Introduce parent and child entries for MemEntryImpl...

net/disk_cache/mem_entry_impl.h

Index: net/disk_cache/mem_entry_impl.h
===================================================================
--- net/disk_cache/mem_entry_impl.h	(revision 18335)
+++ net/disk_cache/mem_entry_impl.h	(working copy)
@@ -1,20 +1,38 @@
-// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
+// Copyright (c) 2006-2009 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 NET_DISK_CACHE_MEM_ENTRY_IMPL_H__
-#define NET_DISK_CACHE_MEM_ENTRY_IMPL_H__
+#ifndef NET_DISK_CACHE_MEM_ENTRY_IMPL_H_
+#define NET_DISK_CACHE_MEM_ENTRY_IMPL_H_
 
 #include "net/disk_cache/disk_cache.h"
+#include "testing/gtest/include/gtest/gtest_prod.h"
 
 namespace disk_cache {
 
 class MemBackendImpl;
 
 // This class implements the Entry interface for the memory-only cache. An
-// object of this class represents a single entry on the cache.
+// object of this class represents a single entry on the cache. We use two
+// types of entries, parent and child to support sparse caching.
+// A parent entry is non-sparse until a sparse method is invoked (i.e.
+// ReadSparseData, WriteSparseData, GetAvailableRange) when sparse information
+// is initialized. It then manages a list of child entries and delegates the
+// sparse API calls to the child entries. It creates and deletes child entries
+// and updates the list when needed.
+// A child entry is used to carry partial cache content, non-sparse methods like
+// ReadData and WriteData cannot be applied to them. The lifetime of a child
+// entry is managed by the parent entry that created it except that the entry
+// can be evicted independently. A child entry does not have a key and it is not
+// registered in the backend's entry map. It is registered in the backend's
+// ranking list to enable eviction of a partial content.
 class MemEntryImpl : public Entry {
  public:
+  enum EntryType {
+    kParentEntry,
+    kChildEntry,
+  };
+
   explicit MemEntryImpl(MemBackendImpl* backend);
 
   // Entry interface.
@@ -39,9 +57,18 @@
   // cache.
   bool CreateEntry(const std::string& key);
 
-  // Permamently destroys this entry
+  // Performs the initialization of a MemEntryImpl as a child entry.
+  // TODO(hclam): this method should be private. Leave this as public because
+  // this is the only way to create a child entry. Move this method to private
+  // once child entries are created by parent entry.
+  bool CreateChildEntry(MemEntryImpl* parent);
+
+  // Permanently destroys this entry.
   void InternalDoom();
 
+  void Open();
+  bool InUse();
+
   MemEntryImpl* next() const {
     return next_;
   }
@@ -58,8 +85,9 @@
     prev_ = prev;
   }
 
-  void Open();
-  bool InUse();
+  EntryType type() const {
+    return type_;
+  }
 
  private:
    enum {
@@ -81,14 +109,16 @@
 
   MemEntryImpl* next_;         // Pointers for the LRU list.
   MemEntryImpl* prev_;
-  base::Time last_modified_;         // LRU information.
+  MemEntryImpl* parent_;       // Pointer to the parent entry.
+  base::Time last_modified_;   // LRU information.
   base::Time last_used_;
   MemBackendImpl* backend_;    // Back pointer to the cache.
   bool doomed_;                // True if this entry was removed from the cache.
+  EntryType type_;             // The type of this entry.
 
   DISALLOW_EVIL_CONSTRUCTORS(MemEntryImpl);
 };
 
 }  // namespace disk_cache
 
-#endif  // NET_DISK_CACHE_MEM_ENTRY_IMPL_H__
+#endif  // NET_DISK_CACHE_MEM_ENTRY_IMPL_H_