Save device choice on spawned control threads

The `RuntimeDeviceTracker` is a thread-local variable that monitors the
devices to use separately on each thread. This is an important feature
to allow different threads to control different devices.

When a tracker is created on a new thread, it was simply reset, which
makes sense. However, the reset does not take into account the device
selected by `vtkm::cont::Initialize`. This means that if VTK-m was used
in a different thread than it was initialized, it would ignore the
`--vtkm-device` parameter.

To get around this problem, keep track of the `RuntimeDeviceTracker` on
the "main" thread. When a `RuntimeDeviceTracker` is created on a new
thread, it copies the state from that tracker.

vtkm/cont/RuntimeDeviceTracker.cxx

@@ -276,32 +276,41 @@ ScopedRuntimeDeviceTracker::~ScopedRuntimeDeviceTracker()
 VTKM_CONT
 vtkm::cont::RuntimeDeviceTracker& GetRuntimeDeviceTracker()
 {
-#if defined(VTKM_CLANG) && defined(__apple_build_version__) && (__apple_build_version__ < 8000000)
-  static std::mutex mtx;
-  static std::map<std::thread::id, vtkm::cont::RuntimeDeviceTracker*> globalTrackers;
-  static std::map<std::thread::id, vtkm::cont::detail::RuntimeDeviceTrackerInternals*>
-    globalTrackerInternals;
-  std::thread::id this_id = std::this_thread::get_id();
-
-  std::unique_lock<std::mutex> lock(mtx);
-  auto iter = globalTrackers.find(this_id);
-  if (iter != globalTrackers.end())
+  using SharedTracker = std::shared_ptr<vtkm::cont::RuntimeDeviceTracker>;
+  static thread_local vtkm::cont::detail::RuntimeDeviceTrackerInternals details;
+  static thread_local SharedTracker runtimeDeviceTracker;
+  static std::weak_ptr<vtkm::cont::RuntimeDeviceTracker> defaultRuntimeDeviceTracker;
+
+  if (runtimeDeviceTracker)
   {
-    return *iter->second;
+    return *runtimeDeviceTracker;
+  }
+
+  // The RuntimeDeviceTracker for this thread has not been created. Create a new one.
+  runtimeDeviceTracker = SharedTracker(new vtkm::cont::RuntimeDeviceTracker(&details, true));
+
+  // Get the default details, which are a global variable, with thread safety
+  static std::mutex mutex;
+  std::unique_lock<std::mutex> lock(mutex);
+
+  SharedTracker defaultTracker = defaultRuntimeDeviceTracker.lock();
+
+  if (defaultTracker)
+  {
+    // We already have a default tracker, so copy the state from there. We don't need to
+    // keep our mutex locked because we already have a safe handle to the defaultTracker.
+    lock.unlock();
+    runtimeDeviceTracker->CopyStateFrom(*defaultTracker);
   }
   else
   {
-    auto* details = new vtkm::cont::detail::RuntimeDeviceTrackerInternals();
-    vtkm::cont::RuntimeDeviceTracker* tracker = new vtkm::cont::RuntimeDeviceTracker(details, true);
-    globalTrackers[this_id] = tracker;
-    globalTrackerInternals[this_id] = details;
-    return *tracker;
+    // There is no default tracker yet. It has never been created (or it was on a thread
+    // that got deleted). Use the current thread's details as the default.
+    defaultRuntimeDeviceTracker = runtimeDeviceTracker;
   }
-#else
-  static thread_local vtkm::cont::detail::RuntimeDeviceTrackerInternals details;
-  static thread_local vtkm::cont::RuntimeDeviceTracker runtimeDeviceTracker(&details, true);
-  return runtimeDeviceTracker;
-#endif
+
+  return *runtimeDeviceTracker;
 }
+
 }
 } // namespace vtkm::cont

vtkm/cont/testing/CMakeLists.txt

@@ -73,6 +73,7 @@ set(unit_tests
   UnitTestDataSetUniform.cxx
   UnitTestDeviceAdapterAlgorithmDependency.cxx
   UnitTestDeviceAdapterAlgorithmGeneral.cxx
+  UnitTestDeviceSelectOnThreads.cxx
   UnitTestDynamicCellSet.cxx
   UnitTestError.cxx
   UnitTestFieldRangeCompute.cxx

vtkm/cont/testing/UnitTestDeviceSelectOnThreads.cxx

+//============================================================================
+//  Copyright (c) Kitware, Inc.
+//  All rights reserved.
+//  See LICENSE.txt for details.
+//
+//  This software is distributed WITHOUT ANY WARRANTY; without even
+//  the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+//  PURPOSE.  See the above copyright notice for more information.
+//============================================================================
+
+#include <vtkm/cont/Initialize.h>
+#include <vtkm/cont/RuntimeDeviceTracker.h>
+
+#include <vtkm/cont/testing/Testing.h>
+
+#include <future>
+#include <vector>
+
+namespace
+{
+
+bool CheckLocalRuntime()
+{
+  if (!vtkm::cont::GetRuntimeDeviceTracker().CanRunOn(vtkm::cont::DeviceAdapterTagSerial{}))
+  {
+    std::cout << "Serial device not runable" << std::endl;
+    return false;
+  }
+
+  for (vtkm::Int8 deviceIndex = 0; deviceIndex < VTKM_MAX_DEVICE_ADAPTER_ID; ++deviceIndex)
+  {
+    vtkm::cont::DeviceAdapterId device = vtkm::cont::make_DeviceAdapterId(deviceIndex);
+    if (!device.IsValueValid() || (deviceIndex == VTKM_DEVICE_ADAPTER_SERIAL))
+    {
+      continue;
+    }
+    if (vtkm::cont::GetRuntimeDeviceTracker().CanRunOn(device))
+    {
+      std::cout << "Device " << device.GetName() << " declared as runnable" << std::endl;
+      return false;
+    }
+  }
+
+  return true;
+}
+
+void DoTest()
+{
+  VTKM_TEST_ASSERT(CheckLocalRuntime(),
+                   "Runtime check failed on main thread. Did you try to set a device argument?");
+
+  // Now check on a new thread. The runtime is a thread-local object so that each thread can
+  // use its own device. But when you start a thread, you want the default to be what the
+  // user selected on the main thread.
+  VTKM_TEST_ASSERT(std::async(std::launch::async, CheckLocalRuntime).get(),
+                   "Runtime loses defaults in spawned thread.");
+}
+
+} // anonymous namespace
+
+int UnitTestDeviceSelectOnThreads(int argc, char* argv[])
+{
+  // This test is checking to make sure that a device selected in the command line
+  // argument is the default for all threads. We will test this by adding an argument
+  // to select the serial device, which is always available. The test might fail if
+  // a different device is also selected.
+  std::string deviceSelectString("--vtkm-device=serial");
+  std::vector<char> deviceSelectArg(deviceSelectString.size());
+  std::copy(deviceSelectString.begin(), deviceSelectString.end(), deviceSelectArg.begin());
+  deviceSelectArg.push_back('\0');
+
+  std::vector<char*> newArgs;
+  for (int i = 0; i < argc; ++i)
+  {
+    newArgs.push_back(argv[i]);
+  }
+  newArgs.push_back(deviceSelectArg.data());
+  newArgs.push_back(nullptr);
+
+  int newArgc = argc + 1;
+
+  return vtkm::cont::testing::Testing::Run(DoTest, newArgc, newArgs.data());
+}