Exposes socket connection error into pony level

packages/net/tcp_connection.pony

@@ -358,7 +358,8 @@ actor TCPConnection
 
         if not _connected and not _closed then
           // We don't have a connection yet.
-          if @pony_os_connected[Bool](fd) then
+          let errnum = @pony_os_socket_error[U32](fd)
+          if errnum == 0 then
             // The connection was successful, make it ours.
             _fd = fd
             _event = event
@@ -375,7 +376,7 @@ actor TCPConnection
             // The connection failed, unsubscribe the event and close.
             @pony_asio_event_unsubscribe(event)
             @pony_os_socket_close[None](fd)
-            _notify_connecting()
+            _notify_connecting(errnum)
           end
         else
           // We're already connected, unsubscribe the event and close.
@@ -655,14 +656,16 @@ actor TCPConnection
       end
     end
 
-  fun ref _notify_connecting() =>
+  fun ref _notify_connecting(errnum: U32 = 0) =>
     """
     Inform the notifier that we're connecting.
+    `errnum` is equal to 0 if no connection attemps has been made.
+    Otherwise it has errno value of failed connection.
     """
     if _connect_count > 0 then
       _notify.connecting(this, _connect_count)
     else
-      _notify.connect_failed(this)
+      _notify.connect_error(this, errnum)
       _hard_close()
     end
 

packages/net/tcp_connection_notify.pony

@@ -33,6 +33,13 @@ interface TCPConnectionNotify
     """
     None
 
+  fun ref connect_error(conn: TCPConnection ref, errnum: U32) =>
+    """
+    Extended version of `connect_failed` which exposes connection error.
+    `errnum` indicated errno value of failed connection.
+    """
+    this.connect_failed(conn)
+
   fun ref auth_failed(conn: TCPConnection ref) =>
     """
     A raw TCPConnection has no authentication mechanism. However, when

src/libponyrt/lang/socket.c

@@ -729,15 +729,16 @@ int pony_os_accept(asio_event_t* ev)
 }
 
 // Check this when a connection gets its first writeable event.
-bool pony_os_connected(int fd)
+// NOTE: error clears after the call
+int pony_os_socket_error(int fd)
 {
-  int val = 0;
-  socklen_t len = sizeof(int);
+  int sockerr;
+  socklen_t errlen = sizeof(sockerr);
 
-  if(getsockopt((SOCKET)fd, SOL_SOCKET, SO_ERROR, (char*)&val, &len) == -1)
-    return false;
+  if(getsockopt((SOCKET)fd, SOL_SOCKET, SO_ERROR, (char*)&sockerr, &errlen))
+    return errno;
 
-  return val == 0;
+  return sockerr;
 }
 
 static int address_family(int length)