synapse/tests/http/server/_base.py

# Copyright 2022 The Matrix.org Foundation C.I.C.
#
# 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
#
# Unles4s 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 inspect
import itertools
import logging
from typing import (
    Any,
    Callable,
    ContextManager,
    Dict,
    List,
    Optional,
    Set,
    Tuple,
    TypeVar,
    Union,
)
from unittest import mock
from unittest.mock import Mock

from twisted.internet.defer import Deferred
from twisted.internet.error import ConnectionDone
from twisted.python.failure import Failure
from twisted.test.proto_helpers import MemoryReactorClock
from twisted.web.server import Site

from synapse.http.server import (
    HTTP_STATUS_REQUEST_CANCELLED,
    respond_with_html_bytes,
    respond_with_json,
)
from synapse.http.site import SynapseRequest
from synapse.logging.context import LoggingContext, make_deferred_yieldable
from synapse.types import JsonDict

from tests.server import FakeChannel, make_request
from tests.unittest import logcontext_clean

logger = logging.getLogger(__name__)


T = TypeVar("T")


def test_disconnect(
    reactor: MemoryReactorClock,
    channel: FakeChannel,
    expect_cancellation: bool,
    expected_body: Union[bytes, JsonDict],
    expected_code: Optional[int] = None,
) -> None:
    """Disconnects an in-flight request and checks the response.

    Args:
        reactor: The twisted reactor running the request handler.
        channel: The `FakeChannel` for the request.
        expect_cancellation: `True` if request processing is expected to be cancelled,
            `False` if the request should run to completion.
        expected_body: The expected response for the request.
        expected_code: The expected status code for the request. Defaults to `200` or
            `499` depending on `expect_cancellation`.
    """
    # Determine the expected status code.
    if expected_code is None:
        if expect_cancellation:
            expected_code = HTTP_STATUS_REQUEST_CANCELLED
        else:
            expected_code = 200

    request = channel.request
    if channel.is_finished():
        raise AssertionError(
            "Request finished before we could disconnect - "
            "ensure `await_result=False` is passed to `make_request`.",
        )

    # We're about to disconnect the request. This also disconnects the channel, so we
    # have to rely on mocks to extract the response.
    respond_method: Callable[..., Any]
    if isinstance(expected_body, bytes):
        respond_method = respond_with_html_bytes
    else:
        respond_method = respond_with_json

    with mock.patch(
        f"synapse.http.server.{respond_method.__name__}", wraps=respond_method
    ) as respond_mock:
        # Disconnect the request.
        request.connectionLost(reason=ConnectionDone())

        if expect_cancellation:
            # An immediate cancellation is expected.
            respond_mock.assert_called_once()
        else:
            respond_mock.assert_not_called()

            # The handler is expected to run to completion.
            reactor.advance(1.0)
            respond_mock.assert_called_once()

        args, _kwargs = respond_mock.call_args
        code, body = args[1], args[2]

        if code != expected_code:
            raise AssertionError(
                f"{code} != {expected_code} : "
                "Request did not finish with the expected status code."
            )

        if request.code != expected_code:
            raise AssertionError(
                f"{request.code} != {expected_code} : "
                "Request did not finish with the expected status code."
            )

        if body != expected_body:
            raise AssertionError(
                f"{body!r} != {expected_body!r} : "
                "Request did not finish with the expected status code."
            )


@logcontext_clean
def make_request_with_cancellation_test(
    test_name: str,
    reactor: MemoryReactorClock,
    site: Site,
    method: str,
    path: str,
    content: Union[bytes, str, JsonDict] = b"",
    *,
    token: Optional[str] = None,
) -> FakeChannel:
    """Performs a request repeatedly, disconnecting at successive `await`s, until
    one completes.

    Fails if:
        * A logging context is lost during cancellation.
        * A logging context get restarted after it is marked as finished, eg. if
            a request's logging context is used by some processing started by the
            request, but the request neglects to cancel that processing or wait for it
            to complete.

            Note that "Re-starting finished log context" errors get raised within the
            request handling code and may or may not get caught. These errors will
            likely manifest as a different logging context error at a later point. When
            debugging logging context failures, setting a breakpoint in
            `logcontext_error` can prove useful.
        * A request gets stuck, possibly due to a previous cancellation.
        * The request does not return a 499 when the client disconnects.
            This implies that a `CancelledError` was swallowed somewhere.

    It is up to the caller to verify that the request returns the correct data when
    it finally runs to completion.

    Note that this function can only cover a single code path and does not guarantee
    that an endpoint is compatible with cancellation on every code path.
    To allow inspection of the code path that is being tested, this function will
    log the stack trace at every `await` that gets cancelled. To view these log
    lines, `trial` can be run with the `SYNAPSE_TEST_LOG_LEVEL=INFO` environment
    variable, which will include the log lines in `_trial_temp/test.log`.
    Alternatively, `_log_for_request` can be modified to write to `sys.stdout`.

    Args:
        test_name: The name of the test, which will be logged.
        reactor: The twisted reactor running the request handler.
        site: The twisted `Site` to use to render the request.
        method: The HTTP request method ("verb").
        path: The HTTP path, suitably URL encoded (e.g. escaped UTF-8 & spaces and
            such).
        content: The body of the request.

    Returns:
        The `FakeChannel` object which stores the result of the final request that
        runs to completion.
    """
    # To process a request, a coroutine run is created for the async method handling
    # the request. That method may then start other coroutine runs, wrapped in
    # `Deferred`s.
    #
    # We would like to trigger a cancellation at the first `await`, re-run the
    # request and cancel at the second `await`, and so on. By patching
    # `Deferred.__next__`, we can intercept `await`s, track which ones we have or
    # have not seen, and force them to block when they wouldn't have.

    # The set of previously seen `await`s.
    # Each element is a stringified stack trace.
    seen_awaits: Set[Tuple[str, ...]] = set()

    _log_for_request(
        0, f"Running make_request_with_cancellation_test for {test_name}..."
    )

    for request_number in itertools.count(1):
        deferred_patch = Deferred__next__Patch(seen_awaits, request_number)

        try:
            with mock.patch(
                "synapse.http.server.respond_with_json", wraps=respond_with_json
            ) as respond_mock:
                with deferred_patch.patch():
                    # Start the request.
                    channel = make_request(
                        reactor,
                        site,
                        method,
                        path,
                        content,
                        await_result=False,
                        access_token=token,
                    )
                    request = channel.request

                    # Run the request until we see a new `await` which we have not
                    # yet cancelled at, or it completes.
                    while not respond_mock.called and not deferred_patch.new_await_seen:
                        previous_awaits_seen = deferred_patch.awaits_seen

                        reactor.advance(0.0)

                        if deferred_patch.awaits_seen == previous_awaits_seen:
                            # We didn't see any progress. Try advancing the clock.
                            reactor.advance(1.0)

                        if deferred_patch.awaits_seen == previous_awaits_seen:
                            # We still didn't see any progress. The request might be
                            # stuck.
                            raise AssertionError(
                                "Request appears to be stuck, possibly due to a "
                                "previous cancelled request"
                            )

                if respond_mock.called:
                    # The request ran to completion and we are done with testing it.

                    # `respond_with_json` writes the response asynchronously, so we
                    # might have to give the reactor a kick before the channel gets
                    # the response.
                    deferred_patch.unblock_awaits()
                    channel.await_result()

                    return channel

                # Disconnect the client and wait for the response.
                request.connectionLost(reason=ConnectionDone())

                _log_for_request(request_number, "--- disconnected ---")

                # Advance the reactor just enough to get a response.
                # We don't want to advance the reactor too far, because we can only
                # detect re-starts of finished logging contexts after we set the
                # finished flag below.
                for _ in range(2):
                    # We may need to pump the reactor to allow `delay_cancellation`s to
                    # finish.
                    if not respond_mock.called:
                        reactor.advance(0.0)

                    # Try advancing the clock if that didn't work.
                    if not respond_mock.called:
                        reactor.advance(1.0)

                    # `delay_cancellation`s may be waiting for processing that we've
                    # forced to block. Try unblocking them, followed by another round of
                    # pumping the reactor.
                    if not respond_mock.called:
                        deferred_patch.unblock_awaits()

                # Mark the request's logging context as finished. If it gets
                # activated again, an `AssertionError` will be raised and bubble up
                # through request handling code. This `AssertionError` may or may not be
                # caught. Eventually some other code will deactivate the logging
                # context which will raise a different `AssertionError` because
                # resource usage won't have been correctly tracked.
                if isinstance(request, SynapseRequest) and request.logcontext:
                    request.logcontext.finished = True

                # Check that the request finished with a 499,
                # ie. the `CancelledError` wasn't swallowed.
                respond_mock.assert_called_once()

                if request.code != HTTP_STATUS_REQUEST_CANCELLED:
                    raise AssertionError(
                        f"{request.code} != {HTTP_STATUS_REQUEST_CANCELLED} : "
                        "Cancelled request did not finish with the correct status code."
                    )
        finally:
            # Unblock any processing that might be shared between requests, if we
            # haven't already done so.
            deferred_patch.unblock_awaits()

    assert False, "unreachable"  # noqa: B011


class Deferred__next__Patch:
    """A `Deferred.__next__` patch that will intercept `await`s and force them
    to block once it sees a new `await`.

    When done with the patch, `unblock_awaits()` must be called to clean up after any
    `await`s that were forced to block, otherwise processing shared between multiple
    requests, such as database queries started by `@cached`, will become permanently
    stuck.

    Usage:
        seen_awaits = set()
        deferred_patch = Deferred__next__Patch(seen_awaits, 1)
        try:
            with deferred_patch.patch():
                # do things
                ...
        finally:
            deferred_patch.unblock_awaits()
    """

    def __init__(self, seen_awaits: Set[Tuple[str, ...]], request_number: int):
        """
        Args:
            seen_awaits: The set of stack traces of `await`s that have been previously
                seen. When the `Deferred.__next__` patch sees a new `await`, it will add
                it to the set.
            request_number: The request number to log against.
        """
        self._request_number = request_number
        self._seen_awaits = seen_awaits

        self._original_Deferred___next__ = Deferred.__next__

        # The number of `await`s on `Deferred`s we have seen so far.
        self.awaits_seen = 0

        # Whether we have seen a new `await` not in `seen_awaits`.
        self.new_await_seen = False

        # To force `await`s on resolved `Deferred`s to block, we make up a new
        # unresolved `Deferred` and return it out of `Deferred.__next__` /
        # `coroutine.send()`. We have to resolve it later, in case the `await`ing
        # coroutine is part of some shared processing, such as `@cached`.
        self._to_unblock: Dict[Deferred, Union[object, Failure]] = {}

        # The last stack we logged.
        self._previous_stack: List[inspect.FrameInfo] = []

    def patch(self) -> ContextManager[Mock]:
        """Returns a context manager which patches `Deferred.__next__`."""

        def Deferred___next__(
            deferred: "Deferred[T]", value: object = None
        ) -> "Deferred[T]":
            """Intercepts `await`s on `Deferred`s and rigs them to block once we have
            seen enough of them.

            `Deferred.__next__` will normally:
                * return `self` if the `Deferred` is unresolved, in which case
                   `coroutine.send()` will return the `Deferred`, and
                   `_defer.inlineCallbacks` will stop running the coroutine until the
                   `Deferred` is resolved.
                * raise a `StopIteration(result)`, containing the result of the `await`.
                * raise another exception, which will come out of the `await`.
            """
            self.awaits_seen += 1

            stack = _get_stack(skip_frames=1)
            stack_hash = _hash_stack(stack)

            if stack_hash not in self._seen_awaits:
                # Block at the current `await` onwards.
                self._seen_awaits.add(stack_hash)
                self.new_await_seen = True

            if not self.new_await_seen:
                # This `await` isn't interesting. Let it proceed normally.

                # Don't log the stack. It's been seen before in a previous run.
                self._previous_stack = stack

                return self._original_Deferred___next__(deferred, value)

            # We want to block at the current `await`.
            if deferred.called and not deferred.paused:
                # This `Deferred` already has a result.
                # We return a new, unresolved, `Deferred` for `_inlineCallbacks` to wait
                # on. This blocks the coroutine that did this `await`.
                # We queue it up for unblocking later.
                new_deferred: "Deferred[T]" = Deferred()
                self._to_unblock[new_deferred] = deferred.result

                _log_await_stack(
                    stack,
                    self._previous_stack,
                    self._request_number,
                    "force-blocked await",
                )
                self._previous_stack = stack

                return make_deferred_yieldable(new_deferred)

            # This `Deferred` does not have a result yet.
            # The `await` will block normally, so we don't have to do anything.
            _log_await_stack(
                stack,
                self._previous_stack,
                self._request_number,
                "blocking await",
            )
            self._previous_stack = stack

            return self._original_Deferred___next__(deferred, value)

        return mock.patch.object(Deferred, "__next__", new=Deferred___next__)

    def unblock_awaits(self) -> None:
        """Unblocks any shared processing that we forced to block.

        Must be called when done, otherwise processing shared between multiple requests,
        such as database queries started by `@cached`, will become permanently stuck.
        """
        to_unblock = self._to_unblock
        self._to_unblock = {}
        for deferred, result in to_unblock.items():
            deferred.callback(result)


def _log_for_request(request_number: int, message: str) -> None:
    """Logs a message for an iteration of `make_request_with_cancellation_test`."""
    # We want consistent alignment when logging stack traces, so ensure the logging
    # context has a fixed width name.
    with LoggingContext(name=f"request-{request_number:<2}"):
        logger.info(message)


def _log_await_stack(
    stack: List[inspect.FrameInfo],
    previous_stack: List[inspect.FrameInfo],
    request_number: int,
    note: str,
) -> None:
    """Logs the stack for an `await` in `make_request_with_cancellation_test`.

    Only logs the part of the stack that has changed since the previous call.

    Example output looks like:
    ```
    delay_cancellation:750 (synapse/util/async_helpers.py:750)
        DatabasePool._runInteraction:768 (synapse/storage/database.py:768)
            > *blocked on await* at DatabasePool.runWithConnection:891 (synapse/storage/database.py:891)
    ```

    Args:
        stack: The stack to log, as returned by `_get_stack()`.
        previous_stack: The previous stack logged, with callers appearing before
            callees.
        request_number: The request number to log against.
        note: A note to attach to the last stack frame, eg. "blocked on await".
    """
    for i, frame_info in enumerate(stack[:-1]):
        # Skip any frames in common with the previous logging.
        if i < len(previous_stack) and frame_info == previous_stack[i]:
            continue

        frame = _format_stack_frame(frame_info)
        message = f"{'  ' * i}{frame}"
        _log_for_request(request_number, message)

    # Always print the final frame with the `await`.
    # If the frame with the `await` started another coroutine run, we may have already
    # printed a deeper stack which includes our final frame. We want to log where all
    # `await`s happen, so we reprint the frame in this case.
    i = len(stack) - 1
    frame_info = stack[i]
    frame = _format_stack_frame(frame_info)
    message = f"{'  ' * i}> *{note}* at {frame}"
    _log_for_request(request_number, message)


def _format_stack_frame(frame_info: inspect.FrameInfo) -> str:
    """Returns a string representation of a stack frame.

    Used for debug logging.

    Returns:
        A string, formatted like
        "JsonResource._async_render:559 (synapse/http/server.py:559)".
    """
    method_name = _get_stack_frame_method_name(frame_info)

    return (
        f"{method_name}:{frame_info.lineno} ({frame_info.filename}:{frame_info.lineno})"
    )


def _get_stack(skip_frames: int) -> List[inspect.FrameInfo]:
    """Captures the stack for a request.

    Skips any twisted frames and stops at `JsonResource.wrapped_async_request_handler`.

    Used for debug logging.

    Returns:
        A list of `inspect.FrameInfo`s, with callers appearing before callees.
    """
    stack = []

    skip_frames += 1  # Also skip `get_stack` itself.

    for frame_info in inspect.stack()[skip_frames:]:
        # Skip any twisted `inlineCallbacks` gunk.
        if "/twisted/" in frame_info.filename:
            continue

        # Exclude the reactor frame, upwards.
        method_name = _get_stack_frame_method_name(frame_info)
        if method_name == "ThreadedMemoryReactorClock.advance":
            break

        stack.append(frame_info)

        # Stop at `JsonResource`'s `wrapped_async_request_handler`, which is the entry
        # point for request handling.
        if frame_info.function == "wrapped_async_request_handler":
            break

    return stack[::-1]


def _get_stack_frame_method_name(frame_info: inspect.FrameInfo) -> str:
    """Returns the name of a stack frame's method.

    eg. "JsonResource._async_render".
    """
    method_name = frame_info.function

    # Prefix the class name for instance methods.
    frame_self = frame_info.frame.f_locals.get("self")
    if frame_self:
        method = getattr(frame_self, method_name, None)
        if method:
            method_name = method.__qualname__
        else:
            # We couldn't find the method on `self`.
            # Make something up. It's useful to know which class "contains" a
            # function anyway.
            method_name = f"{type(frame_self).__name__} {method_name}"

    return method_name


def _hash_stack(stack: List[inspect.FrameInfo]):
    """Turns a stack into a hashable value that can be put into a set."""
    return tuple(_format_stack_frame(frame) for frame in stack)