SimpleCache: open with fewer seeks.

net/disk_cache/simple/simple_entry_impl.h

 // Copyright (c) 2013 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_SIMPLE_SIMPLE_ENTRY_IMPL_H_
 #define NET_DISK_CACHE_SIMPLE_SIMPLE_ENTRY_IMPL_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 61 matching lines @@
   int OpenEntry(Entry** entry, const CompletionCallback& callback);
 
   // Creates this entry, if possible. Returns |this| to |entry|.
   int CreateEntry(Entry** entry, const CompletionCallback& callback);
 
   // Identical to Backend::Doom() except that it accepts a CompletionCallback.
   int DoomEntry(const CompletionCallback& callback);
 
   const std::string& key() const { return key_; }
   uint64_t entry_hash() const { return entry_hash_; }
+
+  // The key is not a constructor parameter to the SimpleEntryImpl, because
+  // during cache iteration, it's necessary to open entries by their hash
+  // alone. In that case, the SimpleSynchronousEntry will read the key from disk
+  // and it will be set.
   void SetKey(const std::string& key);
 
   // From Entry:
   void Doom() override;
   void Close() override;
   std::string GetKey() const override;
   base::Time GetLastUsed() const override;
   base::Time GetLastModified() const override;
   int32_t GetDataSize(int index) const override;
   int ReadData(int stream_index,
@@ skipping 283 matching lines @@
   // 1 on disk, to reduce the number of file descriptors and save disk space.
   // This strategy allows stream 1 to change size easily. Since stream 0 is only
   // used to write HTTP headers, the memory consumption of keeping it in memory
   // is acceptable.
   scoped_refptr<net::GrowableIOBuffer> stream_0_data_;
 };
 
 }  // namespace disk_cache
 
 #endif  // NET_DISK_CACHE_SIMPLE_SIMPLE_ENTRY_IMPL_H_