Improved support for objects inside persistent memory.

base/metrics/field_trial.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.
 
 // FieldTrial is a class for handling details of statistical experiments
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
 // The simplest example is an experiment to see whether one of two options
 // produces "better" results across our user population.  In that scenario, UMA
@@ skipping 119 matching lines @@
     StringPiece group_name;
     bool activated;
 
     State();
     State(const State& other);
     ~State();
   };
 
   // We create one FieldTrialEntry per field trial in shared memory, via
   // AddToAllocatorWhileLocked. The FieldTrialEntry is followed by a
-  // base::Pickle object that we unpickle and read from. Any changes to this
-  // structure requires a bump in kFieldTrialType id defined in the .cc file.
+  // base::Pickle object that we unpickle and read from.
   struct BASE_EXPORT FieldTrialEntry {
+    // SHA1(FieldTrialEntry): Increment this if structure changes!
+    static constexpr uint32_t kPersistentTypeId = 0xABA17E13 + 2;
+
     // Expected size for 32/64-bit check.
     static constexpr size_t kExpectedInstanceSize = 8;
 
     // Whether or not this field trial is activated. This is really just a
     // boolean but using a 32 bit value for portability reasons. It should be
     // accessed via NoBarrier_Load()/NoBarrier_Store() to prevent the compiler
     // from doing unexpected optimizations because it thinks that only one
     // thread is accessing the memory location.
     subtle::Atomic32 activated;
 
@@ skipping 576 matching lines @@
 
   // Tracks whether CreateTrialsFromCommandLine() has been called.
   bool create_trials_from_command_line_called_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_