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[EDIT] FILE: client.py

# Copyright 2016-2023 The NATS Authors
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

from __future__ import annotations

import asyncio
import base64
import ipaddress
import json
import logging
import re
import ssl
import string
import time
from collections import UserString
from dataclasses import dataclass
from email.parser import BytesParser
from io import BytesIO
from pathlib import Path
from random import shuffle
from secrets import token_hex
from typing import Any, Awaitable, Callable, Dict, List, Optional, Tuple, Union
from urllib.parse import ParseResult, urlparse

try:
    from fast_mail_parser import parse_email
except ImportError:
    parse_email = None

import nats.js
from nats import errors
from nats.nuid import NUID
from nats.protocol import command as prot_command
from nats.protocol.parser import (
    AUTHORIZATION_VIOLATION,
    PERMISSIONS_ERR,
    PONG,
    STALE_CONNECTION,
    Parser,
)

from .errors import ErrInvalidUserCredentials, ErrStaleConnection
from .msg import Msg
from .subscription import (
    DEFAULT_SUB_PENDING_BYTES_LIMIT,
    DEFAULT_SUB_PENDING_MSGS_LIMIT,
    Subscription,
)
from .transport import TcpTransport, Transport, WebSocketTransport

try:
    from importlib.metadata import version

__version__ = version("nats-py")
except Exception:
    __version__ = "0.0.0"

__lang__ = "python3"
_logger = logging.getLogger(__name__)
PROTOCOL = 1

INFO_OP = b"INFO"
CONNECT_OP = b"CONNECT"
PING_OP = b"PING"
PONG_OP = b"PONG"
OK_OP = b"+OK"
ERR_OP = b"-ERR"
_CRLF_ = b"\r\n"
_CRLF_LEN_ = len(_CRLF_)
_SPC_ = b" "
_SPC_BYTE_ = 32
EMPTY = ""

PING_PROTO = PING_OP + _CRLF_
PONG_PROTO = PONG_OP + _CRLF_
DEFAULT_INBOX_PREFIX = b"_INBOX"

DEFAULT_PENDING_SIZE = 2 * 1024 * 1024
DEFAULT_BUFFER_SIZE = 32768
DEFAULT_RECONNECT_TIME_WAIT = 2  # in seconds
DEFAULT_MAX_RECONNECT_ATTEMPTS = 60
DEFAULT_PING_INTERVAL = 120  # in seconds
DEFAULT_MAX_OUTSTANDING_PINGS = 2
DEFAULT_MAX_PAYLOAD_SIZE = 1048576
DEFAULT_MAX_FLUSHER_QUEUE_SIZE = 1024
DEFAULT_FLUSH_TIMEOUT = 10  # in seconds
DEFAULT_CONNECT_TIMEOUT = 2  # in seconds
DEFAULT_DRAIN_TIMEOUT = 30  # in seconds
MAX_CONTROL_LINE_SIZE = 1024

NATS_HDR_LINE = bytearray(b"NATS/1.0")
NATS_HDR_LINE_SIZE = len(NATS_HDR_LINE)
NO_RESPONDERS_STATUS = "503"
CTRL_STATUS = "100"
STATUS_MSG_LEN = 3  # e.g. 20x, 40x, 50x

Callback = Callable[[], Awaitable[None]]
ErrorCallback = Callable[[Exception], Awaitable[None]]
JWTCallback = Callable[[], Union[bytearray, bytes]]
SignatureCallback = Callable[[str], bytes]
TokenCallback = Callable[[], str]

class RawCredentials(UserString):
    pass

Credentials = Union[str, Tuple[str, str], RawCredentials, Path]

@dataclass
class Server:
    """
    Server represents a NATS server in the connection pool.
    """

uri: ParseResult
    reconnects: int = 0

@dataclass
class Srv:
    """
    Srv is a helper data structure to hold state of a server.
    """

uri: ParseResult
    reconnects: int = 0
    last_attempt: Optional[float] = None
    did_connect: bool = False
    discovered: bool = False
    tls_name: Optional[str] = None
    server_version: Optional[str] = None

ReconnectToServerHandler = Callable[[List[Server], Dict[str, Any]], Tuple[Optional[Server], float]]

class ServerVersion:
    def __init__(self, server_version: str) -> None:
        self._server_version = server_version
        self._major_version: Optional[int] = None
        self._minor_version: Optional[int] = None
        self._patch_version: Optional[int] = None
        self._prerelease_version: Optional[str] = None
        self._build_version: Optional[str] = None
        self._dev_version: Optional[str] = None

def parse_version(self) -> None:
        # https://semver.org/#is-there-a-suggested-regular-expression-regex-to-check-a-semver-string
        _SEMVER_REGEX = r"""
            ^
            (?P<major>0|[1-9]\d*)
            \.
            (?P<minor>0|[1-9]\d*)
            \.
            (?P<patch>0|[1-9]\d*)
            (?:-(?P<prerelease>
                (?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)
                (?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*
            ))?
            (?:\+(?P<buildmetadata>
                [0-9a-zA-Z-]+
                (?:\.[0-9a-zA-Z-]+)*
            ))?
            $
        """
        _REGEX = re.compile(_SEMVER_REGEX, re.VERBOSE)
        match = _REGEX.match(self._server_version)
        if match is None:
            raise ValueError(f"{self._server_version} is not a valid Semantic Version")
        matches = match.groupdict()
        self._major_version = int(matches["major"])
        self._minor_version = int(matches["minor"])
        self._patch_version = int(matches["patch"])
        self._prerelease_version = matches["prerelease"] or ""
        self._build_version = matches["buildmetadata"] or ""
        if self._build_version:
            self._dev_version = "+".join([self._prerelease_version, self._build_version])
        else:
            self._dev_version = self._prerelease_version

@property
    def major(self) -> int:
        if not self._major_version:
            self.parse_version()
        return self._major_version

@property
    def minor(self) -> int:
        if not self._minor_version:
            self.parse_version()
        return self._minor_version

@property
    def patch(self) -> int:
        if not self._patch_version:
            self.parse_version()
        return self._patch_version

@property
    def prerelease(self) -> int:
        if not self._prerelease_version:
            self.parse_version()
        return self._prerelease_version

@property
    def build(self) -> int:
        if not self._build_version:
            self.parse_version()
        return self._build_version

@property
    def dev(self) -> str:
        if not self._dev_version:
            self.parse_version()
        return self._dev_version or ""

def __repr__(self) -> str:
        return f"<nats server v{self._server_version}>"

async def _default_error_callback(ex: Exception) -> None:
    """
    Provides a default way to handle async errors if the user
    does not provide one.
    """
    _logger.error("nats: encountered error", exc_info=ex)

class Client:
    """
    Asyncio based client for NATS.
    """

msg_class: type[Msg] = Msg

# FIXME: Use an enum instead.
    DISCONNECTED = 0
    CONNECTED = 1
    CLOSED = 2
    RECONNECTING = 3
    CONNECTING = 4
    DRAINING_SUBS = 5
    DRAINING_PUBS = 6

def __repr__(self) -> str:
        return f"<nats client v{__version__}>"

def __init__(self) -> None:
        self._current_server: Optional[Srv] = None
        self._server_info: Dict[str, Any] = {}
        self._server_pool: List[Srv] = []
        self._reading_task: Optional[asyncio.Task] = None
        self._ping_interval_task: Optional[asyncio.Task] = None
        self._pings_outstanding: int = 0
        self._pongs_received: int = 0
        self._pongs: List[asyncio.Future] = []
        self._transport: Optional[Transport] = None
        self._err: Optional[Exception] = None

# callbacks
        self._error_cb: ErrorCallback = _default_error_callback
        self._disconnected_cb: Optional[Callback] = None
        self._closed_cb: Optional[Callback] = None
        self._discovered_server_cb: Optional[Callback] = None
        self._reconnected_cb: Optional[Callback] = None
        self._reconnect_to_server_handler: Optional[ReconnectToServerHandler] = None

self._reconnection_task: Optional[asyncio.Task[None]] = None
        self._reconnection_task_future: Optional[asyncio.Future] = None
        self._max_payload: int = DEFAULT_MAX_PAYLOAD_SIZE

# client id that the NATS server knows about.
        self._client_id: Optional[int] = None
        self._sid: int = 0
        self._subs: Dict[int, Subscription] = {}
        self._status: int = Client.DISCONNECTED
        self._ps: Parser = Parser(self)

# pending queue of commands that will be flushed to the server.
        self._pending: List[bytes] = []

# current size of pending data in total.
        self._pending_data_size: int = 0

# max pending size is the maximum size of the data that can be buffered.
        self._max_pending_size: int = 0

self._flush_queue: Optional[asyncio.Queue[asyncio.Future[Any]]] = None
        self._flusher_task: Optional[asyncio.Task] = None
        self._flush_timeout: Optional[float] = 0
        self._hdr_parser: BytesParser = BytesParser()

# New style request/response
        self._resp_map: Dict[str, asyncio.Future] = {}
        self._resp_sub_prefix: Optional[bytearray] = None
        self._nuid = NUID()
        self._inbox_prefix = bytearray(DEFAULT_INBOX_PREFIX)
        self._auth_configured: bool = False

# NKEYS support
        #
        # user_jwt_cb is used to fetch and return the account
        # signed JWT for this user.
        self._user_jwt_cb: Optional[JWTCallback] = None

# signature_cb is used to sign a nonce from the server while
        # authenticating with nkeys. The user should sign the nonce and
        # return the base64 encoded signature.
        self._signature_cb: Optional[SignatureCallback] = None

# user credentials file can be a tuple or single file.
        self._user_credentials: Optional[Credentials] = None

# file that contains the nkeys seed and its public key as a string.
        self._nkeys_seed: Optional[str] = None
        self._nkeys_seed_str: Optional[str] = None
        self._public_nkey: Optional[str] = None

self.options: Dict[str, Any] = {}
        self.stats = {
            "in_msgs": 0,
            "out_msgs": 0,
            "in_bytes": 0,
            "out_bytes": 0,
            "reconnects": 0,
            "errors_received": 0,
        }

async def connect(
        self,
        servers: Union[str, List[str]] = ["nats://localhost:4222"],
        error_cb: Optional[ErrorCallback] = None,
        disconnected_cb: Optional[Callback] = None,
        closed_cb: Optional[Callback] = None,
        discovered_server_cb: Optional[Callback] = None,
        reconnected_cb: Optional[Callback] = None,
        name: Optional[str] = None,
        pedantic: bool = False,
        verbose: bool = False,
        allow_reconnect: bool = True,
        connect_timeout: int = DEFAULT_CONNECT_TIMEOUT,
        reconnect_time_wait: int = DEFAULT_RECONNECT_TIME_WAIT,
        max_reconnect_attempts: int = DEFAULT_MAX_RECONNECT_ATTEMPTS,
        ping_interval: int = DEFAULT_PING_INTERVAL,
        max_outstanding_pings: int = DEFAULT_MAX_OUTSTANDING_PINGS,
        dont_randomize: bool = False,
        flusher_queue_size: int = DEFAULT_MAX_FLUSHER_QUEUE_SIZE,
        no_echo: bool = False,
        tls: Optional[ssl.SSLContext] = None,
        tls_hostname: Optional[str] = None,
        tls_handshake_first: bool = False,
        user: Optional[str] = None,
        password: Optional[str] = None,
        token: Optional[Union[str, TokenCallback]] = None,
        drain_timeout: int = DEFAULT_DRAIN_TIMEOUT,
        signature_cb: Optional[SignatureCallback] = None,
        user_jwt_cb: Optional[JWTCallback] = None,
        user_credentials: Optional[Credentials] = None,
        nkeys_seed: Optional[str] = None,
        nkeys_seed_str: Optional[str] = None,
        inbox_prefix: Union[str, bytes] = DEFAULT_INBOX_PREFIX,
        pending_size: int = DEFAULT_PENDING_SIZE,
        flush_timeout: Optional[float] = None,
        ws_connection_headers: Optional[Dict[str, List[str]]] = None,
        reconnect_to_server_handler: Optional[ReconnectToServerHandler] = None,
    ) -> None:
        """
        Establishes a connection to NATS.

:param servers: NATS Connection
        :param name: Label the connection with name (shown in NATS monitoring)
        :param error_cb: Callback to report errors.
        :param disconnected_cb: Callback to report disconnection from NATS.
        :param closed_cb: Callback to report when client stops reconnection to NATS.
        :param discovered_server_cb: Callback to report when a new server joins the cluster.
        :param pending_size: Max size of the pending buffer for publishing commands.
        :param flush_timeout: Max duration to wait for a forced flush to occur.

Connecting setting all callbacks::

import asyncio
            import nats

async def main():
                async def disconnected_cb():
                    print('Got disconnected!')

async def reconnected_cb():
                    print(f'Got reconnected to {nc.connected_url.netloc}')

async def error_cb(e):
                    print(f'There was an error: {e}')

async def closed_cb():
                    print('Connection is closed')

# Connect to NATS with logging callbacks.
                nc = await nats.connect('demo.nats.io',
                                         error_cb=error_cb,
                                         reconnected_cb=reconnected_cb,
                                         disconnected_cb=disconnected_cb,
                                         closed_cb=closed_cb,
                                         )

async def handler(msg):
                    print(f'Received a message on {msg.subject} {msg.reply}: {msg.data}')
                    await msg.respond(b'OK')

sub = await nc.subscribe('help.please', cb=handler)

resp = await nc.request('help.please', b'help')
                print('Response:', resp)

await nc.close()

if __name__ == '__main__':
                asyncio.run(main())

Using a context manager::

import asyncio
            import nats

async def main():

is_done = asyncio.Future()

async def closed_cb():
                    print('Connection to NATS is closed.')
                    is_done.set_result(True)

async with (await nats.connect('nats://demo.nats.io:4222', closed_cb=closed_cb)) as nc:
                    print(f'Connected to NATS at {nc.connected_url.netloc}...')

async def subscribe_handler(msg):
                        subject = msg.subject
                        reply = msg.reply
                        data = msg.data.decode()
                        print('Received a message on '{subject} {reply}': {data}'.format(
                            subject=subject, reply=reply, data=data))

await nc.subscribe('discover', cb=subscribe_handler)
                    await nc.flush()

for i in range(0, 10):
                        await nc.publish('discover', b'hello world')
                        await asyncio.sleep(0.1)

await asyncio.wait_for(is_done, 60.0)

if __name__ == '__main__':
                asyncio.run(main())

"""

for cb in [
            error_cb,
            disconnected_cb,
            closed_cb,
            reconnected_cb,
            discovered_server_cb,
        ]:
            if cb and not asyncio.iscoroutinefunction(cb):
                raise errors.InvalidCallbackTypeError

self._setup_server_pool(servers)
        self._error_cb = error_cb or _default_error_callback
        self._closed_cb = closed_cb
        self._discovered_server_cb = discovered_server_cb
        self._reconnected_cb = reconnected_cb
        self._disconnected_cb = disconnected_cb
        self._reconnect_to_server_handler = reconnect_to_server_handler

# Custom inbox prefix
        if isinstance(inbox_prefix, str):
            inbox_prefix = inbox_prefix.encode()
        assert isinstance(inbox_prefix, bytes)
        self._inbox_prefix = bytearray(inbox_prefix)

# NKEYS support
        self._signature_cb = signature_cb
        self._user_jwt_cb = user_jwt_cb
        self._user_credentials = user_credentials
        self._nkeys_seed = nkeys_seed
        self._nkeys_seed_str = nkeys_seed_str

# Customizable options
        self.options["verbose"] = verbose
        self.options["pedantic"] = pedantic
        self.options["name"] = name
        self.options["allow_reconnect"] = allow_reconnect
        self.options["dont_randomize"] = dont_randomize
        self.options["reconnect_time_wait"] = reconnect_time_wait
        self.options["max_reconnect_attempts"] = max_reconnect_attempts
        self.options["ping_interval"] = ping_interval
        self.options["max_outstanding_pings"] = max_outstanding_pings
        self.options["no_echo"] = no_echo
        self.options["user"] = user
        self.options["password"] = password
        self.options["token"] = token
        self.options["connect_timeout"] = connect_timeout
        self.options["drain_timeout"] = drain_timeout
        self.options["tls_handshake_first"] = tls_handshake_first
        self.options["ws_connection_headers"] = ws_connection_headers

if tls:
            self.options["tls"] = tls
        if tls_hostname:
            self.options["tls_hostname"] = tls_hostname

# Check if the username or password was set in the server URI
        server_auth_configured = False
        if len(self._server_pool) > 0:
            for server in self._server_pool:
                if server.uri.username or server.uri.password:
                    server_auth_configured = True
                    break
        if user or password or token or server_auth_configured:
            self._auth_configured = True

if self._user_credentials is not None or self._nkeys_seed is not None or self._nkeys_seed_str is not None:
            self._auth_configured = True
            self._setup_nkeys_connect()

# Queue used to trigger flushes to the socket.
        self._flush_queue = asyncio.Queue(maxsize=flusher_queue_size)

# Max size of buffer used for flushing commands to the server.
        self._max_pending_size = pending_size

# Max duration for a force flush (happens when a buffer is full).
        self._flush_timeout = flush_timeout

if self.options["dont_randomize"] is False:
            shuffle(self._server_pool)

while True:
            try:
                await self._select_next_server()
                await self._process_connect_init()
                assert self._current_server, "the current server must be set by _select_next_server"
                self._current_server.reconnects = 0
                break
            except errors.NoServersError as e:
                if self.options["max_reconnect_attempts"] < 0:
                    # Never stop reconnecting
                    continue
                self._err = e
                raise e
            except (OSError, errors.Error, asyncio.TimeoutError) as e:
                self._err = e
                await self._error_cb(e)

# Bail on first attempt if reconnecting is disallowed.
                if not self.options["allow_reconnect"]:
                    raise e

await self._close(Client.DISCONNECTED, False)
                if self._current_server is not None:
                    self._current_server.last_attempt = time.monotonic()
                    self._current_server.reconnects += 1

def _setup_nkeys_connect(self) -> None:
        if self._user_credentials is not None:
            self._setup_nkeys_jwt_connect()
        else:
            self._setup_nkeys_seed_connect()

def _setup_nkeys_jwt_connect(self) -> None:
        assert self._user_credentials, "_user_credentials required"
        import os

import nkeys

creds: Credentials = self._user_credentials

if isinstance(creds, tuple):
            assert len(creds) == 2

def user_cb() -> bytearray:
                contents = None
                with open(creds[0], "rb") as f:
                    contents = bytearray(os.fstat(f.fileno()).st_size)
                    f.readinto(contents)  # type: ignore[attr-defined]
                return contents

self._user_jwt_cb = user_cb

def sig_cb(nonce: str) -> bytes:
                seed = None
                with open(creds[1], "rb") as f:
                    seed = bytearray(os.fstat(f.fileno()).st_size)
                    f.readinto(seed)  # type: ignore[attr-defined]
                kp = nkeys.from_seed(seed)
                raw_signed = kp.sign(nonce.encode())
                sig = base64.b64encode(raw_signed)

# Best effort attempt to clear from memory.
                kp.wipe()
                del kp
                del seed
                return sig

self._signature_cb = sig_cb
        elif isinstance(creds, str) or isinstance(creds, UserString) or isinstance(creds, Path):
            # Define the functions to be able to sign things using nkeys.
            def user_cb() -> bytearray:
                return self._read_creds_user_jwt(creds)

self._user_jwt_cb = user_cb

def sig_cb(nonce: str) -> bytes:
                user_seed = self._read_creds_user_nkey(creds)
                kp = nkeys.from_seed(user_seed)
                raw_signed = kp.sign(nonce.encode())
                sig = base64.b64encode(raw_signed)

# Delete all state related to the keys.
                kp.wipe()
                del user_seed
                del kp
                return sig

self._signature_cb = sig_cb

def _read_creds_user_nkey(self, creds: str | UserString | Path) -> bytearray:
        def get_user_seed(f):
            for line in f:
                # Detect line where the NKEY would start and end,
                # then seek and read into a fixed bytearray that
                # can be wiped.
                if b"BEGIN USER NKEY SEED" in line:
                    nkey_start_pos = f.tell()
                    try:
                        next(f)
                    except StopIteration:
                        raise ErrInvalidUserCredentials
                    nkey_end_pos = f.tell()
                    nkey_size = nkey_end_pos - nkey_start_pos - 1
                    f.seek(nkey_start_pos)

# Only gather enough bytes for the user seed
                    # into the pre allocated bytearray.
                    user_seed = bytearray(nkey_size)
                    f.readinto(user_seed)  # type: ignore[attr-defined]
                    return user_seed

if isinstance(creds, UserString):
            return get_user_seed(BytesIO(creds.data.encode()))

with open(creds, "rb", buffering=0) as f:
            return get_user_seed(f)

def _read_creds_user_jwt(self, creds: str | RawCredentials | Path):
        def get_user_jwt(f):
            user_jwt = None
            while True:
                line = bytearray(f.readline())
                if b"BEGIN NATS USER JWT" in line:
                    user_jwt = bytearray(f.readline())
                    break
            # Remove trailing line break but reusing same memory view.
            return user_jwt[: len(user_jwt) - 1]

if isinstance(creds, UserString):
            return get_user_jwt(BytesIO(creds.data.encode()))

with open(creds, "rb") as f:
            return get_user_jwt(f)

def _setup_nkeys_seed_connect(self) -> None:
        assert self._nkeys_seed or self._nkeys_seed_str, "Client.connect must be called first"

import nkeys

def _get_nkeys_seed() -> nkeys.KeyPair:
            import os

if self._nkeys_seed_str:
                seed = bytearray(self._nkeys_seed_str.encode())
            else:
                creds = self._nkeys_seed
                with open(creds, "rb") as f:
                    seed = bytearray(os.fstat(f.fileno()).st_size)
                    f.readinto(seed)  # type: ignore[attr-defined]
            key_pair = nkeys.from_seed(seed)
            del seed
            return key_pair

kp = _get_nkeys_seed()
        self._public_nkey = kp.public_key.decode()
        kp.wipe()
        del kp

def sig_cb(nonce: str) -> bytes:
            kp = _get_nkeys_seed()
            raw_signed = kp.sign(nonce.encode())
            sig = base64.b64encode(raw_signed)

# Best effort attempt to clear from memory.
            kp.wipe()
            del kp
            return sig

self._signature_cb = sig_cb

async def close(self) -> None:
        """
        Closes the socket to which we are connected and
        sets the client to be in the CLOSED state.
        No further reconnections occur once reaching this point.
        """
        await self._close(Client.CLOSED)

async def _close(self, status: int, do_cbs: bool = True) -> None:
        if self.is_closed:
            self._status = status
            return
        self._status = Client.CLOSED

# Kick the flusher once again so that Task breaks and avoid pending futures.
        await self._flush_pending()

if self._reading_task is not None and not self._reading_task.cancelled():
            self._reading_task.cancel()

if self._ping_interval_task is not None and not self._ping_interval_task.cancelled():
            self._ping_interval_task.cancel()

if self._flusher_task is not None and not self._flusher_task.cancelled():
            self._flusher_task.cancel()

if self._reconnection_task is not None and not self._reconnection_task.done():
            self._reconnection_task.cancel()

# Wait for the reconnection task to be done which should be soon.
            try:
                if self._reconnection_task_future is not None and not self._reconnection_task_future.cancelled():
                    await asyncio.wait_for(
                        self._reconnection_task_future,
                        self.options["reconnect_time_wait"],
                    )
            except (asyncio.CancelledError, asyncio.TimeoutError):
                pass

# Relinquish control to allow background tasks to wrap up.
        await asyncio.sleep(0)

if self._current_server is not None and self._transport:
            # In case there is any pending data at this point, flush before disconnecting.
            if self._pending_data_size > 0:
                self._transport.writelines(self._pending[:])
                self._pending = []
                self._pending_data_size = 0
                await self._transport.drain()

# Cleanup subscriptions since not reconnecting so no need
        # to replay the subscriptions anymore.
        for sub in self._subs.values():
            # Async subs use join when draining already so just cancel here.
            if sub._wait_for_msgs_task and not sub._wait_for_msgs_task.done():
                sub._wait_for_msgs_task.cancel()
            if sub._message_iterator:
                sub._message_iterator._cancel()
            # Sync subs may have some inflight next_msg calls that could be blocking
            # so cancel them here to unblock them.
            if sub._pending_next_msgs_calls:
                for fut in sub._pending_next_msgs_calls.values():
                    fut.cancel()
                sub._pending_next_msgs_calls.clear()
        self._subs.clear()

if self._transport is not None:
            self._transport.close()
            try:
                await self._transport.wait_closed()
            except Exception as e:
                await self._error_cb(e)

if do_cbs:
            if self._disconnected_cb is not None:
                await self._disconnected_cb()
            if self._closed_cb is not None:
                await self._closed_cb()

# Set the client_id and subscription prefix back to None
        self._client_id = None
        self._resp_sub_prefix = None

async def drain(self) -> None:
        """
        drain will put a connection into a drain state. All subscriptions will
        immediately be put into a drain state. Upon completion, the publishers
        will be drained and can not publish any additional messages. Upon draining
        of the publishers, the connection will be closed. Use the `closed_cb`
        option to know when the connection has moved from draining to closed.

"""
        if self.is_draining:
            return
        if self.is_closed:
            raise errors.ConnectionClosedError
        if self.is_connecting or self.is_reconnecting:
            raise errors.ConnectionReconnectingError

drain_tasks = []
        for sub in self._subs.values():
            coro = sub._drain()
            task = asyncio.get_running_loop().create_task(coro)
            drain_tasks.append(task)

drain_is_done = asyncio.gather(*drain_tasks)

# Start draining the subscriptions.
        # Relinquish CPU to allow drain tasks to start in the background,
        # before setting state to draining.
        await asyncio.sleep(0)
        self._status = Client.DRAINING_SUBS

try:
            await asyncio.wait_for(drain_is_done, self.options["drain_timeout"])
        except asyncio.TimeoutError:
            drain_is_done.exception()
            drain_is_done.cancel()
            await self._error_cb(errors.DrainTimeoutError())
        except asyncio.CancelledError:
            pass
        finally:
            self._status = Client.DRAINING_PUBS
            await self.flush()
            await self._close(Client.CLOSED)

async def publish(
        self,
        subject: str,
        payload: bytes = b"",
        reply: str = "",
        headers: Optional[Dict[str, str]] = None,
    ) -> None:
        """
        Publishes a NATS message.

:param subject: Subject to which the message will be published.
        :param payload: Message data.
        :param reply: Inbox to which a responder can respond.
        :param headers: Optional message header.

import asyncio
            import nats

async def main():
                nc = await nats.connect('demo.nats.io')

# Publish as message with an inbox.
                inbox = nc.new_inbox()
                sub = await nc.subscribe('hello')

# Simple publishing
                await nc.publish('hello', b'Hello World!')

# Publish with a reply
                await nc.publish('hello', b'Hello World!', reply=inbox)

# Publish with headers
                await nc.publish('hello', b'With Headers', headers={'Foo':'Bar'})

while True:
                    try:
                        msg = await sub.next_msg()
                    except:
                        break
                    print('----------------------')
                    print('Subject:', msg.subject)
                    print('Reply  :', msg.reply)
                    print('Data   :', msg.data)
                    print('Headers:', msg.header)

if __name__ == '__main__':
                asyncio.run(main())

"""

if self.is_closed:
            raise errors.ConnectionClosedError
        if self.is_draining_pubs:
            raise errors.ConnectionDrainingError

payload_size = len(payload)
        if not self.is_connected:
            if self._max_pending_size <= 0 or payload_size + self._pending_data_size > self._max_pending_size:
                # Cannot publish during a reconnection when the buffering is disabled,
                # or if pending buffer is already full.
                raise errors.OutboundBufferLimitError

if payload_size > self._max_payload:
            raise errors.MaxPayloadError
        await self._send_publish(subject, reply, payload, payload_size, headers)

async def _send_publish(
        self,
        subject: str,
        reply: str,
        payload: bytes,
        payload_size: int,
        headers: Optional[Dict[str, Any]],
    ) -> None:
        """
        Sends PUB command to the NATS server.
        """
        if subject == "":
            # Avoid sending messages with empty replies.
            raise errors.BadSubjectError

pub_cmd = None
        if headers is None:
            pub_cmd = prot_command.pub_cmd(subject, reply, payload)
        else:
            hdr = bytearray()
            hdr.extend(NATS_HDR_LINE)
            hdr.extend(_CRLF_)
            for k, v in headers.items():
                key = k.strip()
                if not key:
                    # Skip empty keys
                    continue
                hdr.extend(key.encode())
                hdr.extend(b": ")
                value = v.strip()
                hdr.extend(value.encode())
                hdr.extend(_CRLF_)
            hdr.extend(_CRLF_)
            pub_cmd = prot_command.hpub_cmd(subject, reply, hdr, payload)

self.stats["out_msgs"] += 1
        self.stats["out_bytes"] += payload_size
        await self._send_command(pub_cmd)
        if self._flush_queue is not None and self._flush_queue.empty():
            await self._flush_pending()

async def subscribe(
        self,
        subject: str,
        queue: str = "",
        cb: Optional[Callable[[Msg], Awaitable[None]]] = None,
        future: Optional[asyncio.Future] = None,
        max_msgs: int = 0,
        pending_msgs_limit: int = DEFAULT_SUB_PENDING_MSGS_LIMIT,
        pending_bytes_limit: int = DEFAULT_SUB_PENDING_BYTES_LIMIT,
    ) -> Subscription:
        """
        subscribe registers interest in a given subject.

If a callback is provided, messages will be processed asynchronously.

If a callback isn't provided, messages can be retrieved via an
        asynchronous iterator on the returned subscription object.
        """
        if not subject or (" " in subject):
            raise errors.BadSubjectError

if queue and (" " in queue):
            raise errors.BadSubjectError

if self.is_closed:
            raise errors.ConnectionClosedError

if self.is_draining:
            raise errors.ConnectionDrainingError

self._sid += 1
        sid = self._sid

sub = Subscription(
            self,
            sid,
            subject,
            queue=queue,
            cb=cb,
            future=future,
            max_msgs=max_msgs,
            pending_msgs_limit=pending_msgs_limit,
            pending_bytes_limit=pending_bytes_limit,
        )

sub._start(self._error_cb)
        self._subs[sid] = sub
        await self._send_subscribe(sub)
        return sub

def _remove_sub(self, sid: int, max_msgs: int = 0) -> None:
        self._subs.pop(sid, None)

async def _send_subscribe(self, sub: Subscription) -> None:
        sub_cmd = None
        if sub._queue is None:
            sub_cmd = prot_command.sub_cmd(sub._subject, EMPTY, sub._id)
        else:
            sub_cmd = prot_command.sub_cmd(sub._subject, sub._queue, sub._id)
        await self._send_command(sub_cmd)
        await self._flush_pending()

async def _init_request_sub(self) -> None:
        self._resp_map = {}

self._resp_sub_prefix = self._inbox_prefix[:]
        self._resp_sub_prefix.extend(b".")
        self._resp_sub_prefix.extend(self._nuid.next())
        self._resp_sub_prefix.extend(b".")
        resp_mux_subject = self._resp_sub_prefix[:]
        resp_mux_subject.extend(b"*")
        await self.subscribe(resp_mux_subject.decode(), cb=self._request_sub_callback)

async def _request_sub_callback(self, msg: Msg) -> None:
        token = msg.subject[len(self._inbox_prefix) + 22 + 2 :]
        future = self._resp_map.get(token)

if not future:
            return

if not future.done():
            future.set_result(msg)

async def request(
        self,
        subject: str,
        payload: bytes = b"",
        timeout: float = 0.5,
        old_style: bool = False,
        headers: Optional[Dict[str, Any]] = None,
    ) -> Msg:
        """
        Implements the request/response pattern via pub/sub
        using a single wildcard subscription that handles
        the responses.

"""
        if old_style:
            # FIXME: Support headers in old style requests.
            return await self._request_old_style(subject, payload, timeout=timeout)
        else:
            msg = await self._request_new_style(subject, payload, timeout=timeout, headers=headers)
        if msg.headers and msg.headers.get(nats.js.api.Header.STATUS) == NO_RESPONDERS_STATUS:
            raise errors.NoRespondersError
        return msg

async def _request_new_style(
        self,
        subject: str,
        payload: bytes,
        timeout: float = 1,
        headers: Optional[Dict[str, Any]] = None,
    ) -> Msg:
        if self.is_draining_pubs:
            raise errors.ConnectionDrainingError

if not self._resp_sub_prefix:
            await self._init_request_sub()
        assert self._resp_sub_prefix

# Use a new NUID + couple of unique token bytes to identify the request,
        token = self._nuid.next()
        token.extend(token_hex(2).encode())
        inbox = self._resp_sub_prefix[:]
        inbox.extend(token)

# Then use the future to get the response.
        future: asyncio.Future = asyncio.Future()
        future.add_done_callback(lambda f: self._resp_map.pop(token.decode(), None))
        self._resp_map[token.decode()] = future

# Publish the request
        await self.publish(subject, payload, reply=inbox.decode(), headers=headers)

# Wait for the response or give up on timeout.
        try:
            return await asyncio.wait_for(future, timeout)
        except asyncio.TimeoutError:
            raise errors.TimeoutError

def new_inbox(self) -> str:
        """
        new_inbox returns a unique inbox that can be used
        for NATS requests or subscriptions::

# Create unique subscription to receive direct messages.
           inbox = nc.new_inbox()
           sub = await nc.subscribe(inbox)
           nc.publish('broadcast', b'', reply=inbox)
           msg = sub.next_msg()
        """
        next_inbox = self._inbox_prefix[:]
        next_inbox.extend(b".")
        next_inbox.extend(self._nuid.next())
        return next_inbox.decode()

async def _request_old_style(self, subject: str, payload: bytes, timeout: float = 1) -> Msg:
        """
        Implements the request/response pattern via pub/sub
        using an ephemeral subscription which will be published
        with a limited interest of 1 reply returning the response
        or raising a Timeout error.
        """
        inbox = self.new_inbox()

future: asyncio.Future[Msg] = asyncio.Future()
        sub = await self.subscribe(inbox, future=future, max_msgs=1)
        await sub.unsubscribe(limit=1)
        await self.publish(subject, payload, reply=inbox)

try:
            msg = await asyncio.wait_for(future, timeout)
            if msg.headers:
                if msg.headers.get(nats.js.api.Header.STATUS) == NO_RESPONDERS_STATUS:
                    raise errors.NoRespondersError
            return msg
        except asyncio.TimeoutError:
            await sub.unsubscribe()
            future.cancel()
            raise errors.TimeoutError

async def _send_unsubscribe(self, sid: int, limit: int = 0) -> None:
        unsub_cmd = prot_command.unsub_cmd(sid, limit)
        await self._send_command(unsub_cmd)
        await self._flush_pending()

async def flush(self, timeout: int = DEFAULT_FLUSH_TIMEOUT) -> None:
        """
        Sends a ping to the server expecting a pong back ensuring
        what we have written so far has made it to the server and
        also enabling measuring of roundtrip time.
        In case a pong is not returned within the allowed timeout,
        then it will raise nats.errors.TimeoutError
        """
        if timeout <= 0:
            raise errors.BadTimeoutError

if self.is_closed:
            raise errors.ConnectionClosedError

future: asyncio.Future = asyncio.Future()
        try:
            await self._send_ping(future)
            await asyncio.wait_for(future, timeout)
        except asyncio.TimeoutError:
            future.cancel()
            raise errors.FlushTimeoutError

@property
    def connected_url(self) -> Optional[ParseResult]:
        if self._current_server and self.is_connected:
            return self._current_server.uri
        return None

@property
    def servers(self) -> List[ParseResult]:
        servers = []
        for srv in self._server_pool:
            servers.append(srv.uri)
        return servers

@property
    def discovered_servers(self) -> List[ParseResult]:
        servers = []
        for srv in self._server_pool:
            if srv.discovered:
                servers.append(srv.uri)
        return servers

@property
    def server_pool(self) -> List[Server]:
        """
        Returns a copy of the current server pool.
        """
        return [Server(uri=srv.uri, reconnects=srv.reconnects) for srv in self._server_pool]

def set_server_pool(self, servers: List[str]) -> None:
        """
        Replaces the current server pool with the provided list of server URLs.

The new pool will be used on the next reconnect attempt. It does not
        trigger an immediate reconnect. The new pool is subject to the same
        rules as the default one (randomization unless disabled, max reconnect
        attempts, etc).

Unless advertised server discovery is disabled, the client will
        continue to discover and add new servers to the pool as it receives
        INFO messages from the server.

:param servers: List of server URLs to use as the new pool.
        :raises errors.ConnectionClosedError: If the connection is closed.
        """
        if self.is_closed:
            raise errors.ConnectionClosedError

# Parse and validate all URLs first without modifying state.
        new_pool: List[Srv] = []
        for server in servers:
            uri = self._parse_server_uri(server)
            srv = Srv(uri)
            # Preserve state from existing pool entries.
            for old_srv in self._server_pool:
                if old_srv.uri.netloc == uri.netloc:
                    srv.reconnects = old_srv.reconnects
                    srv.did_connect = old_srv.did_connect
                    srv.last_attempt = old_srv.last_attempt
                    break
            new_pool.append(srv)

self._server_pool = new_pool

# Update _current_server to point to the corresponding server
        # in the new pool if it exists, so _select_next_server works correctly.
        if self._current_server is not None:
            current_netloc = self._current_server.uri.netloc
            found = False
            for srv in new_pool:
                if srv.uri.netloc == current_netloc:
                    self._current_server = srv
                    found = True
                    break
            if not found and len(new_pool) > 0:
                self._current_server = new_pool[0]

@property
    def max_payload(self) -> int:
        """
        Returns the max payload which we received from the servers INFO
        """
        return self._max_payload

@property
    def client_id(self) -> Optional[int]:
        """
        Returns the client id which we received from the servers INFO
        """
        return self._client_id

@property
    def last_error(self) -> Optional[Exception]:
        """
        Returns the last error which may have occurred.
        """
        return self._err

@property
    def pending_data_size(self) -> int:
        return self._pending_data_size

@property
    def is_closed(self) -> bool:
        return self._status == Client.CLOSED

@property
    def is_reconnecting(self) -> bool:
        return self._status == Client.RECONNECTING

@property
    def is_connected(self) -> bool:
        return (self._status == Client.CONNECTED) or self.is_draining

@property
    def is_connecting(self) -> bool:
        return self._status == Client.CONNECTING

@property
    def is_draining(self) -> bool:
        return self._status == Client.DRAINING_SUBS or self._status == Client.DRAINING_PUBS

@property
    def is_draining_pubs(self) -> bool:
        return self._status == Client.DRAINING_PUBS

@property
    def connected_server_version(self) -> ServerVersion:
        """
        Returns the ServerVersion of the server to which the client
        is currently connected.
        """
        if self._current_server and self._current_server.server_version:
            return ServerVersion(self._current_server.server_version)
        return ServerVersion("0.0.0-unknown")

@property
    def ssl_context(self) -> ssl.SSLContext:
        ssl_context: Optional[ssl.SSLContext] = None
        if "tls" in self.options:
            ssl_context = self.options.get("tls")
        else:
            ssl_context = ssl.create_default_context()
        if ssl_context is None:
            raise errors.Error("nats: no ssl context provided")
        return ssl_context

async def _send_command(self, cmd: bytes, priority: bool = False) -> None:
        if priority:
            self._pending.insert(0, cmd)
        else:
            self._pending.append(cmd)
        self._pending_data_size += len(cmd)
        if self._max_pending_size > 0 and self._pending_data_size > self._max_pending_size:
            # Only flush force timeout on publish
            await self._flush_pending(force_flush=True)

async def _flush_pending(
        self,
        force_flush: bool = False,
    ) -> Any:
        assert self._flush_queue, "Client.connect must be called first"
        try:
            future: asyncio.Future = asyncio.Future()
            if not self.is_connected:
                future.set_result(None)
                return future

# kick the flusher!
            await self._flush_queue.put(future)

if force_flush:
                try:
                    await asyncio.wait_for(future, self._flush_timeout)
                except asyncio.TimeoutError:
                    # Report to the async callback that there was a timeout.
                    await self._error_cb(errors.FlushTimeoutError())

except asyncio.CancelledError:
            pass

@staticmethod
    def _parse_server_uri(connect_url: str) -> ParseResult:
        """
        Parse a single server URL string into a ParseResult.
        Handles scheme defaults and port defaults.
        """
        try:
            if "nats://" in connect_url or "tls://" in connect_url:
                uri = urlparse(connect_url)
            elif "ws://" in connect_url or "wss://" in connect_url:
                uri = urlparse(connect_url)
            elif ":" in connect_url:
                uri = urlparse(f"nats://{connect_url}")
            else:
                uri = urlparse(f"nats://{connect_url}:4222")

if uri.port is None and uri.scheme not in ("ws", "wss"):
                uri = urlparse(f"nats://{uri.hostname}:4222")
        except ValueError:
            raise errors.Error("nats: invalid connect url option")

if uri.hostname is None or uri.hostname == "none":
            raise errors.Error("nats: invalid hostname in connect url")
        return uri

def _setup_server_pool(self, connect_url: Union[List[str]]) -> None:
        if isinstance(connect_url, str):
            uri = self._parse_server_uri(connect_url)
            self._server_pool.append(Srv(uri))
        elif isinstance(connect_url, list):
            try:
                for server in connect_url:
                    uri = urlparse(server)
                    self._server_pool.append(Srv(uri))
            except ValueError:
                raise errors.Error("nats: invalid connect url option")
            # make sure protocols aren't mixed
            if not (
                all(server.uri.scheme in ("nats", "tls") for server in self._server_pool)
                or all(server.uri.scheme in ("ws", "wss") for server in self._server_pool)
            ):
                raise errors.Error("nats: mixing of websocket and non websocket URLs is not allowed")
        else:
            raise errors.Error("nats: invalid connect url option")

async def _connect_to_server(self, s: Srv) -> None:
        """
        Establishes a TCP/WebSocket connection to the given server.
        """
        s.last_attempt = time.monotonic()
        if not self._transport:
            if s.uri.scheme in ("ws", "wss"):
                self._transport = WebSocketTransport(ws_headers=self.options["ws_connection_headers"])
            else:
                self._transport = TcpTransport()
        if s.uri.scheme == "wss":
            await self._transport.connect_tls(
                s.uri,
                ssl_context=self.ssl_context,
                buffer_size=DEFAULT_BUFFER_SIZE,
                connect_timeout=self.options["connect_timeout"],
            )
        else:
            await self._transport.connect(
                s.uri,
                buffer_size=DEFAULT_BUFFER_SIZE,
                connect_timeout=self.options["connect_timeout"],
            )

async def _select_next_server(self) -> None:
        """
        Looks up in the server pool for an available server
        and attempts to connect.
        """

while True:
            if len(self._server_pool) == 0:
                self._current_server = None
                raise errors.NoServersError

now = time.monotonic()
            s = self._server_pool.pop(0)
            if self.options["max_reconnect_attempts"] > 0:
                if s.reconnects > self.options["max_reconnect_attempts"]:
                    # Discard server since already tried to reconnect too many times
                    continue

# Not yet exceeded max_reconnect_attempts so can still use
            # this server in the future.
            self._server_pool.append(s)
            if s.last_attempt is not None and now < s.last_attempt + self.options["reconnect_time_wait"]:
                # Backoff connecting to server if we attempted recently.
                await asyncio.sleep(self.options["reconnect_time_wait"])
            try:
                await self._connect_to_server(s)
                self._current_server = s
                break
            except Exception as e:
                s.last_attempt = time.monotonic()
                s.reconnects += 1

self._err = e
                await self._error_cb(e)
                continue

async def _process_err(self, err_msg: str) -> None:
        """
        Processes the raw error message sent by the server
        and close connection with current server.
        """
        if STALE_CONNECTION in err_msg:
            await self._process_op_err(errors.StaleConnectionError())
            return

if AUTHORIZATION_VIOLATION in err_msg:
            self._err = errors.AuthorizationError()
        else:
            prot_err = err_msg.strip("'")
            m = f"nats: {prot_err}"
            err = errors.Error(m)
            self._err = err

if PERMISSIONS_ERR in m:
                await self._error_cb(err)
                return

do_cbs = False
        if not self.is_connecting:
            do_cbs = True

# FIXME: Some errors such as 'Invalid Subscription'
        # do not cause the server to close the connection.
        # For now we handle similar as other clients and close.
        asyncio.create_task(self._close(Client.CLOSED, do_cbs))

async def _process_op_err(self, e: Exception) -> None:
        """
        Process errors which occurred while reading or parsing
        the protocol. If allow_reconnect is enabled it will
        try to switch the server to which it is currently connected
        otherwise it will disconnect.
        """
        if self.is_closed or self.is_reconnecting:
            return

if self.options["allow_reconnect"] and (self.is_connected or self.is_connecting):
            self._status = Client.RECONNECTING
            self._ps.reset()

if self._reconnection_task is not None and not self._reconnection_task.cancelled():
                # Cancel the previous task in case it may still be running.
                self._reconnection_task.cancel()

self._reconnection_task = asyncio.get_running_loop().create_task(self._attempt_reconnect())
        else:
            self._process_disconnect()
            self._err = e
            await self._close(Client.CLOSED, True)

async def _attempt_reconnect(self) -> None:
        assert self._current_server, "Client.connect must be called first"
        if self._reading_task is not None and not self._reading_task.cancelled():
            self._reading_task.cancel()

if self._ping_interval_task is not None and not self._ping_interval_task.cancelled():
            self._ping_interval_task.cancel()

if self._flusher_task is not None and not self._flusher_task.cancelled():
            self._flusher_task.cancel()

self._err = None
        if self._disconnected_cb is not None:
            await self._disconnected_cb()

if self.is_closed:
            return

if "dont_randomize" not in self.options or not self.options["dont_randomize"]:
            shuffle(self._server_pool)

# Create a future that the client can use to control waiting
        # on the reconnection attempts.
        self._reconnection_task_future = asyncio.Future()
        while True:
            try:
                if self._reconnect_to_server_handler is not None:
                    # Invoke the user-provided handler to select a server.
                    max_reconnect = self.options["max_reconnect_attempts"]
                    if max_reconnect > 0:
                        eligible = [s for s in self._server_pool if s.reconnects <= max_reconnect]
                    else:
                        eligible = list(self._server_pool)

if len(eligible) == 0:
                        raise errors.NoServersError

server_snapshot = [Server(uri=s.uri, reconnects=s.reconnects) for s in eligible]
                    try:
                        selected, callback_delay = self._reconnect_to_server_handler(
                            server_snapshot, self._server_info.copy()
                        )
                    except Exception as e:
                        await self._error_cb(e)
                        continue

if selected is not None:
                        matched = None
                        for s in eligible:
                            if s.uri.netloc == selected.uri.netloc:
                                matched = s
                                break
                        if matched is not None:
                            self._current_server = matched
                        else:
                            await self._error_cb(errors.ServerNotInPoolError())
                            selected = None

if selected is None:
                        self._current_server = eligible[0]

if callback_delay > 0:
                        await asyncio.sleep(callback_delay)

await self._connect_to_server(self._current_server)
                else:
                    # Default server selection via round-robin.
                    await self._select_next_server()

assert self._transport
                await self._process_connect_init()

# Consider a reconnect to be done once CONNECT was
                # processed by the server successfully.
                self.stats["reconnects"] += 1

# Reset reconnect attempts for this server
                # since have successfully connected.
                self._current_server.did_connect = True
                self._current_server.reconnects = 0

# Replay all the subscriptions in case there were some.
                subs_to_remove = []
                for sid, sub in self._subs.items():
                    max_msgs = 0
                    if sub._max_msgs > 0:
                        # If we already hit the message limit, remove the subscription and don't
                        # resubscribe.
                        if sub._received >= sub._max_msgs:
                            subs_to_remove.append(sid)
                            continue
                        # auto unsubscribe the number of messages we have left
                        max_msgs = sub._max_msgs - sub._received

sub_cmd = prot_command.sub_cmd(sub._subject, sub._queue, sid)
                    self._transport.write(sub_cmd)

if max_msgs > 0:
                        unsub_cmd = prot_command.unsub_cmd(sid, max_msgs)
                        self._transport.write(unsub_cmd)

for sid in subs_to_remove:
                    self._subs.pop(sid)

await self._transport.drain()

# Flush pending data before continuing in connected status.
                # FIXME: Could use future here and wait for an error result
                # to bail earlier in case there are errors in the connection.
                # await self._flush_pending(force_flush=True)
                await self._flush_pending()
                self._status = Client.CONNECTED
                await self.flush()
                if self._reconnected_cb is not None:
                    await self._reconnected_cb()
                self._reconnection_task_future = None
                break
            except errors.NoServersError as e:
                self._err = e
                await self.close()
                break
            except (OSError, errors.Error, asyncio.TimeoutError) as e:
                self._err = e
                await self._error_cb(e)
                self._status = Client.RECONNECTING
                self._current_server.last_attempt = time.monotonic()
                self._current_server.reconnects += 1
            except asyncio.CancelledError:
                break

if self._reconnection_task_future is not None and not self._reconnection_task_future.cancelled():
            self._reconnection_task_future.set_result(True)

def _connect_command(self) -> bytes:
        """
        Generates a JSON string with the params to be used
        when sending CONNECT to the server.

->> CONNECT {"lang": "python3"}

"""
        options = {
            "verbose": self.options["verbose"],
            "pedantic": self.options["pedantic"],
            "lang": __lang__,
            "version": __version__,
            "protocol": PROTOCOL,
        }
        if "headers" in self._server_info:
            options["headers"] = self._server_info["headers"]
            options["no_responders"] = self._server_info["headers"]

if self._auth_configured:
            if "nonce" in self._server_info and self._signature_cb is not None:
                sig = self._signature_cb(self._server_info["nonce"])
                options["sig"] = sig.decode()

if self._user_jwt_cb is not None:
                    jwt = self._user_jwt_cb()
                    options["jwt"] = jwt.decode()
                elif self._public_nkey is not None:
                    options["nkey"] = self._public_nkey
            # In case there is no password, then consider handle
            # sending a token instead.
            elif self.options["user"] is not None and self.options["password"] is not None:
                options["user"] = self.options["user"]
                options["pass"] = self.options["password"]
            elif self.options["token"] is not None:
                token = self.options["token"]
                if callable(token):
                    token = token()
                options["auth_token"] = token
            elif self._current_server and self._current_server.uri.username is not None:
                if self._current_server.uri.password is None:
                    options["auth_token"] = self._current_server.uri.username
                else:
                    options["user"] = self._current_server.uri.username
                    options["pass"] = self._current_server.uri.password

if self.options["name"] is not None:
            options["name"] = self.options["name"]
        if self.options["no_echo"] is not None:
            options["echo"] = not self.options["no_echo"]

connect_opts = json.dumps(options, sort_keys=True)
        return b"".join([CONNECT_OP + _SPC_ + connect_opts.encode() + _CRLF_])

async def _process_ping(self) -> None:
        """
        Process PING sent by server.
        """
        await self._send_command(PONG)
        await self._flush_pending()

async def _process_pong(self) -> None:
        """
        Process PONG sent by server.
        """
        if len(self._pongs) > 0:
            future = self._pongs.pop(0)
            future.set_result(True)
            self._pongs_received += 1
            self._pings_outstanding = 0

def _is_control_message(self, data, header: Dict[str, str]) -> Optional[str]:
        if len(data) > 0:
            return None
        status = header.get(nats.js.api.Header.STATUS)
        if status == CTRL_STATUS:
            return header.get(nats.js.api.Header.DESCRIPTION)
        return None

async def _process_headers(self, headers) -> Optional[Dict[str, str]]:
        if not headers:
            return None

hdr: Optional[Dict[str, str]] = None
        raw_headers = headers[NATS_HDR_LINE_SIZE:]

# If the first character is an empty space, then this is
        # an inline status message sent by the server.
        #
        # NATS/1.0 404\r\n\r\n
        # NATS/1.0 503\r\n\r\n
        # NATS/1.0 404 No Messages\r\n\r\n
        #
        # Note: it is possible to receive a message with both inline status
        # and a set of headers.
        #
        # NATS/1.0 100\r\nIdle Heartbeat\r\nNats-Last-Consumer: 1016\r\nNats-Last-Stream: 1024\r\n\r\n
        #
        if raw_headers[0] == _SPC_BYTE_:
            # Special handling for status messages.
            line = headers[len(NATS_HDR_LINE) + 1 :]
            status = line[:STATUS_MSG_LEN]
            desc = line[STATUS_MSG_LEN + 1 : len(line) - _CRLF_LEN_ - _CRLF_LEN_]
            stripped_status = status.strip().decode()

# Process as status only when it is a valid integer.
            hdr = {}
            if stripped_status.isdigit():
                hdr[nats.js.api.Header.STATUS.value] = stripped_status

# Move the raw_headers to end of line
            i = raw_headers.find(_CRLF_)
            raw_headers = raw_headers[i + _CRLF_LEN_ :]

if len(desc) > 0:
                # Heartbeat messages can have both headers and inline status,
                # check that there are no pending headers to be parsed.
                i = desc.find(_CRLF_)
                if i > 0:
                    hdr[nats.js.api.Header.DESCRIPTION] = desc[:i].decode()
                    parsed_hdr = self._hdr_parser.parsebytes(desc[i + _CRLF_LEN_ :])
                    for k, v in parsed_hdr.items():
                        hdr[k] = v
                else:
                    # Just inline status...
                    hdr[nats.js.api.Header.DESCRIPTION] = desc.decode()

if not len(raw_headers) > _CRLF_LEN_:
            return hdr

#
        # Example header without status:
        #
        # NATS/1.0\r\nfoo: bar\r\nhello: world
        #
        raw_headers = headers[NATS_HDR_LINE_SIZE + _CRLF_LEN_ :]
        try:
            if parse_email:
                parsed_hdr = parse_email(raw_headers).headers
            else:
                parsed_hdr = {k.strip(): v.strip() for k, v in self._hdr_parser.parsebytes(raw_headers).items()}
            if hdr:
                hdr.update(parsed_hdr)
            else:
                hdr = parsed_hdr

if parse_email:
                to_delete = []
                for k in hdr.keys():
                    if any(c in k for c in string.whitespace):
                        to_delete.append(k)
                for k in to_delete:
                    del hdr[k]

except Exception as e:
            await self._error_cb(e)
            return hdr

return hdr or None

async def _process_msg(
        self,
        sid: int,
        subject: bytes,
        reply: bytes,
        data: bytes,
        headers: bytes,
    ) -> None:
        """
        Process MSG sent by server.
        """
        payload_size = len(data)
        self.stats["in_msgs"] += 1
        self.stats["in_bytes"] += payload_size

sub = self._subs.get(sid)
        if not sub:
            # Skip in case no subscription present.
            return

sub._received += 1
        if sub._max_msgs > 0 and sub._received >= sub._max_msgs:
            # Enough messages so can throwaway subscription now, the
            # pending messages will still be in the subscription
            # internal queue and the task will finish once the last
            # message is processed.
            self._subs.pop(sid, None)

hdr = await self._process_headers(headers)
        msg = self._build_message(sid, subject, reply, data, hdr)
        if not msg:
            return

# Process flow control messages in case of using a JetStream context.
        ctrl_msg = None
        fc_reply = None
        if sub._jsi:
            #########################################
            #                                       #
            # JetStream Control Messages Processing #
            #                                       #
            #########################################
            jsi = sub._jsi
            jsi._active = True
            if hdr:
                ctrl_msg = self._is_control_message(data, hdr)

# Check if the heartbeat has a "Consumer Stalled" header, if
                # so, the value is the FC reply to send a nil message to.
                # We will send it at the end of this function.
                if ctrl_msg and ctrl_msg.startswith("Idle"):
                    fc_reply = hdr.get(nats.js.api.Header.CONSUMER_STALLED)

# OrderedConsumer: checkOrderedMsgs
            if not ctrl_msg and jsi._ordered and msg.reply:
                did_reset = None
                tokens = Msg.Metadata._get_metadata_fields(msg.reply)
                # FIXME: Support JS Domains.
                sseq = int(tokens[5])
                dseq = int(tokens[6])
                if dseq != jsi._dseq:
                    # Pick up from where we last left.
                    did_reset = await jsi.reset_ordered_consumer(jsi._sseq + 1)
                else:
                    # Update our tracking
                    jsi._dseq = dseq + 1
                    jsi._sseq = sseq
                if did_reset:
                    return

# Skip processing if this is a control message.
        if not ctrl_msg:
            # Check if it is an old style request.
            if sub._future:
                if sub._future.cancelled():
                    # Already gave up, nothing to do.
                    return
                sub._future.set_result(msg)
                return

# Let subscription wait_for_msgs coroutine process the messages,
            # but in case sending to the subscription task would block,
            # then consider it to be an slow consumer and drop the message.
            try:
                sub._pending_size += payload_size
                # allow setting pending_bytes_limit to 0 to disable
                if sub._pending_bytes_limit > 0 and sub._pending_size >= sub._pending_bytes_limit:
                    # Subtract the bytes since the message will be thrown away
                    # so it would not be pending data.
                    sub._pending_size -= payload_size

await self._error_cb(
                        errors.SlowConsumerError(subject=msg.subject, reply=msg.reply, sid=sid, sub=sub)
                    )
                    return
                sub._pending_queue.put_nowait(msg)
            except asyncio.QueueFull:
                sub._pending_size -= len(msg.data)
                await self._error_cb(errors.SlowConsumerError(subject=msg.subject, reply=msg.reply, sid=sid, sub=sub))

# Store the ACK metadata from the message to
            # compare later on with the received heartbeat.
            if sub._jsi:
                sub._jsi.track_sequences(msg.reply)
        elif ctrl_msg.startswith("Flow") and msg.reply and sub._jsi:
            # This is a flow control message.
            # We will schedule the send of the FC reply once we have delivered the
            # DATA message that was received before this flow control message, which
            # has sequence `jsi.fciseq`. However, it is possible that this message
            # has already been delivered, in that case, we need to send the FC reply now.
            if sub._jsi.get_js_delivered() >= sub._jsi._fciseq:
                fc_reply = msg.reply
            else:
                # Schedule a reply after the previous message is delivered.
                sub._jsi.schedule_flow_control_response(msg.reply)

# Handle flow control response.
        if fc_reply:
            await self.publish(fc_reply)

if ctrl_msg and not msg.reply and ctrl_msg.startswith("Idle"):
            if sub._jsi:
                await sub._jsi.check_for_sequence_mismatch(msg)

def _build_message(
        self,
        sid: int,
        subject: bytes,
        reply: bytes,
        data: bytes,
        headers: Optional[Dict[str, str]],
    ):
        return self.msg_class(
            subject=subject.decode(),
            reply=reply.decode(),
            data=data,
            headers=headers,
            _client=self,
            _sid=sid,
        )

def _process_disconnect(self) -> None:
        """
        Process disconnection from the server and set client status
        to DISCONNECTED.
        """
        self._status = Client.DISCONNECTED

async def _process_info(self, info: Dict[str, Any], initial_connection: bool = False) -> None:
        """
        Process INFO lines sent by the server to reconfigure client
        with latest updates from cluster to enable server discovery.
        """
        assert self._current_server, "Client.connect must be called first"
        if "connect_urls" in info:
            if info["connect_urls"]:
                connect_urls = []
                for connect_url in info["connect_urls"]:
                    scheme = ""
                    if self._current_server.uri.scheme == "tls":
                        scheme = "tls"
                    else:
                        scheme = "nats"

uri = urlparse(f"{scheme}://{connect_url}")
                    srv = Srv(uri)
                    srv.discovered = True

# Check whether we should reuse the original hostname.
                    if (
                        "tls_required" in self._server_info
                        and self._server_info["tls_required"]
                        and self._host_is_ip(uri.hostname)
                    ):
                        srv.tls_name = self._current_server.uri.hostname

# Filter for any similar server in the server pool already.
                    should_add = True
                    for s in self._server_pool:
                        if uri.netloc == s.uri.netloc:
                            should_add = False
                    if should_add:
                        connect_urls.append(srv)

if self.options["dont_randomize"] is not True:
                    shuffle(connect_urls)
                for srv in connect_urls:
                    self._server_pool.append(srv)

if not initial_connection and connect_urls and self._discovered_server_cb:
                    await self._discovered_server_cb()

def _host_is_ip(self, connect_url: Optional[str]) -> bool:
        if connect_url is None:
            return False
        try:
            ipaddress.ip_address(connect_url)
            return True
        except Exception:
            return False

async def _process_connect_init(self) -> None:
        """
        Process INFO received from the server and CONNECT to the server
        with authentication.  It is also responsible of setting up the
        reading and ping interval tasks from the client.
        """
        assert self._transport, "must be called only from Client.connect"
        assert self._current_server, "must be called only from Client.connect"
        self._status = Client.CONNECTING

# Check whether to reuse the original hostname for an implicit route.
        hostname = None
        if "tls_hostname" in self.options:
            hostname = self.options["tls_hostname"]
        elif self._current_server.tls_name is not None:
            hostname = self._current_server.tls_name
        else:
            hostname = self._current_server.uri.hostname

handshake_first = self.options["tls_handshake_first"]
        if handshake_first:
            await self._transport.connect_tls(
                hostname,
                self.ssl_context,
                DEFAULT_BUFFER_SIZE,
                self.options["connect_timeout"],
            )

connection_completed = self._transport.readline()
        info_line = await asyncio.wait_for(connection_completed, self.options["connect_timeout"])
        if INFO_OP not in info_line:
            # FIXME: Handle PING/PONG arriving first as well.
            raise errors.Error("nats: empty response from server when expecting INFO message")

_, info = info_line.split(INFO_OP + _SPC_, 1)

try:
            srv_info = json.loads(info.decode())
            self._server_info = srv_info
        except Exception:
            raise errors.Error("nats: info message, json parse error")

# In case 'auth_required' is part of INFO, then need to send credentials.
        if srv_info.get("auth_required", False):
            self._auth_configured = True

await self._process_info(srv_info, initial_connection=True)

if "version" in self._server_info:
            self._current_server.server_version = self._server_info["version"]

if "max_payload" in self._server_info:
            self._max_payload = self._server_info["max_payload"]

if "client_id" in self._server_info:
            self._client_id = self._server_info["client_id"]

if (
            "tls_required" in self._server_info
            and self._server_info["tls_required"]
            and self._current_server.uri.scheme != "ws"
        ):
            if not handshake_first:
                await self._transport.drain()  # just in case something is left

# connect to transport via tls
                await self._transport.connect_tls(
                    hostname,
                    self.ssl_context,
                    DEFAULT_BUFFER_SIZE,
                    self.options["connect_timeout"],
                )

# Refresh state of parser upon reconnect.
        if self.is_reconnecting:
            self._ps.reset()

assert self._transport
        connect_cmd = self._connect_command()
        self._transport.write(connect_cmd)
        await self._transport.drain()
        if self.options["verbose"]:
            future = self._transport.readline()
            next_op = await asyncio.wait_for(future, self.options["connect_timeout"])
            if OK_OP in next_op:
                # Do nothing
                pass
            elif ERR_OP in next_op:
                err_line = next_op.decode()
                _, err_msg = err_line.split(" ", 1)

# FIXME: Maybe handling could be more special here,
                # checking for errors.AuthorizationError for example.
                # await self._process_err(err_msg)
                raise errors.Error("nats: " + err_msg.rstrip("\r\n"))

self._transport.write(PING_PROTO)
        await self._transport.drain()

future = self._transport.readline()
        next_op = await asyncio.wait_for(future, self.options["connect_timeout"])

if PONG_PROTO in next_op:
            self._status = Client.CONNECTED
        elif ERR_OP in next_op:
            err_line = next_op.decode()
            _, err_msg = err_line.split(" ", 1)

# FIXME: Maybe handling could be more special here,
            # checking for ErrAuthorization for example.
            # await self._process_err(err_msg)
            raise errors.Error("nats: " + err_msg.rstrip("\r\n"))

if PONG_PROTO in next_op:
            self._status = Client.CONNECTED

self._reading_task = asyncio.get_running_loop().create_task(self._read_loop())
        self._pongs = []
        self._pings_outstanding = 0
        self._ping_interval_task = asyncio.get_running_loop().create_task(self._ping_interval())

# Task for kicking the flusher queue
        self._flusher_task = asyncio.get_running_loop().create_task(self._flusher())

async def _send_ping(self, future: Optional[asyncio.Future] = None) -> None:
        assert self._transport, "Client.connect must be called first"
        if future is None:
            future = asyncio.Future()
        self._pongs.append(future)
        self._transport.write(PING_PROTO)
        self._pending_data_size += len(PING_PROTO)
        await self._flush_pending()

async def _flusher(self) -> None:
        """
        Coroutine which continuously tries to consume pending commands
        and then flushes them to the socket.
        """
        assert self._transport, "Client.connect must be called first"
        assert self._flush_queue, "Client.connect must be called first"
        while True:
            if not self.is_connected or self.is_connecting:
                break

future: asyncio.Future = await self._flush_queue.get()

try:
                if self._pending_data_size > 0:
                    self._transport.writelines(self._pending[:])
                    self._pending = []
                    self._pending_data_size = 0
                    await self._transport.drain()
            except OSError as e:
                await self._error_cb(e)
                await self._process_op_err(e)
                break
            except (asyncio.CancelledError, RuntimeError, AttributeError):
                # RuntimeError in case the event loop is closed
                break
            finally:
                # future might have been cancelled.  See issue #624
                if not future.done():
                    future.set_result(None)

async def _ping_interval(self) -> None:
        while True:
            await asyncio.sleep(self.options["ping_interval"])
            if not self.is_connected:
                continue
            try:
                self._pings_outstanding += 1
                if self._pings_outstanding > self.options["max_outstanding_pings"]:
                    await self._process_op_err(ErrStaleConnection())
                    return
                await self._send_ping()
            except (asyncio.CancelledError, RuntimeError, AttributeError):
                break
            # except asyncio.InvalidStateError:
            #     pass

async def _read_loop(self) -> None:
        """
        Coroutine which gathers bytes sent by the server
        and feeds them to the protocol parser.
        In case of error while reading, it will stop running
        and its task has to be rescheduled.
        """
        while True:
            try:
                should_bail = self.is_closed or self.is_reconnecting
                if should_bail or self._transport is None:
                    break
                if self._transport.at_eof():
                    err = errors.UnexpectedEOF()
                    await self._error_cb(err)
                    await self._process_op_err(err)
                    break

b = await self._transport.read(DEFAULT_BUFFER_SIZE)
                await self._ps.parse(b)
            except errors.ProtocolError:
                await self._process_op_err(errors.ProtocolError())
                break
            except OSError as e:
                await self._process_op_err(e)
                break
            except asyncio.CancelledError:
                break
            except Exception as ex:
                _logger.error("nats: encountered error", exc_info=ex)
                break
            # except asyncio.InvalidStateError:
            #     pass

async def __aenter__(self) -> "Client":
        """For when NATS client is used in a context manager"""
        return self

async def __aexit__(self, *exc_info) -> None:
        """Close connection to NATS when used in a context manager"""
        await self._close(Client.CLOSED, do_cbs=True)

def jetstream(self, **opts) -> nats.js.JetStreamContext:
        """
        jetstream returns a context that can be used to produce and consume
        messages from NATS JetStream.

:param prefix: Default JetStream API Prefix.
        :param domain: Optional domain used by the JetStream API.
        :param timeout: Timeout for all JS API actions.

import asyncio
            import nats

async def main():
                nc = await nats.connect()
                js = nc.jetstream()

await js.add_stream(name='hello', subjects=['hello'])
                ack = await js.publish('hello', b'Hello JS!')
                print(f'Ack: stream={ack.stream}, sequence={ack.seq}')
                # Ack: stream=hello, sequence=1
                await nc.close()

if __name__ == '__main__':
                asyncio.run(main())
        """
        return nats.js.JetStreamContext(self, **opts)

def jsm(self, **opts) -> nats.js.JetStreamManager:
        """JetStream context for managing JetStream via JS API"""
        return nats.js.JetStreamManager(self, **opts)

CANCEL