Add comments, fix a subtle bug

include/grpc++/server.h

@@ -69,6 +69,7 @@ class Server GRPC_FINAL : public GrpcLibrary, private CallHook {
     ShutdownInternal(TimePoint<T>(deadline).raw_time());
   }
 
+  // Shutdown the server, waiting for all rpc processing to finish.
   void Shutdown() { ShutdownInternal(gpr_inf_future(GPR_CLOCK_MONOTONIC)); }
 
   // Block waiting for all work to complete (the server must either

src/cpp/server/server.cc

@@ -329,11 +329,15 @@ void Server::ShutdownInternal(gpr_timespec deadline) {
     shutdown_ = true;
     grpc_server_shutdown_and_notify(server_, cq_.cq(), new ShutdownRequest());
     cq_.Shutdown();
+    // Spin, eating requests until the completion queue is completely shutdown.
+    // If the deadline expires then cancel anything that's pending and keep
+    // spinning forever until the work is actually drained.
     SyncRequest* request;
     bool ok;
     while (SyncRequest::AsyncWait(&cq_, &request, &ok, deadline)) {
       if (request == NULL) {  // deadline expired
         grpc_server_cancel_all_calls(server_);
+        deadline = gpr_inf_future(GPR_CLOCK_MONOTONIC);
       } else if (ok) {
         SyncRequest::CallData call_data(this, request);
       }