[ssl] Reading from non-blocking socket wrapped by SSLContext returns an empty bytes object in case if the client stays connected

Bug report

Bug description:

I run my client-server application using SSLContext. And during communication I faced with situation when I receive an empty bytes object from the socket on server side. But in Wireshark dump I can see that client is present and sends data. If I add some delay after I receive empty bytes and try read from socket again I get some data. My code properly works on python 3.8 but it fails on 3.12. Also I tested without ssl on 3.12 and it works fine.

# how the socket initiated:
context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
context.load_cert_chain(*ssl_context)
self.socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
self.socket.bind((host, port))
self.socket.listen(1)
self.socket = context.wrap_socket(self.socket, server_side=True)


# how I accept client:
while not True:
    self.socket.settimeout(1)
    try:
        self.connection, _ = self.socket.accept()
        self.connection.settimeout(0.01)
        break
    except (socket.timeout, ssl.SSLError):
        continue


# how I read the data:
def read_data(self):
    data = b''
    if self.connection is not None:
        try:
            print(f"start recv at: {datetime.now().isoformat()}")
            if data := self.connection.recv(256):
                print(f"finish recv at: {datetime.now().isoformat()}")
                return data
            else:
                print(f"no data at: {datetime.now().isoformat()}")
                self.connection = None
                return b''
        except (socket.timeout, OSError):
            return data
    return data

CPython versions tested on:

3.12

Operating systems tested on:

Linux

payload にはリポジトリのファイルやテストが記載されていません。まず、提示された SSLContext と socket のシーケンスを Linux 上で Python 3.8 および 3.12 を使って再現し、その後、SSL でラップされた socket の読み取りパスを追跡してください。完了の条件は、影響を受けるシナリオで、接続済みクライアントの保留中のデータが空の読み取りとして報告されないことです。