reland 2: Switch back to field trial.

base/memory/shared_memory_mac.cc

Index: base/memory/shared_memory_mac.cc
diff --git a/base/memory/shared_memory_mac.cc b/base/memory/shared_memory_mac.cc
index 799b8e3dfcda0acb8006e119d980bc411a17d6af..8f198dcbf75aaeab13c7ffcad774b0ec7e3ffbb7 100644
--- a/base/memory/shared_memory_mac.cc
+++ b/base/memory/shared_memory_mac.cc
@@ -13,7 +13,10 @@
 #include "base/files/file_util.h"
 #include "base/files/scoped_file.h"
 #include "base/logging.h"
+#include "base/mac/mac_util.h"
 #include "base/mac/scoped_mach_vm.h"
+#include "base/metrics/field_trial.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/posix/safe_strerror.h"
 #include "base/process/process_metrics.h"
@@ -29,6 +32,38 @@ namespace base {
 
 namespace {
 
+const char kTrialName[] = "MacMemoryMechanism";
+const char kTrialMach[] = "Mach";
+const char kTrialPosix[] = "Posix";
+
+SharedMemoryHandle::Type GetABTestMechanism() {
+  static bool found_group = false;
+  static SharedMemoryHandle::Type group = SharedMemoryHandle::MACH;
+
+  if (found_group)
+    return group;
+
+  const std::string group_name =
+      base::FieldTrialList::FindFullName(kTrialName);
+  if (group_name == kTrialMach) {
+    group = SharedMemoryHandle::MACH;
+    found_group = true;
+  } else if (group_name == kTrialPosix) {
+    group = SharedMemoryHandle::POSIX;
+    found_group = true;
+  } else {
+    group = SharedMemoryHandle::MACH;
+  }
+
+  return group;
+}
+
+// Emits a histogram entry indicating which type of SharedMemory was created.
+void EmitMechanism(SharedMemoryHandle::Type type) {
+  UMA_HISTOGRAM_ENUMERATION("OSX.SharedMemory.Mechanism", type,
+                            SharedMemoryHandle::TypeMax);
+}
+
 // Returns whether the operation succeeded.
 // |new_handle| is an output variable, populated on success. The caller takes
 // ownership of the underlying memory object.
@@ -227,6 +262,22 @@ bool SharedMemory::CreateAnonymousPosix(size_t size) {
   return Create(options);
 }
 
+bool SharedMemory::CreateAndMapAnonymousMach(size_t size) {
+  SharedMemoryCreateOptions options;
+
+  if (mac::IsOSLionOrLater()) {
+    // A/B test the mechanism. Once the experiment is over, this will always be
+    // set to SharedMemoryHandle::MACH.
+    // http://crbug.com/547261
+    options.type = GetABTestMechanism();
+  } else {
+    // Mach shared memory isn't supported on OSX 10.6 or older.
+    options.type = SharedMemoryHandle::POSIX;
+  }
+  options.size = size;
+  return Create(options) && Map(size);
+}
+
 // static
 bool SharedMemory::GetSizeFromSharedMemoryHandle(
     const SharedMemoryHandle& handle,
@@ -248,6 +299,8 @@ bool SharedMemory::Create(const SharedMemoryCreateOptions& options) {
   if (options.size > static_cast<size_t>(std::numeric_limits<int>::max()))
     return false;
 
+  EmitMechanism(options.type);
+
   if (options.type == SharedMemoryHandle::MACH) {
     shm_ = SharedMemoryHandle(options.size);
     requested_size_ = options.size;