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

content/browser/indexed_db/leveldb/leveldb_database.h

Index: content/browser/indexed_db/leveldb/leveldb_database.h
diff --git a/content/browser/indexed_db/leveldb/leveldb_database.h b/content/browser/indexed_db/leveldb/leveldb_database.h
index cdcabb54647c83c9d2f56690e2a84ba2fecd87c9..1df138d6a6b3c08b564e40deb1eb4414fd4db18d 100644
--- a/content/browser/indexed_db/leveldb/leveldb_database.h
+++ b/content/browser/indexed_db/leveldb/leveldb_database.h
@@ -8,6 +8,7 @@
 #include <memory>
 #include <string>
 
+#include "base/containers/mru_cache.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/strings/string16.h"
@@ -75,6 +76,11 @@ class CONTENT_EXPORT LevelDBDatabase
     const LevelDBComparator* comparator_;
   };
 
+  static leveldb::Status OpenForTesting(const base::FilePath& file_name,
+                                        const LevelDBComparator* comparator,
+                                        std::unique_ptr<LevelDBDatabase>* db,
+                                        size_t max_open_cursors);
+
   static leveldb::Status Open(const base::FilePath& file_name,
                               const LevelDBComparator* comparator,
                               std::unique_ptr<LevelDBDatabase>* db,
@@ -102,8 +108,13 @@ class CONTENT_EXPORT LevelDBDatabase
   bool OnMemoryDump(const base::trace_event::MemoryDumpArgs& args,
                     base::trace_event::ProcessMemoryDump* pmd) override;
 
+  void NotifyIteratorCreated(LevelDBIterator*);
+  void NotifyIteratorUsed(LevelDBIterator*);
+  void NotifyIteratorDestroyed(LevelDBIterator*);
+
  protected:
   LevelDBDatabase();
+  LevelDBDatabase(size_t max_open_iterators);
 
  private:
   friend class LevelDBSnapshot;
@@ -115,6 +126,13 @@ class CONTENT_EXPORT LevelDBDatabase
   std::unique_ptr<leveldb::DB> db_;
   std::unique_ptr<const leveldb::FilterPolicy> filter_policy_;
   const LevelDBComparator* comparator_;
+
+  // Despite the type name, this object uses LRU eviction.
+  base::MRUCache<LevelDBIterator*, LevelDBIterator*> iterator_lru_;
+  // Recorded for UMA reporting.
+  uint32_t num_iterators_ = 0;
+  uint32_t max_iterators_ = 0;
+
   std::string file_name_for_tracing;
 };