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. This version of the API doesn't activate
+  // the trial, which is what we want, since serializing to child processes
+  // shouldn't activate the trial.
   bool GetFieldTrialParams(const std::string& trial_name,
                            FieldTrialParams* params);
 
+  // Same as above, except doesn't search in FieldTrialList for the group name,
+  // since that can cause a deadlock if calling from FieldTrialList.
+  bool GetFieldTrialParamsWithGroupName(const std::string& trial_name,
+                                        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_