Use tbb::task_group::defer in FinalWaitingTask

- This will allow the task_group wait to stay open until the
  FinalWaitingTask is recycled
- Added FinalWaitingTask::wait method to handle the usual usage
  pattern of throwing after wait if needed.

FWCore/Concurrency/interface/FinalWaitingTask.h

@@ -0,0 +1,60 @@
+#ifndef FWCore_Concurrency_FinalWaitingTask_h
+#define FWCore_Concurrency_FinalWaitingTask_h
+// -*- C++ -*-
+//
+// Package:     FWCore/Concurrency
+// Class  :     FinalWaitingTask
+//
+/**\class FinalWaitingTask FinalWaitingTask.h "FWCore/Concurrency/interface/FinalWaitingTask.h"
+
+ Description: [one line class summary]
+
+ Usage:
+   Use this class on the stack to signal the final task to be run.
+   Call done() to check to see if the task was run and check value of
+   exceptionPtr() to see if an exception was thrown by any task in the group.
+
+*/
+//
+// Original Author:  Christopher Jones
+//         Created:  Tue, 12 Jul 2022 18:45:15 GMT
+//
+
+// system include files
+#include "oneapi/tbb/task_group.h"
+
+// user include files
+#include "FWCore/Concurrency/interface/WaitingTask.h"
+
+// forward declarations
+namespace edm {
+  class FinalWaitingTask : public WaitingTask {
+  public:
+    FinalWaitingTask() = delete;
+    explicit FinalWaitingTask(tbb::task_group& iGroup)
+        : m_group{&iGroup}, m_handle{iGroup.defer([]() {})}, m_done{false} {}
+
+    void execute() final { m_done = true; }
+
+    [[nodiscard]] bool done() const noexcept { return m_done.load(); }
+
+    void wait() {
+      m_group->wait();
+      if (exceptionPtr()) {
+        std::rethrow_exception(exceptionPtr());
+      }
+    }
+    std::exception_ptr waitNoThrow() {
+      m_group->wait();
+      return exceptionPtr();
+    }
+
+  private:
+    void recycle() final { m_group->run(std::move(m_handle)); }
+    tbb::task_group* m_group;
+    tbb::task_handle m_handle;
+    std::atomic<bool> m_done;
+  };
+
+}  // namespace edm
+#endif

FWCore/Concurrency/interface/WaitingTask.h

@@ -68,23 +68,6 @@ namespace edm {
     std::atomic<bool> m_ptrSet = false;
   };
 
-  /** Use this class on the stack to signal the final task to be run.
-   Call done() to check to see if the task was run and check value of
-   exceptionPtr() to see if an exception was thrown by any task in the group.
-   */
-  class FinalWaitingTask : public WaitingTask {
-  public:
-    FinalWaitingTask() : m_done{false} {}
-
-    void execute() final { m_done = true; }
-
-    bool done() const { return m_done.load(); }
-
-  private:
-    void recycle() final {}
-    std::atomic<bool> m_done;
-  };
-
   template <typename F>
   class FunctorWaitingTask : public WaitingTask {
   public: