Use SharedPersistentMemoryAllocator to share field trial state

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 99e705a1fa82dd80a34701cbad08e857c589d9f7..8732374a38f4eaa8e407e95c6569e20157f2a86f 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -69,6 +69,7 @@
 #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"
@@ -324,6 +325,9 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
 // 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;
@@ -479,14 +483,20 @@ class BASE_EXPORT FieldTrialList {
       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
@@ -508,6 +518,10 @@ class BASE_EXPORT FieldTrialList {
   // 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();