Add SimpleCache index file heuristics

net/disk_cache/simple/simple_index_file.cc

Index: net/disk_cache/simple/simple_index_file.cc
diff --git a/net/disk_cache/simple/simple_index_file.cc b/net/disk_cache/simple/simple_index_file.cc
index a2913202276b22ef724a04abb6ffd0cdc34aa419..fc33833a0a58040fd9dc9fb175dc45fd44a60971 100644
--- a/net/disk_cache/simple/simple_index_file.cc
+++ b/net/disk_cache/simple/simple_index_file.cc
@@ -66,6 +66,9 @@ bool SimpleIndexFile::IndexMetadata::CheckIndexMetadata() {
 // static
 scoped_ptr<SimpleIndex::EntrySet> SimpleIndexFile::LoadFromDisk(
     const base::FilePath& index_filename) {
+  // Only load if the index is not stale.
+  if (SimpleIndexFile::IsIndexFileStale(index_filename))
+    return scoped_ptr<SimpleIndex::EntrySet>(NULL);
   std::string contents;
   if(!file_util::ReadFileToString(index_filename, &contents)) {
     LOG(WARNING) << "Could not read Simple Index file.";
@@ -161,4 +164,21 @@ void SimpleIndexFile::WriteToDisk(const base::FilePath& index_filename,
   DCHECK(result);
 }
 
+// static
+bool SimpleIndexFile::IsIndexFileStale(const base::FilePath& index_filename) {
+  base::PlatformFileInfo dir_info;
+  base::PlatformFileInfo index_info;
+  if (!file_util::GetFileInfo(index_filename.DirName(), &dir_info))
+    return false;
+  DCHECK(dir_info.is_directory);
+  if (!file_util::GetFileInfo(index_filename, &index_info))
+    return false;
+
+  // Index file last_modified must be equal to the directory last_modified since
+  // the last operation we do is ReplaceFile in the
+  // SimpleIndexFile::WriteToDisk().
+  // If not true, we need to restore the index.
+  return index_info.last_modified >= dir_info.last_modified;
+}
+
 }  // namespace disk_cache