Call alpaka::wait() in EDMetadata destructor only when its Queue has …

…not been reused by a consuming module

If the consuming module does not produce any device-side products, the
EDMetadata destructor should result in blocking alpaka::wait() call.

HeterogeneousCore/AlpakaCore/src/alpaka/EDMetadata.cc

@@ -14,7 +14,14 @@ namespace ALPAKA_ACCELERATOR_NAMESPACE {
     // TODO: a callback notifying a WaitingTaskHolder (or similar)
     // would avoid blocking the CPU, but would also require more work.
 
-    if (event_ and not eventComplete_) {
+    // If event_ is null, the EDMetadata was either
+    // default-constructed, or fully synchronized before leaving the
+    // produce() call, so no synchronization is needed.
+    // If the queue was re-used, then some other EDMetadata object in
+    // the same edm::Event records the event_ (in the same queue) and
+    // calls alpaka::wait(), and therefore this wait() call can be
+    // skipped).
+    if (event_ and not eventComplete_ and mayReuseQueue_) {
       // Must not throw in a destructor, and if there were an
       // exception could not really propagate it anyway.
       CMS_SA_ALLOW try { alpaka::wait(*event_); } catch (...) {