[SYCL] Create static shared-release handler after loading plugins (#4843)

To enforce default contexts to be released before global
variables in plugins and associated libraries, these changes intializes
a function-local static variable after the plugins have been loaded.
Due to the guarantees of std::exit the destructor of the new
function-local variable will be called before the destructor of all
global variables initialized prior to the function-local static
variable's initialization.

Signed-off-by: Steffen Larsen <steffen.larsen@intel.com>

Author: steffenlarsen

sycl/source/detail/global_handler.cpp

@@ -89,7 +89,7 @@ std::mutex &GlobalHandler::getHandlerExtendedMembersMutex() {
   return getOrCreate(MHandlerExtendedMembersMutex);
 }
 
-void releaseSharedGlobalHandles() {
+void releaseDefaultContexts() {
   // Release shared-pointers to SYCL objects.
 #ifndef _WIN32
   GlobalHandler::instance().MPlatformToDefaultContextCache.Inst.reset(nullptr);
@@ -101,11 +101,26 @@ void releaseSharedGlobalHandles() {
   // routines will be called in the end.
   GlobalHandler::instance().MPlatformToDefaultContextCache.Inst.release();
 #endif
-  GlobalHandler::instance().MPlatformCache.Inst.reset(nullptr);
+}
+
+struct DefaultContextReleaseHandler {
+  ~DefaultContextReleaseHandler() { releaseDefaultContexts(); }
+};
+
+void GlobalHandler::registerDefaultContextReleaseHandler() {
+  static DefaultContextReleaseHandler handler{};
 }
 
 void shutdown() {
+  // If default contexts are requested after the first default contexts have
+  // been released there may be a new default context. These must be released
+  // prior to closing the plugins.
+  // Note: Releasing a default context here may cause failures in plugins with
+  // global state as the global state may have been released.
+  releaseDefaultContexts();
+
   // First, release resources, that may access plugins.
+  GlobalHandler::instance().MPlatformCache.Inst.reset(nullptr);
   GlobalHandler::instance().MScheduler.Inst.reset(nullptr);
   GlobalHandler::instance().MProgramManager.Inst.reset(nullptr);
 
@@ -135,7 +150,6 @@ extern "C" __SYCL_EXPORT BOOL WINAPI DllMain(HINSTANCE hinstDLL,
   // Perform actions based on the reason for calling.
   switch (fdwReason) {
   case DLL_PROCESS_DETACH:
-    releaseSharedGlobalHandles();
     shutdown();
     break;
   case DLL_PROCESS_ATTACH:
@@ -146,14 +160,6 @@ extern "C" __SYCL_EXPORT BOOL WINAPI DllMain(HINSTANCE hinstDLL,
   return TRUE; // Successful DLL_PROCESS_ATTACH.
 }
 #else
-// Release shared SYCL object implementation handles at normal destructor
-// priority to avoid the global handler from keeping the objects alive after
-// the backends have destroyed any state they may rely on to correctly handle
-// further operations.
-__attribute__((destructor)) static void syclPreunload() {
-  releaseSharedGlobalHandles();
-}
-
 // Setting low priority on destructor ensures it runs after all other global
 // destructors. Priorities 0-100 are reserved by the compiler. The priority
 // value 110 allows SYCL users to run their destructors after runtime library

sycl/source/detail/global_handler.hpp

@@ -68,8 +68,10 @@ class GlobalHandler {
   XPTIRegistry &getXPTIRegistry();
   std::mutex &getHandlerExtendedMembersMutex();
 
+  static void registerDefaultContextReleaseHandler();
+
 private:
-  friend void releaseSharedGlobalHandles();
+  friend void releaseDefaultContexts();
   friend void shutdown();
 
   // Constructor and destructor are declared out-of-line to allow incomplete

sycl/source/detail/platform_impl.cpp

@@ -127,6 +127,16 @@ std::vector<platform> platform_impl::get_platforms() {
     }
   }
 
+  // Register default context release handler after plugins have been loaded and
+  // after the first calls to each plugin. This initializes a function-local
+  // variable that should be destroyed before any global variables in the
+  // plugins are destroyed. This is done after the first call to the backends to
+  // ensure any lazy-loaded dependencies are loaded prior to the handler
+  // variable's initialization. Note: The default context release handler is not
+  // guaranteed to be destroyed before function-local static variables as they
+  // may be initialized after.
+  GlobalHandler::registerDefaultContextReleaseHandler();
+
   // The host platform should always be available unless not allowed by the
   // SYCL_DEVICE_FILTER
   detail::device_filter_list *FilterList =