Store field trial parameters in shared memory

base/metrics/field_trial_param_associator.h

 // Copyright 2016 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 BASE_METRICS_FIELD_TRIAL_PARAM_ASSOCIATOR_H_
 #define BASE_METRICS_FIELD_TRIAL_PARAM_ASSOCIATOR_H_
 
 #include <map>
 #include <string>
 #include <utility>
 
 #include "base/base_export.h"
 #include "base/memory/singleton.h"
+#include "base/metrics/field_trial.h"
 #include "base/synchronization/lock.h"
 
 namespace base {
 
 // Keeps track of the parameters of all field trials and ensures access to them
 // is thread-safe.
 class BASE_EXPORT FieldTrialParamAssociator {
  public:
   FieldTrialParamAssociator();
   ~FieldTrialParamAssociator();
 
   // Key-value mapping type for field trial parameters.
   typedef std::map<std::string, std::string> FieldTrialParams;
 
   // Retrieve the singleton.
   static FieldTrialParamAssociator* GetInstance();
 
   // Sets parameters for the given field trial name and group.
   bool AssociateFieldTrialParams(const std::string& trial_name,
                                  const std::string& group_name,
                                  const FieldTrialParams& params);
 
-  // Gets the parameters for a field trial and its chosen group.
+  // Gets the parameters for a field trial and its chosen group. If not found in
+  // field_trial_params_, then tries to looks it up in shared memory.
   bool GetFieldTrialParams(const std::string& trial_name,
                            FieldTrialParams* params);
 
+  // Gets the parameters for a field trial and its chosen group. Does not
+  // fallback to looking it up in shared memory. This should only be used if you
+  // know for sure the params are in the mapping, like if you're in the browser
+  // process, and even then you should probably just use GetFieldTrialParams().

[bcwhite, 2016/11/23 21:03:52]

So...  When _should_ this method be used?

[lawrencewu, 2016/11/23 21:49:20]

Pretty much never, except for where it's currently used (in PickleFieldTrial()).
But since that's in a different module this has to be public.

+  bool GetFieldTrialParamsWithoutFallback(const std::string& trial_name,
+                                          const std::string& group_name,
+                                          FieldTrialParams* params);
+
+  // Clears the internal field_trial_params_ mapping, plus removes all params in
+  // shared memory.
+  void ClearAllParamsForTesting();
+
   // Clears the internal field_trial_params_ mapping.
-  void ClearAllParamsForTesting();
+  void ClearAllCachedParamsForTesting();
 
  private:
   friend struct DefaultSingletonTraits<FieldTrialParamAssociator>;
 
   // (field_trial_name, field_trial_group)
   typedef std::pair<std::string, std::string> FieldTrialKey;
 
   Lock lock_;
   std::map<FieldTrialKey, FieldTrialParams> field_trial_params_;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialParamAssociator);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_PARAM_ASSOCIATOR_H_