# Copyright 2018 New Vector Ltd
#
# 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.

import abc
import logging
import re
import urllib.parse
from inspect import signature
from typing import TYPE_CHECKING, Any, Awaitable, Callable, Dict, List, Tuple

from prometheus_client import Counter, Gauge

from twisted.internet.error import ConnectError, DNSLookupError
from twisted.web.server import Request

from synapse.api.errors import HttpResponseException, SynapseError
from synapse.http import RequestTimedOutError
from synapse.http.server import HttpServer
from synapse.http.site import SynapseRequest
from synapse.logging import opentracing
from synapse.logging.opentracing import trace_with_opname
from synapse.types import JsonDict
from synapse.util.caches.response_cache import ResponseCache
from synapse.util.cancellation import is_function_cancellable
from synapse.util.stringutils import random_string

if TYPE_CHECKING:
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)

_pending_outgoing_requests = Gauge(
    "synapse_pending_outgoing_replication_requests",
    "Number of active outgoing replication requests, by replication method name",
    ["name"],
)

_outgoing_request_counter = Counter(
    "synapse_outgoing_replication_requests",
    "Number of outgoing replication requests, by replication method name and result",
    ["name", "code"],
)


class ReplicationEndpoint(metaclass=abc.ABCMeta):
    """Helper base class for defining new replication HTTP endpoints.

    This creates an endpoint under `/_synapse/replication/:NAME/:PATH_ARGS..`
    (with a `/:txn_id` suffix for cached requests), where NAME is a name,
    PATH_ARGS are a tuple of parameters to be encoded in the URL.

    For example, if `NAME` is "send_event" and `PATH_ARGS` is `("event_id",)`,
    with `CACHE` set to true then this generates an endpoint:

        /_synapse/replication/send_event/:event_id/:txn_id

    For POST/PUT requests the payload is serialized to json and sent as the
    body, while for GET requests the payload is added as query parameters. See
    `_serialize_payload` for details.

    Incoming requests are handled by overriding `_handle_request`. Servers
    must call `register` to register the path with the HTTP server.

    Requests can be sent by calling the client returned by `make_client`.
    Requests are sent to master process by default, but can be sent to other
    named processes by specifying an `instance_name` keyword argument.

    Attributes:
        NAME (str): A name for the endpoint, added to the path as well as used
            in logging and metrics.
        PATH_ARGS (tuple[str]): A list of parameters to be added to the path.
            Adding parameters to the path (rather than payload) can make it
            easier to follow along in the log files.
        METHOD (str): The method of the HTTP request, defaults to POST. Can be
            one of POST, PUT or GET. If GET then the payload is sent as query
            parameters rather than a JSON body.
        CACHE (bool): Whether server should cache the result of the request/
            If true then transparently adds a txn_id to all requests, and
            `_handle_request` must return a Deferred.
        RETRY_ON_TIMEOUT(bool): Whether or not to retry the request when a 504
            is received.
        RETRY_ON_CONNECT_ERROR (bool): Whether or not to retry the request when
            a connection error is received.
        RETRY_ON_CONNECT_ERROR_ATTEMPTS (int): Number of attempts to retry when
            receiving connection errors, each will backoff exponentially longer.
    """

    NAME: str = abc.abstractproperty()  # type: ignore
    PATH_ARGS: Tuple[str, ...] = abc.abstractproperty()  # type: ignore
    METHOD = "POST"
    CACHE = True
    RETRY_ON_TIMEOUT = True
    RETRY_ON_CONNECT_ERROR = True
    RETRY_ON_CONNECT_ERROR_ATTEMPTS = 5  # =63s (2^6-1)

    def __init__(self, hs: "HomeServer"):
        if self.CACHE:
            self.response_cache: ResponseCache[str] = ResponseCache(
                hs.get_clock(), "repl." + self.NAME, timeout_ms=30 * 60 * 1000
            )

        # We reserve `instance_name` as a parameter to sending requests, so we
        # assert here that sub classes don't try and use the name.
        assert (
            "instance_name" not in self.PATH_ARGS
        ), "`instance_name` is a reserved parameter name"
        assert (
            "instance_name"
            not in signature(self.__class__._serialize_payload).parameters
        ), "`instance_name` is a reserved parameter name"

        assert self.METHOD in ("PUT", "POST", "GET")

        self._replication_secret = None
        if hs.config.worker.worker_replication_secret:
            self._replication_secret = hs.config.worker.worker_replication_secret

    def _check_auth(self, request: Request) -> None:
        # Get the authorization header.
        auth_headers = request.requestHeaders.getRawHeaders(b"Authorization")

        if not auth_headers:
            raise RuntimeError("Missing Authorization header.")
        if len(auth_headers) > 1:
            raise RuntimeError("Too many Authorization headers.")
        parts = auth_headers[0].split(b" ")
        if parts[0] == b"Bearer" and len(parts) == 2:
            received_secret = parts[1].decode("ascii")
            if self._replication_secret == received_secret:
                # Success!
                return

        raise RuntimeError("Invalid Authorization header.")

    @abc.abstractmethod
    async def _serialize_payload(**kwargs) -> JsonDict:
        """Static method that is called when creating a request.

        Concrete implementations should have explicit parameters (rather than
        kwargs) so that an appropriate exception is raised if the client is
        called with unexpected parameters. All PATH_ARGS must appear in
        argument list.

        Returns:
            If POST/PUT request then dictionary must be JSON serialisable,
            otherwise must be appropriate for adding as query args.
        """
        return {}

    @abc.abstractmethod
    async def _handle_request(
        self, request: Request, **kwargs: Any
    ) -> Tuple[int, JsonDict]:
        """Handle incoming request.

        This is called with the request object and PATH_ARGS.

        Returns:
            HTTP status code and a JSON serialisable dict to be used as response
            body of request.
        """

    @classmethod
    def make_client(cls, hs: "HomeServer") -> Callable:
        """Create a client that makes requests.

        Returns a callable that accepts the same parameters as
        `_serialize_payload`, and also accepts an optional `instance_name`
        parameter to specify which instance to hit (the instance must be in
        the `instance_map` config).
        """
        clock = hs.get_clock()
        client = hs.get_simple_http_client()
        local_instance_name = hs.get_instance_name()

        # The value of these option should match the replication listener settings
        master_host = hs.config.worker.worker_replication_host
        master_port = hs.config.worker.worker_replication_http_port
        master_tls = hs.config.worker.worker_replication_http_tls

        instance_map = hs.config.worker.instance_map

        outgoing_gauge = _pending_outgoing_requests.labels(cls.NAME)

        replication_secret = None
        if hs.config.worker.worker_replication_secret:
            replication_secret = hs.config.worker.worker_replication_secret.encode(
                "ascii"
            )

        @trace_with_opname("outgoing_replication_request")
        async def send_request(*, instance_name: str = "master", **kwargs: Any) -> Any:
            with outgoing_gauge.track_inprogress():
                if instance_name == local_instance_name:
                    raise Exception("Trying to send HTTP request to self")
                if instance_name == "master":
                    host = master_host
                    port = master_port
                    tls = master_tls
                elif instance_name in instance_map:
                    host = instance_map[instance_name].host
                    port = instance_map[instance_name].port
                    tls = instance_map[instance_name].tls
                else:
                    raise Exception(
                        "Instance %r not in 'instance_map' config" % (instance_name,)
                    )

                data = await cls._serialize_payload(**kwargs)

                url_args = [
                    urllib.parse.quote(kwargs[name], safe="") for name in cls.PATH_ARGS
                ]

                if cls.CACHE:
                    txn_id = random_string(10)
                    url_args.append(txn_id)

                if cls.METHOD == "POST":
                    request_func: Callable[
                        ..., Awaitable[Any]
                    ] = client.post_json_get_json
                elif cls.METHOD == "PUT":
                    request_func = client.put_json
                elif cls.METHOD == "GET":
                    request_func = client.get_json
                else:
                    # We have already asserted in the constructor that a
                    # compatible was picked, but lets be paranoid.
                    raise Exception(
                        "Unknown METHOD on %s replication endpoint" % (cls.NAME,)
                    )

                # Here the protocol is hard coded to be http by default or https in case the replication
                # port is set to have tls true.
                scheme = "https" if tls else "http"
                uri = "%s://%s:%s/_synapse/replication/%s/%s" % (
                    scheme,
                    host,
                    port,
                    cls.NAME,
                    "/".join(url_args),
                )

                headers: Dict[bytes, List[bytes]] = {}
                # Add an authorization header, if configured.
                if replication_secret:
                    headers[b"Authorization"] = [b"Bearer " + replication_secret]
                opentracing.inject_header_dict(headers, check_destination=False)

                try:
                    # Keep track of attempts made so we can bail if we don't manage to
                    # connect to the target after N tries.
                    attempts = 0
                    # We keep retrying the same request for timeouts. This is so that we
                    # have a good idea that the request has either succeeded or failed
                    # on the master, and so whether we should clean up or not.
                    while True:
                        try:
                            result = await request_func(uri, data, headers=headers)
                            break
                        except RequestTimedOutError:
                            if not cls.RETRY_ON_TIMEOUT:
                                raise

                            logger.warning("%s request timed out; retrying", cls.NAME)

                            # If we timed out we probably don't need to worry about backing
                            # off too much, but lets just wait a little anyway.
                            await clock.sleep(1)
                        except (ConnectError, DNSLookupError) as e:
                            if not cls.RETRY_ON_CONNECT_ERROR:
                                raise
                            if attempts > cls.RETRY_ON_CONNECT_ERROR_ATTEMPTS:
                                raise

                            delay = 2**attempts
                            logger.warning(
                                "%s request connection failed; retrying in %ds: %r",
                                cls.NAME,
                                delay,
                                e,
                            )

                            await clock.sleep(delay)
                            attempts += 1
                except HttpResponseException as e:
                    # We convert to SynapseError as we know that it was a SynapseError
                    # on the main process that we should send to the client. (And
                    # importantly, not stack traces everywhere)
                    _outgoing_request_counter.labels(cls.NAME, e.code).inc()
                    raise e.to_synapse_error()
                except Exception as e:
                    _outgoing_request_counter.labels(cls.NAME, "ERR").inc()
                    raise SynapseError(
                        502, f"Failed to talk to {instance_name} process"
                    ) from e

                _outgoing_request_counter.labels(cls.NAME, 200).inc()
                return result

        return send_request

    def register(self, http_server: HttpServer) -> None:
        """Called by the server to register this as a handler to the
        appropriate path.
        """

        url_args = list(self.PATH_ARGS)
        method = self.METHOD

        if self.CACHE and is_function_cancellable(self._handle_request):
            raise Exception(
                f"{self.__class__.__name__} has been marked as cancellable, but CACHE "
                "is set. The cancellable flag would have no effect."
            )

        if self.CACHE:
            url_args.append("txn_id")

        args = "/".join("(?P<%s>[^/]+)" % (arg,) for arg in url_args)
        pattern = re.compile("^/_synapse/replication/%s/%s$" % (self.NAME, args))

        http_server.register_paths(
            method,
            [pattern],
            self._check_auth_and_handle,
            self.__class__.__name__,
        )

    async def _check_auth_and_handle(
        self, request: SynapseRequest, **kwargs: Any
    ) -> Tuple[int, JsonDict]:
        """Called on new incoming requests when caching is enabled. Checks
        if there is a cached response for the request and returns that,
        otherwise calls `_handle_request` and caches its response.
        """
        # We just use the txn_id here, but we probably also want to use the
        # other PATH_ARGS as well.

        # Check the authorization headers before handling the request.
        if self._replication_secret:
            self._check_auth(request)

        if self.CACHE:
            txn_id = kwargs.pop("txn_id")

            # We ignore the `@cancellable` flag, since cancellation wouldn't interupt
            # `_handle_request` and `ResponseCache` does not handle cancellation
            # correctly yet. In particular, there may be issues to do with logging
            # context lifetimes.

            return await self.response_cache.wrap(
                txn_id, self._handle_request, request, **kwargs
            )

        # The `@cancellable` decorator may be applied to `_handle_request`. But we
        # told `HttpServer.register_paths` that our handler is `_check_auth_and_handle`,
        # so we have to set up the cancellable flag ourselves.
        request.is_render_cancellable = is_function_cancellable(self._handle_request)

        return await self._handle_request(request, **kwargs)
