Make SafeBrowsingDatabase's PrefixSets only updatable by swapping a new one in.

chrome/browser/safe_browsing/prefix_set.h

 // Copyright (c) 2012 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.
 //
 // A read-only set implementation for |SBPrefix| items.  Prefixes are
 // sorted and stored as 16-bit deltas from the previous prefix.  An
 // index structure provides quick random access, and also handles
 // cases where 16 bits cannot encode a delta.
 //
 // For example, the sequence {20, 25, 41, 65432, 150000, 160000} would
@@ skipping 53 matching lines @@
 
 class PrefixSet {
  public:
   ~PrefixSet();
 
   // |true| if |hash| is in the hashes passed to the set's builder, or if
   // |hash.prefix| is one of the prefixes passed to the set's builder.
   bool Exists(const SBFullHash& hash) const;
 
   // Persist the set on disk.
-  static scoped_ptr<PrefixSet> LoadFile(const base::FilePath& filter_name);
+  static scoped_ptr<const PrefixSet> LoadFile(
+      const base::FilePath& filter_name);
   bool WriteFile(const base::FilePath& filter_name) const;
 
  private:
   friend class PrefixSetBuilder;
 
   friend class PrefixSetTest;
   FRIEND_TEST_ALL_PREFIXES(PrefixSetTest, AllBig);
   FRIEND_TEST_ALL_PREFIXES(PrefixSetTest, EdgeCases);
   FRIEND_TEST_ALL_PREFIXES(PrefixSetTest, Empty);
   FRIEND_TEST_ALL_PREFIXES(PrefixSetTest, FullHashBuild);
@@ skipping 69 matching lines @@
   // Helper for unit tests and format conversion.
   explicit PrefixSetBuilder(const std::vector<SBPrefix>& prefixes);
 
   // Add a prefix to the set.  Prefixes must arrive in ascending order.
   // Duplicate prefixes are dropped.
   void AddPrefix(SBPrefix prefix);
 
   // Flush any buffered prefixes, and return the final PrefixSet instance.
   // |hashes| are sorted and stored in |full_hashes_|.  Any call other than the
   // destructor is illegal after this call.
-  scoped_ptr<PrefixSet> GetPrefixSet(const std::vector<SBFullHash>& hashes);
+  scoped_ptr<const PrefixSet> GetPrefixSet(
+      const std::vector<SBFullHash>& hashes);
 
   // Helper for clients which only track prefixes.  Calls GetPrefixSet() with
   // empty hash vector.
-  scoped_ptr<PrefixSet> GetPrefixSetNoHashes();
+  scoped_ptr<const PrefixSet> GetPrefixSetNoHashes();
 
  private:
   // Encode a run of deltas for |AddRun()|.  The run is broken by a too-large
   // delta, or kMaxRun, whichever comes first.
   void EmitRun();
 
   // Buffers prefixes until enough are avaliable to emit a run.
   std::vector<SBPrefix> buffer_;
 
   // The PrefixSet being built.
   scoped_ptr<PrefixSet> prefix_set_;
 };
 
 }  // namespace safe_browsing
 
 #endif  // CHROME_BROWSER_SAFE_BROWSING_PREFIX_SET_H_