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>
@@ skipping 15 matching lines @@
   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. If the parameters are not in the
+  // mapping, then check the field trial entry in shared memory for the
+  // parameters and cache them here.
   bool GetFieldTrialParams(const std::string& trial_name,
                            FieldTrialParams* params);
 
+  // Same as above.
+  bool GetFieldTrialParams(const std::string& trial_name,

[Alexei Svitkine (slow), 2016/11/01 19:13:27]

Overloads are discouraged by the style guide. Change the name - e.g. add a
ByGroupName suffix. Also expand comment since it's not quite the same as above.

One important thing to mention is this version of the API doesn't activate the
trial. This is actually what we want for this use case - since serializing to
child processes shouldn't activate the trial. But it's important to explicitly
note this in the comment.

[lawrencewu, 2016/11/01 19:59:58]

Expanded the comment and changed the function to
GetFieldTrialParamsWithGroupName().

+                           const std::string& group_name,
+                           FieldTrialParams* params);
+
   // Clears the internal field_trial_params_ mapping.
   void ClearAllParamsForTesting();
 
  private:
   friend struct DefaultSingletonTraits<FieldTrialParamAssociator>;
 
   // (field_trial_name, field_trial_group)
   typedef std::pair<std::string, std::string> FieldTrialKey;
 
+  // Looks up the field trial entry in shared memory and gets the params from
+  // there.
+  bool GetParamsFromSharedMemory(const std::string& trial_name,
+                                 const std::string& group_name,
+                                 FieldTrialParams* params);
+
+  // Sets the parameters for |trial_name| and |group_name|. Caller must lock.
+  bool AssociateParamsWhileLocked(const std::string& trial_name,
+                                  const std::string& group_name,
+                                  const FieldTrialParams& params);
+
   Lock lock_;
   std::map<FieldTrialKey, FieldTrialParams> field_trial_params_;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialParamAssociator);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_PARAM_ASSOCIATOR_H_