core: abstract reactor::listen() as a virtual networking_stack method

Since every connection (thus far) is established via listen(), this allows
us to abstract the entire stack.

core/reactor.cc

@@ -96,7 +96,7 @@ reactor::posix_listen(socket_address sa, listen_options opts) {
 
 server_socket
 reactor::listen(socket_address sa, listen_options opt) {
-    return server_socket(std::make_unique<posix_server_socket_impl>(posix_listen(sa, opt)));
+    return _networking_stack->listen(sa, opt);
 }
 
 class posix_connected_socket_impl final : public connected_socket_impl {
@@ -440,4 +440,9 @@ posix_data_sink_impl::do_write(size_t idx) {
     });
 }
 
+server_socket
+posix_networking_stack::listen(socket_address sa, listen_options opt) {
+    return server_socket(std::make_unique<posix_server_socket_impl>(the_reactor.posix_listen(sa, opt)));
+}
+
 reactor the_reactor;

core/reactor.hh

@@ -211,14 +211,23 @@ public:
 
 class server_socket {
     std::unique_ptr<server_socket_impl> _ssi;
-private:
+public:
     explicit server_socket(std::unique_ptr<server_socket_impl> ssi)
         : _ssi(std::move(ssi)) {}
-public:
     future<connected_socket, socket_address> accept() {
         return _ssi->accept();
     }
-    friend class reactor;
+};
+
+class networking_stack {
+public:
+    virtual ~networking_stack() {}
+    virtual server_socket listen(socket_address sa, listen_options opts) = 0;
+};
+
+class posix_networking_stack : public networking_stack {
+public:
+    virtual server_socket listen(socket_address sa, listen_options opts) override;
 };
 
 class readable_eventfd {
@@ -297,6 +306,8 @@ class reactor {
     };
     std::unordered_map<int, signal_handler> _signal_handlers;
     bool _stopped = false;
+    std::unique_ptr<networking_stack> _networking_stack
+        = std::make_unique<posix_networking_stack>();
 public:
     file_desc _epollfd;
     readable_eventfd _io_eventfd;