More graceful stop of nghttp2::asio_http2::server::http2

Explicit io_service::stop() will prevent running streams from
finishing their task. That means if there are already reposnes
that we have called end(std::string) on them and they have not
finished sending back their data, they will be closed with a
NGHTTP2_INTERNAL_ERROR
Instead, we can stop accepting connections and destroy all
io_service::work objects to signals end of work.

src/asio_io_service_pool.cc

@@ -75,13 +75,18 @@ void io_service_pool::join() {
   }
 }
 
-void io_service_pool::stop() {
+void io_service_pool::force_stop() {
   // Explicitly stop all io_services.
   for (auto &iosv : io_services_) {
     iosv->stop();
   }
 }
 
+void io_service_pool::stop() {
+  // Destroy all work objects to signals end of work
+  work_.clear();
+}
+
 boost::asio::io_service &io_service_pool::get_io_service() {
   // Use a round-robin scheme to choose the next io_service to use.
   auto &io_service = *io_services_[next_io_service_];

src/asio_io_service_pool.h

@@ -62,6 +62,9 @@ public:
   void run(bool asynchronous = false);
 
   /// Stop all io_service objects in the pool.
+  void force_stop();
+
+  /// Destroy all work objects to signals end of work
   void stop();
 
   /// Join on all io_service objects in the pool.

src/asio_server.cc

@@ -124,6 +124,11 @@ boost::system::error_code server::bind_and_listen(boost::system::error_code &ec,
 
 void server::start_accept(boost::asio::ssl::context &tls_context,
                           tcp::acceptor &acceptor, serve_mux &mux) {
+
+  if (!acceptor.is_open()) {
+    return;
+  }
+
   auto new_connection = std::make_shared<connection<ssl_socket>>(
       mux, tls_handshake_timeout_, read_timeout_,
       io_service_pool_.get_io_service(), tls_context);
@@ -158,6 +163,11 @@ void server::start_accept(boost::asio::ssl::context &tls_context,
 }
 
 void server::start_accept(tcp::acceptor &acceptor, serve_mux &mux) {
+
+   if (!acceptor.is_open()) {
+    return;
+  }
+
   auto new_connection = std::make_shared<connection<tcp::socket>>(
       mux, tls_handshake_timeout_, read_timeout_,
       io_service_pool_.get_io_service());
@@ -177,10 +187,10 @@ void server::start_accept(tcp::acceptor &acceptor, serve_mux &mux) {
 }
 
 void server::stop() {
-  io_service_pool_.stop();
   for (auto &acceptor : acceptors_) {
     acceptor.close();
   }
+  io_service_pool_.stop();
 }
 
 void server::join() { io_service_pool_.join(); }