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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?