pdns_control's nonblocking connection to the socket is not correct

[Habbie]

DynMessenger::DynMessenger uses timed_connect which calls connect once, assuming it will either get

• 0 - the connection succeeded
• EINPROGRESS - the connection is pending, so we call select and assume that when that returns writability, we are connected
• any other error - the connection failed

However, connect(2) (from Linux man-pages 6.03) notes:

EINPROGRESS
The socket is nonblocking and the connection cannot be completed immediately. (UNIX domain sockets failed with EAGAIN instead.)

The control socket is a UNIX socket, so this code is wrong. Usually, the control socket has room in the accept backlog, so connections do succeed. But if the control thread is tied up (which might be a bug in itself), the backlog may fill, and non blocking connections will not succeed immediately.

In that situation, we get EAGAIN instead of EINPROGRESS, and return error. If I change the code to handle EAGAIN in the same way as EINPROGRESS, the connection still fails, because apparently select considers the socket writable even when it is not yet connected.