6 +362,9 @@ StringPiece LevelDBTransaction::TransactionIterator::Key() const {
   DCHECK(IsValid());
   if (data_changed_)
     RefreshDataIterator();
+  if (current_ == db_iterator_.get() && is_evicted_) {
+    return key_after_eviction_;
+  }
   return current_->Key();
 }
 
@@ -361,9 +372,20 @@ StringPiece LevelDBTransaction::TransactionIterator::Value() const {
   DCHECK(IsValid());
   if (data_changed_)
     RefreshDataIterator();
+  if (current_ == db_iterator_.get()) {
+    const_cast<LevelDBTransaction::TransactionIterator*>(this)
+        ->LoadDBIteratorIfEvicted();
+  }
   return current_->Value();
 }
 
+void LevelDBTransaction::TransactionIterator::EvictWorkingMemory() {
+  DCHECK(!is_evicted_);
+  is_evicted_ = true;
+  key_after_eviction_ = db_iterator_->Key().as_string();
+  db_iterator_.reset();
+}
+
 void LevelDBTransaction::TransactionIterator::DataChanged() {
   data_changed_ = true;
 }
@@ -411,14 +433,28 @@ void LevelDBTransaction::TransactionIterator::RefreshDataIterator() const {
 }
 
 bool LevelDBTransaction::TransactionIterator::DataIteratorIsLower() const {
+  DCHECK(!is_evicted_);
   return comparator_->Compare(data_iterator_->Key(), db_iterator_->Key()) < 0;
 }
 
 bool LevelDBTransaction::TransactionIterator::DataIteratorIsHigher() const {
+  DCHECK(!is_evicted_);
   return comparator_->Compare(data_iterator_->Key(), db_iterator_->Key()) > 0;
 }
 
+void LevelDBTransaction::TransactionIterator::LoadDBIteratorIfEvicted() {
+  transaction_->db_->NotifyIteratorUsed(this);
+  if (!is_evicted_)
+    return;
+
+  db_iterator_ = transaction_->db_->CreateIterator(&transaction_->snapshot_);
+  db_iterator_->Seek(key_after_eviction_);
+  key_after_eviction_.clear();
+  is_evicted_ = false;
+}
+
 void LevelDBTransaction::TransactionIterator::HandleConflictsAndDeletes() {
+  LoadDBIteratorIfEvicted();
   bool loop = true;
 
   while (loop) {
@@ -452,6 +488,7 @@ void LevelDBTransaction::TransactionIterator::HandleConflictsAndDeletes() {
 
 void
 LevelDBTransaction::TransactionIterator::SetCurrentIteratorToSmallestKey() {
+  DCHECK(!is_evicted_);
   LevelDBIterator* smallest = nullptr;
 
   if (data_iterator_->IsValid())
@@ -489,6 +526,7 @@ void LevelDBTransaction::RegisterIterator(TransactionIterator* iterator) {
 void LevelDBTransaction::UnregisterIterator(TransactionIterator* iterator) {
   DCHECK(iterators_.find(iterator) != iterators_.end());
   iterators_.erase(iterator);
+  db_->NotifyIteratorDestroyed(iterator);
 }
 
 void LevelDBTransaction::NotifyIterators() {