reland 1: "mac: Run A/B experiment on SharedMemory mechanism for resource buffers."

The first CL used a new UMA feature FeatureList. FeatureList is not yet ready
for production use, so I've switched back to FieldTrial.

> The CL changes the default behavior of resource buffer to use a Mach-backed
> SharedMemory region. This CL adds a field trial to measure the effect of Mach vs
> POSIX backed SharedMemory regions.
>
> BUG=547261, 466437
> Committed: https://crrev.com/e0e2fd398f3d07c8eebbe662d71a0f9286862476
> Cr-Commit-Position: refs/heads/master@{#356513}

BUG=547261, 466437
TBR=mark@chromium.org, isherman@chromium.org, asvitkine@chromium.org, avi@chromium.org

Review URL: https://codereview.chromium.org/1412923003

Cr-Commit-Position: refs/heads/master@{#356620}

3 files changed, 54 insertions, 0 deletions

diff --git a/base/memory/shared_memory.h b/base/memory/shared_memory.h
index 18416c0..5cdcb0d 100644
--- a/base/memory/shared_memory.h
+++ b/base/memory/shared_memory.h
@@ -140,6 +140,10 @@ class BASE_EXPORT SharedMemory {
   // http://crbug.com/466437.
   bool CreateAndMapAnonymousPosix(size_t size);
   bool CreateAnonymousPosix(size_t size);
+
+  // This method is an analog of CreateAndMapAnonymous that forces the
+  // underlying OS primitive to be a Mach memory object.
+  bool CreateAndMapAnonymousMach(size_t size);
 #endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
   // Creates an anonymous shared memory segment of size size.
diff --git a/base/memory/shared_memory_handle.h b/base/memory/shared_memory_handle.h
index b3dfc8f..49398a4 100644
--- a/base/memory/shared_memory_handle.h
+++ b/base/memory/shared_memory_handle.h
@@ -75,12 +75,15 @@ class BASE_EXPORT SharedMemoryHandle {
 #else
 class BASE_EXPORT SharedMemoryHandle {
  public:
+  // The values of these enums must not change, as they are used by the
+  // histogram OSX.SharedMemory.Mechanism.
   enum Type {
     // The SharedMemoryHandle is backed by a POSIX fd.
     POSIX,
     // The SharedMemoryHandle is backed by the Mach primitive "memory object".
     MACH,
   };
+  static const int TypeMax = 2;
 
   // The format that should be used to transmit |Type| over the wire.
   typedef int TypeWireFormat;
diff --git a/base/memory/shared_memory_mac.cc b/base/memory/shared_memory_mac.cc
index 799b8e3..47b8a89 100644
--- a/base/memory/shared_memory_mac.cc
+++ b/base/memory/shared_memory_mac.cc
@@ -14,6 +14,8 @@
 #include "base/files/scoped_file.h"
 #include "base/logging.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 +31,38 @@ 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 +261,17 @@ bool SharedMemory::CreateAnonymousPosix(size_t size) {
   return Create(options);
 }
 
+bool SharedMemory::CreateAndMapAnonymousMach(size_t size) {
+  SharedMemoryCreateOptions options;
+
+  // 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();
+  options.size = size;
+  return Create(options) && Map(size);
+}
+
 // static
 bool SharedMemory::GetSizeFromSharedMemoryHandle(
     const SharedMemoryHandle& handle,
@@ -248,6 +293,8 @@ 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;