[SYCL] Fix handling of discarded events in preview lib

The queue implementation assumed that handler.finalize()
will return either a valid (non-discarded) event or nullptr
(that's why parseEvent is nop in preview mode).

However, that was not always true. It was possible for
scheduler to only mark an event as discarded after
handler.finalize() completed (during actual command
execution). This resulted in discarded event being
stored in LastEventPtr in the queue and calls to wait()
on that discarded event (which is not allowed) in MT
scenarios.

Fix this, by modyfing addCG() to mark the event as
discarded immediately. This allows handler to return
nullptr for preview mode.

Author: igchor

sycl/source/detail/scheduler/commands.cpp

@@ -3132,22 +3132,17 @@ ur_result_t ExecCGCommand::enqueueImpQueue() {
   auto RawEvents = getUrEvents(EventImpls);
   flushCrossQueueDeps(EventImpls, MWorkerQueue);
 
-  // We can omit creating a UR event and create a "discarded" event if the
-  // command has been explicitly marked as not needing an event, e.g. if the
-  // user did not ask for one, and there are no requirements.
-  bool DiscardUrEvent = MQueue && !MEventNeeded &&
-                        MQueue->supportsDiscardingPiEvents() &&
-                        MCommandGroup->getRequirements().size() == 0;
-
   ur_event_handle_t UREvent = nullptr;
-  ur_event_handle_t *Event = DiscardUrEvent ? nullptr : &UREvent;
-  detail::event_impl *EventImpl = DiscardUrEvent ? nullptr : MEvent.get();
+  ur_event_handle_t *Event = !MEventNeeded ? nullptr : &UREvent;
+  detail::event_impl *EventImpl = !MEventNeeded ? nullptr : MEvent.get();
 
   auto SetEventHandleOrDiscard = [&]() {
-    if (Event)
+    if (!MEventNeeded) {
+      assert(MEvent->isDiscarded());
+    } else {
+      assert(!MEvent->isDiscarded());
       MEvent->setHandle(*Event);
-    else
-      MEvent->setStateDiscarded();
+    }
   };
 
   switch (MCommandGroup->getType()) {

sycl/source/detail/scheduler/scheduler.cpp

@@ -133,6 +133,9 @@ EventImplPtr Scheduler::addCG(
     }
     NewEvent = NewCmd->getEvent();
     NewEvent->setSubmissionTime();
+    if (!EventNeeded) {
+      NewEvent->setStateDiscarded();
+    }
   }
 
   enqueueCommandForCG(NewEvent, AuxiliaryCmds);

sycl/source/detail/scheduler/scheduler.hpp

@@ -376,7 +376,8 @@ class Scheduler {
   /// directly to the queue.
   /// \param Dependencies Optional list of dependency
   /// sync points when enqueuing to a command buffer.
-  /// \return an event object to wait on for command group completion.
+  /// \return an event object to wait on for command group completion. It can
+  /// be a discarded event.
   EventImplPtr addCG(
       std::unique_ptr<detail::CG> CommandGroup, const QueueImplPtr &Queue,
       bool EventNeeded, ur_exp_command_buffer_handle_t CommandBuffer = nullptr,

sycl/source/handler.cpp

@@ -895,11 +895,18 @@ event handler::finalize() {
 #endif
   }
 
+  bool DiscardEvent = !impl->MEventNeeded && Queue &&
+                      Queue->supportsDiscardingPiEvents() &&
+                      CommandGroup->getRequirements().size() == 0;
+
   detail::EventImplPtr Event = detail::Scheduler::getInstance().addCG(
-      std::move(CommandGroup), Queue->shared_from_this(), impl->MEventNeeded);
+      std::move(CommandGroup), Queue->shared_from_this(), !DiscardEvent);
 
 #ifdef __INTEL_PREVIEW_BREAKING_CHANGES
-  MLastEvent = Event;
+  // For preview mode, handler.finalize() is expected to return nullptr
+  // if the event is discarded.
+  MLastEvent = DiscardEvent ? nullptr : Event;
+  assert(MLastEvent || !MLastEvent->isDiscarded());
 #else
   MLastEvent = detail::createSyclObjFromImpl<event>(Event);
 #endif