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 34b9782bee0fe06c51abf1383c3d98c247c4a958..658ca1187613348e74683c12d505847a3c176366 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -513,6 +513,12 @@ 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

[Alexei Svitkine (slow), 2016/11/24 17:44:34]

I don't think this should mention child_process_launcher.cc - which is in a
different layer.

I suggest just describing this more generally - i.e. the file descriptor that
should be shared with the child process.

[lawrencewu, 2016/11/24 18:16:13]

Done.

+  // we can add it to the mapping there. Returns -1 if no handle exists or was
+  // not initialized properly.
+  static int GetFieldTrialHandle();
 #endif
 
   // Adds a switch to the command line containing the field trial state as a
@@ -557,12 +563,14 @@ class BASE_EXPORT FieldTrialList {
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest,
                            DoNotAddSimulatedFieldTrialsToAllocator);
 
-#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
-  // CreateTrialsFromSharedMemory(). Returns true on success, false on failure.
-  static bool CreateTrialsFromWindowsHandle(HANDLE handle);
-#endif
+  // Takes in |handle_switch| from the command line which represents the shared
+  // memory handle for field trials, parses it, and creates the field trials.
+  // Returns true on success, false on failure.
+  static bool CreateTrialsFromHandleSwitch(const std::string& handle_switch);
+
+  // 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.
@@ -632,6 +640,8 @@ 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;

[Alexei Svitkine (slow), 2016/11/24 17:44:34]

Hmm, how about having this be the same var on Win and Posix and use PlatformFile
as the type - which is typedef'd to int on POSIX and HANDLE on Windows.

Them, you can use kInvalidPlatformFile for the default value and document that
as the return value of the other function.

[lawrencewu, 2016/11/24 18:16:13]

That's much cleaner, done.

 #endif
 
   // Tracks whether CreateTrialsFromCommandLine() has been called.