Bring back enqueue front

src/rt/ds/wrapindex.h

@@ -20,6 +20,16 @@ class WrapIndex
     return index;
   }
 
+  size_t operator--(int)
+  {
+    auto result = index;
+    if (result == 0)
+      index = N - 1;
+    else
+      index--;
+    return result;
+  }
+
   operator size_t() const
   {
     return index;

src/rt/sched/mpmcq.h

@@ -126,8 +126,17 @@ namespace verona::rt
 
     void enqueue_front(T* node)
     {
-      // No longer support put on the back.
-      enqueue_segment({node, &node->next_in_queue});
+      auto old_front = acquire_front();
+      if (old_front == nullptr)
+      {
+        // Post to back.
+        enqueue(node);
+        return;
+      }
+
+      // Link into the front.
+      node->next_in_queue.store(old_front, std::memory_order_relaxed);
+      front.store(node, std::memory_order_release);
     }
 
     /**

src/rt/sched/workstealingqueue.h

@@ -47,8 +47,7 @@ namespace verona::rt
 
     void enqueue_front(Work* work)
     {
-      // TODO should this be --dequeue_index?
-      queues[++enqueue_index].enqueue_front(work);
+      queues[dequeue_index--].enqueue_front(work);
     }
 
     // Dequeue a single node from any of the queues.