Terrasync: don't busy wait when no requests are running.

simgear/scene/tsync/terrasync.cxx

@@ -643,6 +643,12 @@ void SGTerraSync::SvnThread::runInternal()
         }
 
         _busy = anySlotBusy;
+        if (!anySlotBusy) {
+            // wait on the blocking deque here, otherwise we spin
+            // the loop very fast, since _http::update with no connections
+            // active returns immediately.
+            waitingTiles.waitOnNotEmpty();
+        }
     } // of thread running loop
 }
 

simgear/threads/SGQueue.hxx

@@ -386,6 +386,11 @@ public:
         return this->queue.size();
     }
 
+    void waitOnNotEmpty() {
+    	SGGuard<SGMutex> g(mutex);
+    	while (this->queue.empty())
+    	    not_empty.wait(mutex);
+    }
 private:
 
     /**