Add barrier concept to autowiring::parallel

There are cases, especially when dealing with lambdas that do not return anything, where the user doesn't care to iterate over each available item and just wants to return when everything currently outstanding is completed.

autowiring/Parallel.h

@@ -183,6 +183,17 @@ class parallel {
     return parallel_collection<T> { begin<T>(), end<T>() };
   }
 
+  // Blocks until all outstanding work is done
+  void barrier(void) {
+    std::unique_lock<std::mutex> lk(m_queueMutex);
+    m_queueUpdated.wait(lk, [this] {
+      size_t totalReady = m_nVoidEntries;
+      for (auto& entry : m_queue)
+        totalReady += entry.second.size();
+      return m_outstandingCount == totalReady;
+    });
+  }
+
   // Get an iterator to the begining of out queue of job results
   template<typename T>
   parallel_iterator<T> begin(void) {
@@ -210,6 +221,7 @@ class parallel {
   // For void entries we don't need a queue, we can just keep a general count of "done"
   size_t m_nVoidEntries = 0;
 
+  // Total number of entries currently outstanding:
   size_t m_outstandingCount = 0;
 
   AutoCurrentContext m_ctxt;

src/autowiring/test/ParallelTest.cpp

@@ -84,3 +84,15 @@ TEST_F(ParallelTest, VoidReturnAll) {
   ASSERT_EQ(100UL, i) << "A sufficient number of empty lambdas were not encountered";
   ASSERT_EQ(100, *val) << "Not all pended lambda functions were called as expected";
 }
+
+TEST_F(ParallelTest, Barrier) {
+  AutoCurrentContext()->Initiate();
+  autowiring::parallel p;
+
+  std::atomic<size_t> x{ 0 };
+  for (size_t i = 0; i < 1000; i++)
+    p += [&x] { x++; };
+
+  p.barrier();
+  ASSERT_EQ(1000, x) << "Not all parallel watchers were completed on return from join";
+}