Supporting command line argument to force field trials

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
diff --git a/base/metrics/field_trial.h b/base/metrics/field_trial.h
index 50508d241425e558f4f4d737cf352947a86f0bef..e1c6c5f4a050f2b0d4a280d66df6a47fc557194c 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
@@ -103,26 +103,6 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
   // assignment (and hence is not yet participating in the trial).
   static const int kNotFinalized;
 
-  // This is the group number of the 'default' group. This provides an easy way
-  // to assign all the remaining probability to a group ('default').
-  static const int kDefaultGroupNumber;
-
-  // The 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).  |name| and |default_group_name| may not be empty.
-  //
-  // Group probabilities that are later supplied must sum to less than or equal
-  // to the total_probability. Arguments year, month and day_of_month specify
-  // the expiration time. If the build time is after the expiration time then
-  // the field trial reverts to the 'default' group.
-  //
-  // Using this constructor creates a startup-randomized FieldTrial. If you
-  // want a one-time randomized trial, call UseOneTimeRandomization() right
-  // after construction.
-  FieldTrial(const std::string& name, Probability total_probability,
-             const std::string& default_group_name, const int year,
-             const int month, const int day_of_month);
-
   // Changes the field trial to use one-time randomization, i.e. produce the
   // same result for the current trial on every run of this client. Must be
   // called right after construction.
@@ -145,7 +125,6 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
   std::string name() const { return name_; }
 
   // Return the randomly selected group number that was assigned.
-  // Return kDefaultGroupNumber if the instance is in the 'default' group.
   // Note that this will force an instance to participate, and make it illegal
   // to attempt to probabilistically add any other groups to the trial.
   int group();
@@ -156,8 +135,8 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
 
   // Gets the unique identifier of the Field Trial, but only if a group was
   // officially chosen, otherwise name_group_id is left untouched and false
-  // is returned. When true is returned, the name and group ids were successfuly
-  // set in name_group_id.
+  // is returned. When true is returned, the name and group ids were
+  // successfully set in name_group_id.
   bool GetNameGroupId(NameGroupId* name_group_id);
 
   // Return the default group name of the FieldTrial.
@@ -193,6 +172,14 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
 
   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, Probability total_probability,
+             const std::string& default_group_name, const int year,
+             const int month, const int day_of_month);
   virtual ~FieldTrial();
 
   // Returns the group_name. A winner need not have been chosen.
@@ -247,6 +234,10 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
   // group.
   bool enable_field_trial_;
 
+  // When forced_ is true, we return the chosen group from AppendGroup when
+  // appropriate.
+  bool forced_;
+
   // When benchmarking is enabled, field trials all revert to the 'default'
   // group.
   static bool enable_benchmarking_;
@@ -263,7 +254,7 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
 // Only one instance of this class exists.
 class BASE_EXPORT FieldTrialList {
  public:
-  // Define a separator charactor to use when creating a persistent form of an
+  // Define a separator character to use when creating a persistent form of an
   // instance.  This is intended for use as a command line argument, passed to a
   // second process to mimic our state (i.e., provide the same group name).
   static const char kPersistentStringSeparator;  // Currently a slash.
@@ -292,9 +283,26 @@ class BASE_EXPORT FieldTrialList {
   // Destructor Release()'s references to all registered FieldTrial instances.
   ~FieldTrialList();
 
-  // Register() stores a pointer to the given trial in a global map.
-  // This method also AddRef's the indicated trial.
-  static void Register(FieldTrial* trial);
+  // Creates a new FieldTrial instance if none exists already. If one exists,
+  // it has to be because it was forced by CreateFieldTrial.
+  //
+  // The 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).  |name| and |default_group_name| may not be empty but

[Alexei Svitkine (slow), 2012/03/29 19:19:07]

Nit: Either remove the |'s around |name| and |default_group_name| (And start
the sentence with "Arguments name and default_group_name .."), or put |'s around
the other uses of variable names in this comment, for consistency.

[MAD, 2012/04/02 18:31:15]

Done.

+  // default_group_number can be NULL if the value is not needed.

[Alexei Svitkine (slow), 2012/03/29 19:19:07]

Please explain what |default_group_number| is for, rather than just mentioning
it can be NULL.

[MAD, 2012/04/02 18:31:15]

Done.

+  //
+  // Group probabilities that are later supplied must sum to less than or equal
+  // to the total_probability. Arguments year, month and day_of_month specify
+  // the expiration time. If the build time is after the expiration time then
+  // the field trial reverts to the 'default' group.
+  //
+  // Use this static method to get a startup-randomized FieldTrial or a
+  // previously created forced FieldTrial. If you want a one-time randomized
+  // trial, call UseOneTimeRandomization() right after creation.
+  static FieldTrial* GetFieldTrialInstance(
+      const std::string& name, FieldTrial::Probability total_probability,
+      const std::string& default_group_name, int* default_group_number,
+      const int year, const int month, const int day_of_month);
 
   // The Find() method can be used to test to see if a named Trial was already
   // registered, or to retrieve a pointer to it from the global map.
@@ -311,11 +319,12 @@ class BASE_EXPORT FieldTrialList {
   // Returns true if the named trial has been registered.
   static bool TrialExists(const std::string& name);
 
-  // Create a persistent representation of all FieldTrial instances and the
-  // |client_id()| state 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
-  // |client_id()|, the names, the trial name, and a "/" separator.
+  // Creates a persistent representation of all 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 for all trials,
+  // with "/" used to separate all names and to terminate the string. This
+  // string is parsed by CreateTrialsFromString().
   static void StatesToString(std::string* output);
 
   // Returns an array of Unique IDs for each Field Trial that has a chosen
@@ -324,24 +333,20 @@ class BASE_EXPORT FieldTrialList {
   static void GetFieldTrialNameGroupIds(
       std::vector<FieldTrial::NameGroupId>* name_group_ids);
 
-  // Use a previously generated state string (re: StatesToString()) augment the
-  // current list of field tests to include the supplied tests, and using a 100%
-  // probability for each test, force them to have the same group string.  This
-  // is commonly used in a non-browser process, to carry randomly selected state
-  // in a browser process into this non-browser process. This method calls
-  // CreateFieldTrial to create the FieldTrial in the non-browser process.
-  // Currently only the group_name_ and name_ are restored, as well as the
-  // |client_id()| state, that could be used for one-time randomized trials
-  // set up only in child processes.
-  static bool CreateTrialsInChildProcess(const std::string& prior_trials);
+  // Use a state string (re: StatesToString()) to augment the current list of
+  // field tests to include the supplied tests, and using a 100% probability for
+  // each test, force them to have the same group string.  This is commonly used
+  // in a non-browser process, to carry randomly selected state in a browser
+  // process into this non-browser process, but could also be invoked through a
+  // command line argument to the browser process.
+  static bool CreateTrialsFromString(const std::string& prior_trials);
 
   // Create a FieldTrial with the given |name| and using 100% probability for
   // the FieldTrial, force FieldTrial to have the same group string as
   // |group_name|. This is commonly used in a non-browser process, to carry
   // randomly selected state in a browser process into this non-browser process.
-  // Currently only the group_name_ and name_ are set in the FieldTrial. It
-  // returns NULL if there is a FieldTrial that is already registered with the
-  // same |name| but has different finalized group string (|group_name|).
+  // It returns NULL if there is a FieldTrial that is already registered with
+  // the same |name| but has different finalized group string (|group_name|).
   static FieldTrial* CreateFieldTrial(const std::string& name,
                                       const std::string& group_name);
 
@@ -390,6 +395,11 @@ class BASE_EXPORT FieldTrialList {
   // 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