Store field trial parameters in shared 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 208 matching lines @@
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedTurnFeatureOn);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedChangeDefault_Default);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedChangeDefault_NonDefault);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, FloatBoundariesGiveEqualGroupSizes);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, DoesNotSurpassTotalProbability);
 
   friend class base::FieldTrialList;
 
   friend class RefCounted<FieldTrial>;
 
+  friend class FieldTrialParamAssociator;
+
   // This is the group number of the 'default' group when a choice wasn't forced
   // by a call to FieldTrialList::CreateFieldTrial. It is kept private so that
   // consumers don't use it by mistake in cases where the group was forced.
   static const int kDefaultGroupNumber;
 
   // Creates a field trial with the specified parameters. Group assignment will
   // be done based on |entropy_value|, which must have a range of [0, 1).
   FieldTrial(const std::string& trial_name,
              Probability total_probability,
              const std::string& default_group_name,
@@ skipping 187 matching lines @@
   // Returns the group number chosen for the named trial, or
   // FieldTrial::kNotFinalized if the trial does not exist.
   static int FindValue(const std::string& trial_name);
 
   // Returns the group name chosen for the named trial, or the empty string if
   // the trial does not exist. The first call of this function on a given field
   // trial will mark it as active, so that its state will be reported with usage
   // metrics, crashes, etc.
   static std::string FindFullName(const std::string& trial_name);
 
+  // Looks in the allocator for |field_trial|'s params via its ref_, and stores
+  // it in |params|.

[Alexei Svitkine (slow), 2016/11/23 17:23:39]

This is talking about implementation detail.

Please change this comment to talk about what it does without implementation
detail - since it's a public function.

Also, move these new functions to the end of the list of public functions - they
shouldn't be in the middle of commonly used ones that are expected to be used by
many callers.

[lawrencewu, 2016/11/23 19:07:33]

Done.

+  static bool GetParamsFromSharedMemory(
+      FieldTrial* field_trial,
+      std::map<std::string, std::string>* params);
+
+  // Clears all the params in the allocator.
+  static void ClearParamsFromSharedMemoryForTesting();
+
   // Returns true if the named trial has been registered.
   static bool TrialExists(const std::string& trial_name);
 
   // Returns true if the named trial exists and has been activated.
   static bool IsTrialActive(const std::string& trial_name);
 
   // Creates a persistent representation of active FieldTrial instances for
   // resurrection in another process. This allows randomization to be done in
   // one process, and secondary processes can be synchronized on the result.
   // The resulting string contains the name and group name pairs of all
@@ skipping 91 matching lines @@
   // Notify all observers that a group has been finalized for |field_trial|.
   static void NotifyFieldTrialGroupSelection(FieldTrial* field_trial);
 
   // Return the number of active field trials.
   static size_t GetFieldTrialCount();
 
  private:
   // Allow tests to access our innards for testing purposes.
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, InstantiateAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AddTrialsToAllocator);
+  FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AssociateFieldTrialParams);
 
 #if defined(OS_WIN)
   // Takes in |handle| that should have been retrieved from the command line and
   // creates a SharedMemoryHandle from it, and then calls
   // CreateTrialsFromSharedMemory(). Returns true on success, false on failure.
   static bool CreateTrialsFromWindowsHandle(HANDLE handle);
 #endif
 
   // Expects a mapped piece of shared memory |shm| that was created from the
   // browser process's field_trial_allocator and shared via the command line.
@@ skipping 33 matching lines @@
 
   static FieldTrialList* global_;  // The singleton of this class.
 
   // This will tell us if there is an attempt to register a field
   // trial or check if one-time randomization is enabled without
   // creating the FieldTrialList. This is not an error, unless a
   // FieldTrialList is created after that.
   static bool used_without_global_;
 
   // Lock for access to registered_.
-  base::Lock lock_;
+  Lock lock_;
   RegistrationMap registered_;
 
   std::map<std::string, std::string> seen_states_;
 
   // Entropy provider to be used for one-time randomized field trials. If NULL,
   // one-time randomization is not supported.
   std::unique_ptr<const FieldTrial::EntropyProvider> entropy_provider_;
 
   // List of observers to be notified when a group is selected for a FieldTrial.
   scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
@@ skipping 10 matching lines @@
   // AppendFieldTrialHandleIfNeeded().
   HANDLE readonly_allocator_handle_ = nullptr;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_