path: root/synapse/logging/tracing.py

# Copyright 2019 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
#
# 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.


# NOTE This is a small wrapper around opentelemetry because tracing is optional
# and not always packaged downstream. Since opentelemetry instrumentation is
# fairly invasive it was awkward to make it optional. As a result we opted to
# encapsulate all opentelemetry state in these methods which effectively noop if
# opentelemetry is not present. We should strongly consider encouraging the
# downstream distributers to package opentelemetry and making opentelemetry a
# full dependency. In order to facilitate this move the methods have work very
# similarly to opentelemetry's and it should only be a matter of few regexes to
# move over to opentelemetry's access patterns proper.

"""
============================
Using OpenTelemetry in Synapse
============================

Python-specific tracing concepts are at
https://opentelemetry.io/docs/instrumentation/python/. Note that Synapse wraps
OpenTelemetry in a small module (this one) in order to make the OpenTelemetry
dependency optional. That means that some access patterns are different to those
demonstrated in the OpenTelemetry guides. However, it is still useful to know,
especially if OpenTelemetry is included as a full dependency in the future or if
you are modifying this module.


OpenTelemetry is encapsulated so that no span objects from OpenTelemetry are
exposed in Synapse's code. This allows OpenTelemetry to be easily disabled in
Synapse and thereby have OpenTelemetry as an optional dependency. This does
however limit the number of modifiable spans at any point in the code to one.
From here out references to `tracing` in the code snippets refer to the Synapses
module. Most methods provided in the module have a direct correlation to those
provided by OpenTelemetry. Refer to docs there for a more in-depth documentation
on some of the args and methods.

Tracing
-------

In Synapse, it is not possible to start a non-active span. Spans can be started
using the ``start_active_span`` method. This returns a context manager that
needs to be entered and exited to expose the ``span``. This is usually done by
using a ``with`` statement.

.. code-block:: python

   from synapse.logging.tracing import start_active_span

   with start_active_span("operation name"):
       # Do something we want to trace

Forgetting to enter or exit a scope will result in unstarted and unfinished
spans that will not be reported (exported).

At anytime where there is an active span ``set_attribute`` can be
used to set a tag on the current active span.

Tracing functions
-----------------

Functions can be easily traced using decorators. The name of the function
becomes the operation name for the span.

.. code-block:: python

   from synapse.logging.tracing import trace

   # Start a span using 'interesting_function' as the operation name
   @trace
   def interesting_function(*args, **kwargs):
       # Does all kinds of cool and expected things return
       something_usual_and_useful


Operation names can be explicitly set for a function by using
``trace_with_opname``:

.. code-block:: python

   from synapse.logging.tracing import trace_with_opname

   @trace_with_opname("a_better_operation_name")
   def interesting_badly_named_function(*args, **kwargs):
       # Does all kinds of cool and expected things return
       something_usual_and_useful

Setting Tags
------------

To set a tag on the active span do

.. code-block:: python

   from synapse.logging.tracing import set_attribute

   set_attribute(tag_name, tag_value)

There's a convenient decorator to tag all the args of the method. It uses
inspection in order to use the formal parameter names prefixed with 'ARG_' as
tag names. It uses kwarg names as tag names without the prefix.

.. code-block:: python
   from synapse.logging.tracing import tag_args
   @tag_args
   def set_fates(clotho, lachesis, atropos, father="Zues", mother="Themis"):
       pass
   set_fates("the story", "the end", "the act")
   # This will have the following tags
   #  - ARG_clotho: "the story"
   #  - ARG_lachesis: "the end"
   #  - ARG_atropos: "the act"
   #  - father: "Zues"
   #  - mother: "Themis"

Contexts and carriers
---------------------

There are a selection of wrappers for injecting and extracting contexts from
carriers provided. We use these to inject of OpenTelemetry Contexts into
Twisted's http headers, EDU contents and our database tables. Please refer to
the end of ``logging/tracing.py`` for the available injection and extraction
methods.

Homeserver whitelisting
-----------------------

Most of the whitelist checks are encapsulated in the modules's injection and
extraction method but be aware that using custom carriers or crossing
unchartered waters will require the enforcement of the whitelist.
``logging/tracing.py`` has a ``whitelisted_homeserver`` method which takes
in a destination and compares it to the whitelist.

Most injection methods take a 'destination' arg. The context will only be
injected if the destination matches the whitelist or the destination is None.

=======
Gotchas
=======

- Checking whitelists on span propagation
- Inserting pii
- Forgetting to enter or exit a scope
- Span source: make sure that the span you expect to be active across a function
  call really will be that one. Does the current function have more than one
  caller? Will all of those calling functions have be in a context with an
  active span?
"""
import contextlib
import inspect
import logging
import re
from abc import ABC
from functools import wraps
from typing import (
    TYPE_CHECKING,
    Any,
    Callable,
    ContextManager,
    Dict,
    Generator,
    Iterable,
    List,
    Optional,
    Pattern,
    Sequence,
    TypeVar,
    Union,
    cast,
    overload,
)

from typing_extensions import ParamSpec

from twisted.internet import defer
from twisted.web.http import Request
from twisted.web.http_headers import Headers

from synapse.api.constants import EventContentFields
from synapse.config import ConfigError
from synapse.util import json_decoder

if TYPE_CHECKING:
    from synapse.http.site import SynapseRequest
    from synapse.server import HomeServer

# Helper class

T = TypeVar("T")

# Matches the number suffix in an instance name like "matrix.org client_reader-8"
STRIP_INSTANCE_NUMBER_SUFFIX_REGEX = re.compile(r"[_-]?\d+$")


class _DummyLookup(object):
    """This will always returns the fixed value given for any accessed property"""

    def __init__(self, value: T) -> None:
        self.value = value

    def __getattribute__(self, name: str) -> T:
        return object.__getattribute__(self, "value")


class DummyLink(ABC):
    """Dummy placeholder for `opentelemetry.trace.Link`"""

    def __init__(self) -> None:
        self.not_implemented_message = (
            "opentelemetry isn't installed so this is just a dummy link placeholder"
        )

    @property
    def context(self) -> None:
        raise NotImplementedError(self.not_implemented_message)

    @property
    def attributes(self) -> None:
        raise NotImplementedError(self.not_implemented_message)


# These dependencies are optional so they can fail to import
# and we
try:
    import opentelemetry
    import opentelemetry.exporter.jaeger.thrift
    import opentelemetry.propagate
    import opentelemetry.sdk.resources
    import opentelemetry.sdk.trace
    import opentelemetry.sdk.trace.export
    import opentelemetry.semconv.trace
    import opentelemetry.trace
    import opentelemetry.trace.propagation
    import opentelemetry.trace.status

    SpanKind = opentelemetry.trace.SpanKind
    SpanAttributes = opentelemetry.semconv.trace.SpanAttributes
    StatusCode = opentelemetry.trace.status.StatusCode
    Link = opentelemetry.trace.Link
except ImportError:
    opentelemetry = None  # type: ignore[assignment]
    SpanKind = _DummyLookup(0)  # type: ignore
    SpanAttributes = _DummyLookup("fake-attribute")  # type: ignore
    StatusCode = _DummyLookup(0)  # type: ignore
    Link = DummyLink  # type: ignore


logger = logging.getLogger(__name__)


class SynapseTags:
    """FIXME: Rename to `SynapseAttributes` so it matches OpenTelemetry `SpanAttributes`"""

    # The message ID of any to_device message processed
    TO_DEVICE_MESSAGE_ID = "to_device.message_id"

    # Whether the sync response has new data to be returned to the client.
    SYNC_RESULT = "sync.new_data"

    # The Synapse instance name
    INSTANCE_NAME = "instance_name"

    # incoming HTTP request ID  (as written in the logs)
    REQUEST_ID = "request_id"

    # HTTP request tag (used to distinguish full vs incremental syncs, etc)
    REQUEST_TAG = "request_tag"

    # Text description of a database transaction
    DB_TXN_DESC = "db.txn_desc"

    # Uniqueish ID of a database transaction
    DB_TXN_ID = "db.txn_id"

    # The name of the external cache
    CACHE_NAME = "cache.name"

    # Used to tag function arguments
    #
    # Tag a named arg. The name of the argument should be appended to this prefix.
    FUNC_ARG_PREFIX = "ARG."
    # Tag extra variadic number of positional arguments (`def foo(first, second, *extras)`)
    FUNC_ARGS = "args"
    # Tag keyword args
    FUNC_KWARGS = "kwargs"

    # Some intermediate result that's interesting to the function. The label for
    # the result should be appended to this prefix.
    RESULT_PREFIX = "RESULT."


class SynapseBaggage:
    FORCE_TRACING = "synapse-force-tracing"


# Block everything by default
# A regex which matches the server_names to expose traces for.
# None means 'block everything'.
_homeserver_whitelist: Optional[Pattern[str]] = None

# Util methods


P = ParamSpec("P")
R = TypeVar("R")


def only_if_tracing(func: Callable[P, R]) -> Callable[P, Optional[R]]:
    """Decorator function that executes the function only if we're tracing. Otherwise returns None."""

    @wraps(func)
    def _only_if_tracing_inner(*args: P.args, **kwargs: P.kwargs) -> Optional[R]:
        if opentelemetry:
            return func(*args, **kwargs)
        else:
            return None

    return _only_if_tracing_inner


@overload
def ensure_active_span(
    message: str,
) -> Callable[[Callable[P, R]], Callable[P, Optional[R]]]:
    ...


@overload
def ensure_active_span(
    message: str, ret: T
) -> Callable[[Callable[P, R]], Callable[P, Union[T, R]]]:
    ...


def ensure_active_span(
    message: str, ret: Optional[T] = None
) -> Callable[[Callable[P, R]], Callable[P, Union[Optional[T], R]]]:
    """Executes the operation only if opentelemetry is enabled and there is an active span.
    If there is no active span it logs message at the error level.

    Args:
        message: Message which fills in "There was no active span when trying to %s"
            in the error log if there is no active span and opentelemetry is enabled.
        ret: return value if opentelemetry is None or there is no active span.

    Returns:
        The result of the func, falling back to ret if opentelemetry is disabled or there
        was no active span.
    """

    def ensure_active_span_inner_1(
        func: Callable[P, R]
    ) -> Callable[P, Union[Optional[T], R]]:
        @wraps(func)
        def ensure_active_span_inner_2(
            *args: P.args, **kwargs: P.kwargs
        ) -> Union[Optional[T], R]:
            if not opentelemetry:
                return ret

            if not opentelemetry.trace.get_current_span():
                logger.error(
                    "There was no active span when trying to %s."
                    " Did you forget to start one or did a context slip?",
                    message,
                    stack_info=True,
                )

                return ret

            return func(*args, **kwargs)

        return ensure_active_span_inner_2

    return ensure_active_span_inner_1


# Setup


def init_tracer(hs: "HomeServer") -> None:
    """Set the whitelists and initialise the OpenTelemetry tracer"""
    global opentelemetry
    if not hs.config.tracing.tracing_enabled:
        # We don't have a tracer
        opentelemetry = None  # type: ignore[assignment]
        return

    if not opentelemetry:
        raise ConfigError(
            "The server has been configured to use OpenTelemetry but OpenTelemetry is not "
            "installed."
        )

    # Pull out of the config if it was given. Otherwise set it to something sensible.
    set_homeserver_whitelist(hs.config.tracing.homeserver_whitelist)

    # Instance names are opaque strings but by stripping off the number suffix,
    # we can get something that looks like a "worker type", e.g.
    # "client_reader-1" -> "client_reader" so we don't spread the traces across
    # so many services.
    instance_name_by_type = re.sub(
        STRIP_INSTANCE_NUMBER_SUFFIX_REGEX, "", hs.get_instance_name()
    )

    resource = opentelemetry.sdk.resources.Resource(
        attributes={
            opentelemetry.sdk.resources.SERVICE_NAME: f"{hs.config.server.server_name} {instance_name_by_type}"
        }
    )

    # TODO: `force_tracing_for_users` is not compatible with OTEL samplers
    # because you can only determine `opentelemetry.trace.TraceFlags.SAMPLED`
    # and whether it uses a recording span when the span is created and we don't
    # have enough information at that time (we can determine in
    # `synapse/api/auth.py`). There isn't a way to change the trace flags after
    # the fact so there is no way to programmatically force
    # recording/tracing/sampling like there was in opentracing.
    sampler = opentelemetry.sdk.trace.sampling.ParentBasedTraceIdRatio(
        hs.config.tracing.sample_rate
    )

    tracer_provider = opentelemetry.sdk.trace.TracerProvider(
        resource=resource, sampler=sampler
    )

    # consoleProcessor = opentelemetry.sdk.trace.export.BatchSpanProcessor(
    #     opentelemetry.sdk.trace.export.ConsoleSpanExporter()
    # )
    # tracer_provider.add_span_processor(consoleProcessor)

    jaeger_exporter = opentelemetry.exporter.jaeger.thrift.JaegerExporter(
        **hs.config.tracing.jaeger_exporter_config
    )
    jaeger_processor = opentelemetry.sdk.trace.export.BatchSpanProcessor(
        jaeger_exporter
    )
    tracer_provider.add_span_processor(jaeger_processor)

    # Sets the global default tracer provider
    opentelemetry.trace.set_tracer_provider(tracer_provider)


# Whitelisting


@only_if_tracing
def set_homeserver_whitelist(homeserver_whitelist: Iterable[str]) -> None:
    """Sets the homeserver whitelist

    Args:
        homeserver_whitelist: regexes specifying whitelisted homeservers
    """
    global _homeserver_whitelist
    if homeserver_whitelist:
        # Makes a single regex which accepts all passed in regexes in the list
        _homeserver_whitelist = re.compile(
            "({})".format(")|(".join(homeserver_whitelist))
        )


@only_if_tracing
def whitelisted_homeserver(destination: str) -> bool:
    """Checks if a destination matches the whitelist

    Args:
        destination
    """

    if _homeserver_whitelist:
        return _homeserver_whitelist.match(destination) is not None
    return False


# Start spans and scopes


def use_span(
    span: "opentelemetry.trace.Span",
    end_on_exit: bool = True,
) -> ContextManager["opentelemetry.trace.Span"]:
    if opentelemetry is None:
        return contextlib.nullcontext()  # type: ignore[unreachable]

    return opentelemetry.trace.use_span(span=span, end_on_exit=end_on_exit)


def create_non_recording_span() -> "opentelemetry.trace.Span":
    """Create a no-op span that does not record or become part of a recorded trace"""

    return opentelemetry.trace.NonRecordingSpan(
        opentelemetry.trace.INVALID_SPAN_CONTEXT
    )


def start_span(
    name: str,
    *,
    context: Optional["opentelemetry.context.context.Context"] = None,
    kind: Optional["opentelemetry.trace.SpanKind"] = SpanKind.INTERNAL,
    attributes: "opentelemetry.util.types.Attributes" = None,
    links: Optional[Sequence["opentelemetry.trace.Link"]] = None,
    start_time: Optional[int] = None,
    record_exception: bool = True,
    set_status_on_exception: bool = True,
    end_on_exit: bool = True,
    # For testing only
    tracer: Optional["opentelemetry.trace.Tracer"] = None,
) -> "opentelemetry.trace.Span":
    if opentelemetry is None:
        raise Exception("Not able to create span without opentelemetry installed.")

    if tracer is None:
        tracer = opentelemetry.trace.get_tracer(__name__)

    # TODO: Why is this necessary to satisfy this error? It has a default?
    #  ` error: Argument "kind" to "start_span" of "Tracer" has incompatible type "Optional[SpanKind]"; expected "SpanKind"  [arg-type]`
    if kind is None:
        kind = SpanKind.INTERNAL

    return tracer.start_span(
        name=name,
        context=context,
        kind=kind,
        attributes=attributes,
        links=links,
        start_time=start_time,
        record_exception=record_exception,
        set_status_on_exception=set_status_on_exception,
    )


def start_active_span(
    name: str,
    *,
    context: Optional["opentelemetry.context.context.Context"] = None,
    kind: Optional["opentelemetry.trace.SpanKind"] = SpanKind.INTERNAL,
    attributes: "opentelemetry.util.types.Attributes" = None,
    links: Optional[Sequence["opentelemetry.trace.Link"]] = None,
    start_time: Optional[int] = None,
    record_exception: bool = True,
    set_status_on_exception: bool = True,
    end_on_exit: bool = True,
    # For testing only
    tracer: Optional["opentelemetry.trace.Tracer"] = None,
) -> ContextManager["opentelemetry.trace.Span"]:
    if opentelemetry is None:
        return contextlib.nullcontext()  # type: ignore[unreachable]

    # TODO: Why is this necessary to satisfy this error? It has a default?
    #  ` error: Argument "kind" to "start_span" of "Tracer" has incompatible type "Optional[SpanKind]"; expected "SpanKind"  [arg-type]`
    if kind is None:
        kind = SpanKind.INTERNAL

    span = start_span(
        name=name,
        context=context,
        kind=kind,
        attributes=attributes,
        links=links,
        start_time=start_time,
        record_exception=record_exception,
        set_status_on_exception=set_status_on_exception,
        tracer=tracer,
    )

    # Equivalent to `tracer.start_as_current_span`
    return opentelemetry.trace.use_span(
        span,
        end_on_exit=end_on_exit,
        record_exception=record_exception,
        set_status_on_exception=set_status_on_exception,
    )


def start_active_span_from_edu(
    operation_name: str,
    *,
    edu_content: Dict[str, Any],
) -> ContextManager["opentelemetry.trace.Span"]:
    """
    Extracts a span context from an edu and uses it to start a new active span

    Args:
        operation_name: The label for the chunk of time used to process the given edu.
        edu_content: an edu_content with a `context` field whose value is
            canonical json for a dict which contains tracing information.
    """
    if opentelemetry is None:
        return contextlib.nullcontext()  # type: ignore[unreachable]

    carrier = json_decoder.decode(
        edu_content.get(EventContentFields.TRACING_CONTEXT, "{}")
    )

    context = extract_text_map(carrier)

    return start_active_span(name=operation_name, context=context)


# OpenTelemetry setters for attributes, logs, etc
@only_if_tracing
def get_active_span() -> Optional["opentelemetry.trace.Span"]:
    """Get the currently active span, if any"""
    return opentelemetry.trace.get_current_span()


def get_span_context_from_context(
    context: "opentelemetry.context.context.Context",
) -> Optional["opentelemetry.trace.SpanContext"]:
    """Utility function to convert a `Context` to a `SpanContext`

    Based on https://github.com/open-telemetry/opentelemetry-python/blob/43288ca9a36144668797c11ca2654836ec8b5e99/opentelemetry-api/src/opentelemetry/trace/propagation/tracecontext.py#L99-L102
    """
    span = opentelemetry.trace.get_current_span(context=context)
    span_context = span.get_span_context()
    if span_context == opentelemetry.trace.INVALID_SPAN_CONTEXT:
        return None
    return span_context


def get_context_from_span(
    span: "opentelemetry.trace.Span",
) -> "opentelemetry.context.context.Context":
    # This doesn't affect the current context at all, it just converts a span
    # into `Context` object basically (bad name).
    ctx = opentelemetry.trace.propagation.set_span_in_context(span)
    return ctx


@ensure_active_span("set a tag")
def set_attribute(key: str, value: Union[str, bool, int, float]) -> None:
    """Sets a tag on the active span"""
    active_span = get_active_span()
    assert active_span is not None
    active_span.set_attribute(key, value)


@ensure_active_span("set the status")
def set_status(
    status_code: "opentelemetry.trace.status.StatusCode", exc: Optional[Exception]
) -> None:
    """Sets a tag on the active span"""
    active_span = get_active_span()
    assert active_span is not None
    active_span.set_status(opentelemetry.trace.status.Status(status_code=status_code))
    if exc:
        active_span.record_exception(exc)


DEFAULT_LOG_NAME = "log"


@ensure_active_span("log")
def log_kv(key_values: Dict[str, Any], timestamp: Optional[int] = None) -> None:
    """Log to the active span"""
    active_span = get_active_span()
    assert active_span is not None
    event_name = key_values.get("event", DEFAULT_LOG_NAME)
    active_span.add_event(event_name, attributes=key_values, timestamp=timestamp)


@only_if_tracing
def force_tracing(span: Optional["opentelemetry.trace.Span"] = None) -> None:
    """Force sampling for the active/given span and its children.

    Args:
        span: span to force tracing for. By default, the active span.
    """
    # TODO
    pass


def is_context_forced_tracing(
    context: "opentelemetry.context.context.Context",
) -> bool:
    """Check if sampling has been force for the given span context."""
    # TODO
    return False


# Injection and extraction


@ensure_active_span("inject the active tracing context into a header dict")
def inject_active_tracing_context_into_header_dict(
    headers: Dict[bytes, List[bytes]],
    destination: Optional[str] = None,
    check_destination: bool = True,
) -> None:
    """
    Injects the active tracing context into a dict of HTTP headers

    Args:
        headers: the dict to inject headers into
        destination: address of entity receiving the span context. Must be given unless
            `check_destination` is False.
        check_destination (bool): If False, destination will be ignored and the context
            will always be injected. If True, the context will only be injected if the
            destination matches the tracing allowlist

    Note:
        The headers set by the tracer are custom to the tracer implementation which
        should be unique enough that they don't interfere with any headers set by
        synapse or twisted.
    """
    if check_destination:
        if destination is None:
            raise ValueError(
                "destination must be given unless check_destination is False"
            )
        if not whitelisted_homeserver(destination):
            return

    active_span = get_active_span()
    assert active_span is not None
    ctx = get_context_from_span(active_span)

    propagator = opentelemetry.propagate.get_global_textmap()
    # Put all of SpanContext properties into the headers dict
    propagator.inject(headers, context=ctx)


def inject_trace_id_into_response_headers(response_headers: Headers) -> None:
    """Inject the current trace id into the HTTP response headers"""
    if not opentelemetry:
        return
    active_span = get_active_span()
    if not active_span:
        return

    trace_id = active_span.get_span_context().trace_id

    if trace_id is not None:
        response_headers.addRawHeader("Synapse-Trace-Id", f"{trace_id:x}")


@ensure_active_span(
    "get the active span context as a dict", ret=cast(Dict[str, str], {})
)
def get_active_span_text_map(destination: Optional[str] = None) -> Dict[str, str]:
    """
    Gets the active tracing Context serialized as a dict. This can be used
    instead of manually injecting a span into an empty carrier.

    Args:
        destination: the name of the remote server.

    Returns:
        dict: the serialized active span's context if opentelemetry is enabled, otherwise
        empty.
    """
    if destination and not whitelisted_homeserver(destination):
        return {}

    active_span = get_active_span()
    assert active_span is not None
    ctx = get_context_from_span(active_span)

    carrier_text_map: Dict[str, str] = {}
    propagator = opentelemetry.propagate.get_global_textmap()
    # Put all of Context properties onto the carrier text map that we can return
    propagator.inject(carrier_text_map, context=ctx)

    return carrier_text_map


def context_from_request(
    request: Request,
) -> Optional["opentelemetry.context.context.Context"]:
    """Extract an opentelemetry context from the headers on an HTTP request

    This is useful when we have received an HTTP request from another part of our
    system, and want to link our spans to those of the remote system.
    """
    if not opentelemetry:
        return None
    header_dict = {
        k.decode(): v[0].decode() for k, v in request.requestHeaders.getAllRawHeaders()
    }

    # Extract all of the relevant values from the headers to construct a
    # SpanContext to return.
    return extract_text_map(header_dict)


@only_if_tracing
def extract_text_map(
    carrier: Dict[str, str]
) -> Optional["opentelemetry.context.context.Context"]:
    """
    Wrapper method for opentelemetry's propagator.extract for TEXT_MAP.
    Args:
        carrier: a dict possibly containing a context.

    Returns:
        The active context extracted from carrier.
    """
    propagator = opentelemetry.propagate.get_global_textmap()
    # Extract all of the relevant values from the `carrier` to construct a
    # Context to return.
    return propagator.extract(carrier)


# Tracing decorators


def _custom_sync_async_decorator(
    func: Callable[P, R],
    wrapping_logic: Callable[[Callable[P, R], Any, Any], ContextManager[None]],
) -> Callable[P, R]:
    """
    Decorates a function that is sync or async (coroutines), or that returns a Twisted
    `Deferred`. The custom business logic of the decorator goes in `wrapping_logic`.
    Example usage:
    ```py
    # Decorator to time the function and log it out
    def duration(func: Callable[P, R]) -> Callable[P, R]:
        @contextlib.contextmanager
        def _wrapping_logic(func: Callable[P, R], *args: P.args, **kwargs: P.kwargs) -> Generator[None, None, None]:
            start_ts = time.time()
            try:
                yield
            finally:
                end_ts = time.time()
                duration = end_ts - start_ts
                logger.info("%s took %s seconds", func.__name__, duration)
        return _custom_sync_async_decorator(func, _wrapping_logic)
    ```
    Args:
        func: The function to be decorated
        wrapping_logic: The business logic of your custom decorator.
            This should be a ContextManager so you are able to run your logic
            before/after the function as desired.
    """

    if inspect.iscoroutinefunction(func):

        @wraps(func)
        async def _wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
            with wrapping_logic(func, *args, **kwargs):
                return await func(*args, **kwargs)  # type: ignore[misc]

    else:
        # The other case here handles both sync functions and those
        # decorated with inlineDeferred.
        @wraps(func)
        def _wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
            scope = wrapping_logic(func, *args, **kwargs)
            scope.__enter__()

            try:
                result = func(*args, **kwargs)
                if isinstance(result, defer.Deferred):

                    def call_back(result: R) -> R:
                        scope.__exit__(None, None, None)
                        return result

                    def err_back(result: R) -> R:
                        scope.__exit__(None, None, None)
                        return result

                    result.addCallbacks(call_back, err_back)

                else:
                    if inspect.isawaitable(result):
                        logger.error(
                            "@trace may not have wrapped %s correctly! "
                            "The function is not async but returned a %s.",
                            func.__qualname__,
                            type(result).__name__,
                        )

                    scope.__exit__(None, None, None)

                return result

            except Exception as e:
                scope.__exit__(type(e), None, e.__traceback__)
                raise

    return _wrapper  # type: ignore[return-value]


def trace_with_opname(
    opname: str,
    *,
    tracer: Optional["opentelemetry.trace.Tracer"] = None,
) -> Callable[[Callable[P, R]], Callable[P, R]]:
    """
    Decorator to trace a function with a custom opname.
    See the module's doc string for usage examples.
    """
    # type-ignore: mypy bug, see https://github.com/python/mypy/issues/12909
    @contextlib.contextmanager  # type: ignore[arg-type]
    def _wrapping_logic(
        func: Callable[P, R], *args: P.args, **kwargs: P.kwargs
    ) -> Generator[None, None, None]:
        with start_active_span(opname, tracer=tracer):
            yield

    def _decorator(func: Callable[P, R]) -> Callable[P, R]:
        if not opentelemetry:
            return func

        return _custom_sync_async_decorator(func, _wrapping_logic)

    return _decorator


def trace(func: Callable[P, R]) -> Callable[P, R]:
    """
    Decorator to trace a function.

    Sets the operation name to that of the function's name.

    See the module's doc string for usage examples.
    """

    return trace_with_opname(func.__name__)(func)


def tag_args(func: Callable[P, R]) -> Callable[P, R]:
    """
    Tags all of the args to the active span.
    """

    if not opentelemetry:
        return func

    # type-ignore: mypy bug, see https://github.com/python/mypy/issues/12909
    @contextlib.contextmanager  # type: ignore[arg-type]
    def _wrapping_logic(
        func: Callable[P, R], *args: P.args, **kwargs: P.kwargs
    ) -> Generator[None, None, None]:
        argspec = inspect.getfullargspec(func)
        # We use `[1:]` to skip the `self` object reference and `start=1` to
        # make the index line up with `argspec.args`.
        #
        # FIXME: We could update this to handle any type of function by ignoring the
        #   first argument only if it's named `self` or `cls`. This isn't fool-proof
        #   but handles the idiomatic cases.
        for i, arg in enumerate(args[1:], start=1):  # type: ignore[index]
            set_attribute(SynapseTags.FUNC_ARG_PREFIX + argspec.args[i], str(arg))
        set_attribute(SynapseTags.FUNC_ARGS, str(args[len(argspec.args) :]))  # type: ignore[index]
        set_attribute(SynapseTags.FUNC_KWARGS, str(kwargs))
        yield

    return _custom_sync_async_decorator(func, _wrapping_logic)


@contextlib.contextmanager
def trace_servlet(
    request: "SynapseRequest", extract_context: bool = False
) -> Generator[None, None, None]:
    """Returns a context manager which traces a request. It starts a span
    with some servlet specific tags such as the request metrics name and
    request information.

    Args:
        request
        extract_context: Whether to attempt to extract the tracing
            context from the request the servlet is handling.
    """

    if opentelemetry is None:
        yield  # type: ignore[unreachable]
        return

    attrs = {
        SynapseTags.REQUEST_ID: request.get_request_id(),
        SpanAttributes.HTTP_METHOD: request.get_method(),
        SpanAttributes.HTTP_URL: request.get_redacted_uri(),
        SpanAttributes.HTTP_HOST: request.getClientAddress().host,
    }

    request_name = request.request_metrics.name
    tracing_context = context_from_request(request) if extract_context else None

    # This is will end up being the root span for all of servlet traces and we
    # aren't able to determine whether to force tracing yet. We can determine
    # whether to force trace later in `synapse/api/auth.py`.
    with start_active_span(
        request_name,
        kind=SpanKind.SERVER,
        context=tracing_context,
        attributes=attrs,
        # we configure the span not to finish immediately on exiting the scope,
        # and instead pass the span into the SynapseRequest (via
        # `request.set_tracing_span(span)`), which will finish it once we've
        # finished sending the response to the client.
        end_on_exit=False,
    ) as span:
        request.set_tracing_span(span)

        inject_trace_id_into_response_headers(request.responseHeaders)
        try:
            yield
        finally:
            # We set the operation name again in case its changed (which happens
            # with JsonResource).
            span.update_name(request.request_metrics.name)

            if request.request_metrics.start_context.tag is not None:
                span.set_attribute(
                    SynapseTags.REQUEST_TAG, request.request_metrics.start_context.tag
                )