# Buggy code
def _receive_messages(
    sock: ssl.SSLSocket,
    logger: Logger,
    receive_buffer_size: int = 1024,
    all_message_trace_enabled: bool = False,
) -> List[Tuple[Optional[FrameHeader], bytes]]:
    def receive(specific_buffer_size: Optional[int] = None):
        size = (
            specific_buffer_size
            if specific_buffer_size is not None
            else receive_buffer_size
        )
        received_bytes = sock.recv(size)
        if all_message_trace_enabled:
            if len(received_bytes) > 0:
                logger.debug(f"Received bytes: {received_bytes}")
        return received_bytes

    return _fetch_messages(
        messages=[],
        receive=receive,
        remaining_bytes=None,
        current_mask_key=None,
        current_header=None,
        current_data=bytes(),
        logger=logger,
    )


# Traceback (SlackApiError)
2021-02-14 18:19:49,579 ERROR    slack_sdk.rtm.v2          Failed to establish a connection (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81, error: _ssl.c:1108: The handshake operation timed out)
Traceback (most recent call last):
File "/opt/errbot/lib/python3.8/site-packages/slack_sdk/socket_mode/builtin/connection.py", line 109, in connect
sock.connect((hostname, port))
File "/usr/lib/python3.8/ssl.py", line 1342, in connect
self._real_connect(addr, False)
File "/usr/lib/python3.8/ssl.py", line 1333, in _real_connect
self.do_handshake()
File "/usr/lib/python3.8/ssl.py", line 1309, in do_handshake
self._sslobj.do_handshake()
socket.timeout: _ssl.c:1108: The handshake operation timed out
2021-02-14 18:19:49,579 INFO     slack_sdk.rtm.v2          The connection has been closed (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:19:49,579 INFO     slack_sdk.rtm.v2          The connection has been closed (session id: b88f63ee-d644-45a8-bf18-be36c8673728)
2021-02-14 18:19:49,579 INFO     slack_sdk.rtm.v2          A new session has been established (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:19:49,579 INFO     slack_sdk.rtm.v2          Connected to a new endpoint...
2021-02-14 18:19:59,590 DEBUG    slack_sdk.rtm.v2          This connection is already closed. (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:19:59,590 INFO     slack_sdk.rtm.v2          The session seems to be already closed. Going to reconnect... (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:19:59,590 INFO     slack_sdk.rtm.v2          Connecting to a new endpoint...
2021-02-14 18:20:00,075 ERROR    slack_sdk.rtm.v2          Failed to retrieve WSS URL: The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'ratelimited'}
2021-02-14 18:20:00,075 ERROR    slack_sdk.rtm.v2          Failed to check the current session or reconnect to the server (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81, error: SlackApiError, message: The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'ratelimited'})
2021-02-14 18:20:10,092 DEBUG    slack_sdk.rtm.v2          This connection is already closed. (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:20:10,117 INFO     slack_sdk.rtm.v2          The session seems to be already closed. Going to reconnect... (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:20:10,121 INFO     slack_sdk.rtm.v2          Connecting to a new endpoint...
2021-02-14 18:20:10,339 ERROR    slack_sdk.rtm.v2          Failed to retrieve WSS URL: The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'ratelimited'}
2021-02-14 18:20:10,339 ERROR    slack_sdk.rtm.v2          Failed to check the current session or reconnect to the server (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81, error: SlackApiError, message: The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'ratelimited'})
2021-02-14 18:20:20,350 DEBUG    slack_sdk.rtm.v2          This connection is already closed. (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:20:20,350 INFO     slack_sdk.rtm.v2          The session seems to be already closed. Going to reconnect... (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:20:20,350 INFO     slack_sdk.rtm.v2          Connecting to a new endpoint...
2021-02-14 18:20:20,556 ERROR    slack_sdk.rtm.v2          Failed to retrieve WSS URL: The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'ratelimited'}
2021-02-14 18:20:20,557 ERROR    slack_sdk.rtm.v2          Failed to check the current session or reconnect to the server (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81, error: SlackApiError, message: The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'ratelimited'})
2021-02-14 18:20:30,567 DEBUG    slack_sdk.rtm.v2          This connection is already closed. (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:20:30,567 INFO     slack_sdk.rtm.v2          The session seems to be already closed. Going to reconnect... (session id: 18e8f520-5e64-443c-98ed-089ab1ef6f81)
2021-02-14 18:20:30,567 INFO     slack_sdk.rtm.v2          Connecting to a new endpoint...

# Fixed code
def _receive_messages(
    sock: ssl.SSLSocket,
    sock_receive_lock: Lock,
    logger: Logger,
    receive_buffer_size: int = 1024,
    all_message_trace_enabled: bool = False,
) -> List[Tuple[Optional[FrameHeader], bytes]]:
    def receive(specific_buffer_size: Optional[int] = None):
        size = (
            specific_buffer_size
            if specific_buffer_size is not None
            else receive_buffer_size
        )
        with sock_receive_lock:
            try:
                received_bytes = sock.recv(size)
                if all_message_trace_enabled:
                    if len(received_bytes) > 0:
                        logger.debug(f"Received bytes: {received_bytes}")
                return received_bytes
            except OSError as e:
                if e.errno == errno.EBADF:
                    logger.debug("The connection seems to be already closed.")
                    return bytes()
                raise e

    return _fetch_messages(
        messages=[],
        receive=receive,
        remaining_bytes=None,
        current_mask_key=None,
        current_header=None,
        current_data=bytes(),
        logger=logger,
    )