Use a different algorithm with the low entropy source for field trials.

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 76 matching lines @@
 #include "base/time.h"
 
 namespace base {
 
 class FieldTrialList;
 
 class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
  public:
   typedef int Probability;  // Probability type for being selected in a trial.
 
+  // EntropyProvider is an interface for providing entropy for one-time
+  // randomized (persistent) field trials.
+  class BASE_EXPORT EntropyProvider {
+   public:
+    virtual ~EntropyProvider() {}

[Ilya Sherman, 2012/08/17 07:34:28]

nit: Virtual functions should never be inlined.

[Alexei Svitkine (slow), 2012/08/17 14:08:59]

Done. I also fixed this for FieldTrialList::Observer below.

+
+    // Returns a double in the range of [0, 1) based on |trial_name| that will
+    // be used for the dice roll for the specified field trial. A given instance
+    // should always return the same value given the same input |trial_name|.
+    virtual double GetEntropyForTrial(const std::string& trial_name) = 0;
+  };
+
   // A pair representing a Field Trial and its selected group.
   struct SelectedGroup {
     std::string trial;
     std::string group;
   };
 
   typedef std::vector<SelectedGroup> SelectedGroups;
 
   // A return value to indicate that a given instance has not yet had a group
   // assignment (and hence is not yet participating in the trial).
@@ skipping 74 matching lines @@
 
   friend class base::FieldTrialList;
 
   friend class RefCounted<FieldTrial>;
 
   // 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;
 
-  FieldTrial(const std::string& name,
+  FieldTrial(EntropyProvider* entropy_provider,

[Ilya Sherman, 2012/08/17 07:34:28]

nit: Unless you're planning to modify the |entropy_provider| (I'd expect not),
this should be passed as a const pointer.

[Alexei Svitkine (slow), 2012/08/17 14:08:59]

Done.

+             const std::string& name,
              Probability total_probability,
              const std::string& default_group_name);
   virtual ~FieldTrial();
 
   // Return the default group name of the FieldTrial.
   std::string default_group_name() const { return default_group_name_; }
 
   // Sets the group_name as well as group_name_hash to make sure they are sync.
   void SetGroupChoice(const std::string& name, int number);
 
   // Returns the group_name. A winner need not have been chosen.
   std::string group_name_internal() const { return group_name_; }
 
-  // Calculates a uniformly-distributed double between [0.0, 1.0) given
-  // a |client_id| and a |trial_name| (the latter is used as salt to avoid
-  // separate one-time randomized trials from all having the same results).
-  static double HashClientId(const std::string& client_id,
-                             const std::string& trial_name);
+  // Entropy provider that will be used if the field trial uses one-time
+  // randomization. Weak pointer, owned by FieldTrialList.
+  EntropyProvider* entropy_provider_;

[Ilya Sherman, 2012/08/17 07:34:28]

nit: const EntropyProvider* const?

[Alexei Svitkine (slow), 2012/08/17 14:08:59]

Done.

 
   // The name of the field trial, as can be found via the FieldTrialList.
   const std::string name_;
 
   // The maximum sum of all probabilities supplied, which corresponds to 100%.
   // This is the scaling factor used to adjust supplied probabilities.
   const Probability divisor_;
 
   // The name of the default group.
   const std::string default_group_name_;
@@ skipping 51 matching lines @@
     // Notify observers when FieldTrials's group is selected.
     virtual void OnFieldTrialGroupFinalized(const std::string& trial_name,
                                             const std::string& group_name) = 0;
 
    protected:
     virtual ~Observer() {}
   };
 
   // This singleton holds the global list of registered FieldTrials.
   //
-  // |client_id| should be an opaque, diverse ID for this client that does not
-  // change between sessions, to enable one-time randomized trials. The empty
-  // string may be provided, in which case one-time randomized trials will
-  // not be available.
-  explicit FieldTrialList(const std::string& client_id);
+  // To support one-time randomized field trials, specify a non-NULL
+  // |entropy_provider| which should be a source of uniformly distributed
+  // entropy values.
+  explicit FieldTrialList(FieldTrial::EntropyProvider* entropy_provider);
+
   // Destructor Release()'s references to all registered FieldTrial instances.
   ~FieldTrialList();
 
   // Get a FieldTrial instance from the factory.
   //
   // |name| is used to register the instance with the FieldTrialList class,
   // and can be used to find the trial (only one trial can be present for each
   // name). |default_group_name| is the name of the default group which will
   // be chosen if none of the subsequent appended groups get to be chosen.
   // |default_group_number| can receive the group number of the default group as
@@ skipping 90 matching lines @@
   }
 
   // Return the number of active field trials.
   static size_t GetFieldTrialCount();
 
   // Returns true if you can call |FieldTrial::UseOneTimeRandomization()|
   // without error, i.e. if a non-empty string was provided as the client_id
   // when constructing the FieldTrialList singleton.
   static bool IsOneTimeRandomizationEnabled();
 
-  // Returns an opaque, diverse ID for this client that does not change
-  // between sessions.
-  //
-  // Returns the empty string if one-time randomization is not enabled.
-  static const std::string& client_id();
-
  private:
   // A map from FieldTrial names to the actual instances.
   typedef std::map<std::string, FieldTrial*> RegistrationList;
 
   // Helper function should be called only while holding lock_.
   FieldTrial* PreLockedFind(const std::string& name);
 
   // Register() stores a pointer to the given trial in a global map.
   // This method also AddRef's the indicated trial.
   // This should always be called after creating a new FieldTrial instance.
   static void Register(FieldTrial* trial);
 
   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_;
 
   // A helper value made available to users, that shows when the FieldTrialList
   // was initialized.  Note that this is a singleton instance, and hence is a
   // good approximation to the start of the process.
   TimeTicks application_start_time_;
 
   // Lock for access to registered_.
   base::Lock lock_;
   RegistrationList registered_;
 
-  // An opaque, diverse ID for this client that does not change
-  // between sessions, or the empty string if not initialized.
-  std::string client_id_;
+  // Entropy provider to be used for one-time randomized field trials. If NULL,
+  // one-time randomization is not supported.
+  scoped_ptr<FieldTrial::EntropyProvider> entropy_provider_;
 
   // List of observers to be notified when a group is selected for a FieldTrial.
   scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_