Use SharedPersistentMemoryAllocator to share field trial state

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 51 matching lines @@
 #include <set>
 #include <string>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/command_line.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/shared_memory.h"
+#include "base/metrics/persistent_memory_allocator.h"
 #include "base/observer_list_threadsafe.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
 
 namespace base {
 
 class FieldTrialList;
 
 class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
@@ skipping 235 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrial);
 };
 
 //------------------------------------------------------------------------------
 // Class with a list of all active field trials.  A trial is active if it has
 // been registered, which includes evaluating its state based on its probaility.
 // Only one instance of this class exists.
 class BASE_EXPORT FieldTrialList {
  public:
+  // Allocator used instantiate field trial in child processes.
+  static base::SharedPersistentMemoryAllocator* field_trial_allocator;

[bcwhite, 2016/10/13 14:28:02]

I don't suppose there is a more generic place to hold this?  It could be
"child_process_info_allocator" or something, the idea being that it could be
then (re-)used if/when other information comes along that would benefit from
being passed this way.

[lawrencewu, 2016/10/14 04:54:10]

While that would be nice, I'm reluctant to do that now since I don't see an
immediate need for that yet and it would probably require extra effort to hoist
this variable up into a more appropriate class. (This CL itself is already kind
of an effort to hoist the shared memory from gpu_process_host.cc and
render_process_host_impl.cc)

[bcwhite, 2016/10/14 13:56:56]

That's fine.  Add a comment saying that such _could_ be done if extra data
passing is desired.  And keep it in mind as you develop as sometimes there are
choices that would make such a future effort easier.

[lawrencewu, 2016/10/19 16:21:28]

Done.

+
   // Year that is guaranteed to not be expired when instantiating a field trial
   // via |FactoryGetFieldTrial()|.  Set to two years from the build date.
   static int kNoExpirationYear;
 
   // Observer is notified when a FieldTrial's group is selected.
   class BASE_EXPORT Observer {
    public:
     // Notify observers when FieldTrials's group is selected.
     virtual void OnFieldTrialGroupFinalized(const std::string& trial_name,
                                             const std::string& group_name) = 0;
@@ skipping 135 matching lines @@
   // If using shared memory to pass around the list of field trials, then
   // expects |field_trial_handle_switch| command line argument to
   // contain the shared memory handle.
   // If not, then create the trials as before (using the kForceFieldTrials
   // switch). Needs the |field_trial_handle_switch| argument to be passed in
   // since base/ can't depend on content/.
   static void CreateTrialsFromCommandLine(
       const base::CommandLine& cmd_line,
       const char* field_trial_handle_switch);
 
+  // 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.
+  // This function recreates the allocator, iterates through all the field
+  // trials in it, and creates them via CreateFieldTrial().
+  static void CreateTrialsFromSharedMemory(
+      std::unique_ptr<base::SharedMemory> shm);

[Alexei Svitkine (slow), 2016/10/12 20:41:00]

If you're only calling this from the .cc file, it should be in the anon
namespace in that file. Unless it needs to reference private functions/fields of
the class, in which case you can keep it a static member, but make it private.

[lawrencewu, 2016/10/14 04:54:09]

It needs to reference regstration_ when creating the field trial, so I made it
private instead.

+
   // Adds a switch to the command line containing the field trial state as a
   // string (if not using shared memory to share field trial state), or the
   // shared memory handle + length.
   // Needs the |field_trial_handle_switch| argument to be passed in since base/
   // can't depend on content/.
-  static std::unique_ptr<base::SharedMemory> CopyFieldTrialStateToFlags(
-      const char* field_trial_handle_switch,
-      base::CommandLine* cmd_line);
+  static void CopyFieldTrialStateToFlags(const char* field_trial_handle_switch,
+                                         base::CommandLine* cmd_line);
 
   // 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.
   // 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);
 
   // Add an observer to be notified when a field trial is irrevocably committed
   // to being part of some specific field_group (and hence the group_name is
   // also finalized for that field_trial).
   static void AddObserver(Observer* observer);
 
   // Remove an observer.
   static void RemoveObserver(Observer* observer);
 
   // Notify all observers that a group has been finalized for |field_trial|.
   static void NotifyFieldTrialGroupSelection(FieldTrial* field_trial);
 
+  // Called in NotifyFieldTrialGroupSelection(). Adds a new entry to
+  // field_trial_allocator containing the newly-selected field trial and group.
+  static void UpdateFieldTrialAllocator(FieldTrial* field_trial);

[bcwhite, 2016/10/13 14:28:02]

Your comment says "adds" but the name says "update".

[lawrencewu, 2016/10/14 04:54:09]

It is kind of an "update the list of field trials by adding this new one", but I
changed the comment to more clearly reflect that.

+
   // Return the number of active field trials.
   static size_t GetFieldTrialCount();
 
  private:
   // A map from FieldTrial names to the actual instances.
   typedef std::map<std::string, FieldTrial*> RegistrationMap;
 
   // If one-time randomization is enabled, returns a weak pointer to the
   // corresponding EntropyProvider. Otherwise, returns NULL.
   static const FieldTrial::EntropyProvider*
@@ skipping 27 matching lines @@
 
   // 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_