Share field trial allocator on zygote-using Linuxes

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 fa283271beb05da1a1f60d74eb5e5370c0155487..af5f68958a82b5222a16ac0acbc0159c4185853f 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -511,6 +511,11 @@ class BASE_EXPORT FieldTrialList {
   // list of handles to be inherited.
   static void AppendFieldTrialHandleIfNeeded(
       base::HandlesToInheritVector* handles);
+#elif defined(OS_POSIX)
+  // On POSIX, we also need to explicitly pass down any fds to be inherited via
+  // |files_to_register| in child_process_launcher.cc. This gets the handle so
+  // we can add it to the mapping there.

[Alexei Svitkine (slow), 2016/11/23 17:49:55]

You check the return value of this for -1 but don't document that. Please do.

[lawrencewu, 2016/11/23 18:59:44]

Done.

+  static int FieldTrialHandle();

[Alexei Svitkine (slow), 2016/11/23 17:49:55]

Functions should use a verb. GetFieldTrialHandle()?

[lawrencewu, 2016/11/23 18:59:44]

Done.

 #endif
 
   // Adds a switch to the command line containing the field trial state as a
@@ -554,12 +559,22 @@ class BASE_EXPORT FieldTrialList {
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AddTrialsToAllocator);
 
 #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
+  // Takes in |cli_switch| that should have been retrieved from the command line
+  // and constructs a SharedMemoryHandle from it, and then calls
   // CreateTrialsFromSharedMemory(). Returns true on success, false on failure.
-  static bool CreateTrialsFromWindowsHandle(HANDLE handle);
+  static bool CreateTrialsFromWindowsSwitch(std::string cli_switch);

[Alexei Svitkine (slow), 2016/11/23 17:49:55]

Don't pass strings by value.

const std::string&

Also, avoid abbreviations/acronyms - so command_line_switch - or handle_switch.

[lawrencewu, 2016/11/23 18:59:44]

Done.

+#elif defined(OS_POSIX)
+  // Takes in |cli_switch| that should have been retrieved from the command
+  // line, looks up the fd using GlobalDescriptor, constructs a
+  // SharedMemoryHandle from it, and then calls CreateTrialsFromSharedMemory().
+  // Returns true on success, false on failure.
+  static bool CreateTrialsFromPosixSwitch(std::string cli_switch);

[Alexei Svitkine (slow), 2016/11/23 17:49:55]

Do we need two separate functions? Why not keeping it the same one and have the
details be in the implementation?

[lawrencewu, 2016/11/23 18:59:44]

Yeah, that works better. Done.

 #endif
 
+  // Takes an unmapped SharedMemoryHandle, creates a SharedMemory object from it
+  // and maps it with the correct size.
+  static bool CreateTrialsFromSharedMemoryHandle(SharedMemoryHandle shm_handle);
+
   // 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
@@ -628,6 +643,8 @@ class BASE_EXPORT FieldTrialList {
   // because it's needed from both CopyFieldTrialStateToFlags() and
   // AppendFieldTrialHandleIfNeeded().
   HANDLE readonly_allocator_handle_ = nullptr;
+#elif defined(OS_POSIX)
+  int readonly_allocator_handle_ = -1;
 #endif
 
   // Tracks whether CreateTrialsFromCommandLine() has been called.