Another example: some asyncio (run with uvloop) code:
 
        conn, _ = lsock.accept()
        f = loop.create_task(
            loop.connect_accepted_socket(
                proto_factory, conn))
        # More code
        loop.run_forever()
        conn.close()

Suppose the above snippet of code is some real-world program.

Now, in Python 3.5 everything works as expected.  In Python 3.6, "conn.close()" will raise an exception.

Why: uvloop passes the FD of "conn" to libuv, which does its thing and closes the connection when it should be closed.

Now:

> 1. Your code should call sock0.detach() rather than fileno(), so that sock0 no longer “owns” the file descriptor, or

I can't modify "connect_accepted_socket" to call "detach" on "conn", as it would make "conn" unusable right after the call.  This option can't be implemented, as it would break the backwards compatibility.

> 2. libuv should not close file descriptors that it doesn’t own.

This is not just about uvloop/libuv.  It's about any Python program out there that will break in 3.6.  A lot of code extracts the FD out of socket objects and does something with it.  We can't just make socket.close() to raise in 3.6 -- that's not how backwards compatibility promise works.

Guido, what do you think about this issue?