* Do not save the last_error if there are no sockets waiting to be connected
In this implementation, the results of both name resolution and connection attempts are awaited using select(2).
When it returned, the implementation attempted to check for connections even if there were no sockets currently attempting to connect, treating the absence of connected sockets as a connection failure.
With this fix, it will no longer check for connections when there are no sockets waiting to be connected.
Additionally, the following minor fixes have been made:
* Handle failure of getsockopt(2) and removed unnecessary continue in the loop
* Tweak: Use common API to check in_progress_fds
* Safely call TCPServer.new in test
* Set empty writefds when there is no socket waiting to be connected
* Enable fast_fallback option
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