[SYCL] Fix event status query during queue flushing (#7147)

Some events may have no native handle even at the point of cross queue
dependency flushing (e.g. those produced by a 0 size memset, where no PI
call is actually made). This patch changes the assertion of native
handle's existence into a check so that we handle this case properly.

sycl/source/detail/event_impl.cpp

@@ -402,7 +402,9 @@ std::vector<EventImplPtr> event_impl::getWaitList() {
 }
 
 void event_impl::flushIfNeeded(const QueueImplPtr &UserQueue) {
-  if (MIsFlushed)
+  // Some events might not have a native handle underneath even at this point,
+  // e.g. those produced by memset with 0 size (no PI call is made).
+  if (MIsFlushed || !MEvent)
     return;
 
   QueueImplPtr Queue = MQueue.lock();
@@ -416,7 +418,6 @@ void event_impl::flushIfNeeded(const QueueImplPtr &UserQueue) {
     return;
 
   // Check if the task for this event has already been submitted.
-  assert(MEvent != nullptr);
   pi_event_status Status = PI_EVENT_QUEUED;
   getPlugin().call<PiApiKind::piEventGetInfo>(
       MEvent, PI_EVENT_INFO_COMMAND_EXECUTION_STATUS, sizeof(pi_int32), &Status,

sycl/unittests/scheduler/QueueFlushing.cpp

@@ -30,6 +30,7 @@ static pi_result redefinedEventGetInfo(pi_event event, pi_event_info param_name,
                                        size_t param_value_size,
                                        void *param_value,
                                        size_t *param_value_size_ret) {
+  EXPECT_NE(event, nullptr);
   if (param_name == PI_EVENT_INFO_COMMAND_EXECUTION_STATUS) {
     auto *Status = reinterpret_cast<pi_event_status *>(param_value);
     *Status = EventStatus;
@@ -278,4 +279,15 @@ TEST_F(SchedulerTest, QueueFlushing) {
                               access::mode::read_write};
     testEventStatusCheck(&CmdC, QueueImplB, MockReq, PI_EVENT_COMPLETE);
   }
+
+  // Check that nullptr pi_events are handled correctly.
+  {
+    resetTestCtx();
+    detail::MapMemObject CmdA{&AllocaCmd, MockReq, &MockHostPtr, QueueImplA,
+                              access::mode::read_write};
+    MockCommand DepCmd(QueueImplB);
+    (void)CmdA.addDep(detail::DepDesc{&DepCmd, &MockReq, nullptr}, ToCleanUp);
+    MockScheduler::enqueueCommand(&CmdA, Res, detail::NON_BLOCKING);
+    EXPECT_FALSE(EventStatusQueried);
+  }
 }