Support hosted/mounted sub applications on FastAPI

[OleksandrRakovets]

How do you use Sentry?

Sentry Saas (sentry.io)

Version

1.39.1

Steps to Reproduce

In order to handle different hostnames, FastAPI application can serve sub applications via host or mount options. Methods signature in Starlette

For example:

app = FastAPI()
sub_app_1 = FastAPI()
sub_app_2 = FastAPI()
# setup routes for each app

app.host(HOST_1, sub_app_1)
app.host(HOST_2, sub_app_2)

This leads to child routes being substituted with Host or Mount object respectively.

Expected Result

FastAPI & Starlette integrations do not crash.

Ideally, it should capture the actual URL, if it is possible from additional objects.

Actual Result

After adding StarletteIntegration with default arguments (transaction_style="url") application crashes with the following logs:

2024-01-10 14:05:59,236 ERROR [httptools_impl.py] : Exception in ASGI application
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/uvicorn/protocols/http/httptools_impl.py", line 419, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 78, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/fastapi/applications.py", line 1106, in __call__
    await super().__call__(scope, receive, send)
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 364, in _sentry_patched_asgi_app
    return await middleware(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 146, in _run_asgi3
    return await self._run_app(scope, receive, send, asgi_version=3)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 241, in _run_app
    raise exc from None
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 234, in _run_app
    return await self.app(
           ^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/starlette/applications.py", line 122, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 157, in _create_span_call
    return await old_call(app, scope, new_receive, new_send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/starlette/middleware/errors.py", line 184, in __call__
    raise exc
  File "/usr/local/lib/python3.11/site-packages/starlette/middleware/errors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 157, in _create_span_call
    return await old_call(app, scope, new_receive, new_send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/starlette/middleware/base.py", line 108, in __call__
    response = await self.dispatch_func(request, call_next)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/code/webapp/app.py", line 297, in init_db_session
    response = await call_next(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/starlette/middleware/base.py", line 84, in call_next
    raise app_exc
  File "/usr/local/lib/python3.11/site-packages/starlette/middleware/base.py", line 70, in coro
    await self.app(scope, receive_or_disconnect, send_no_error)
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 146, in _run_asgi3
    return await self._run_app(scope, receive, send, asgi_version=3)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 161, in _run_app
    raise exc from None
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 157, in _run_app
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 256, in _sentry_exceptionmiddleware_call
    await old_call(self, scope, receive, send)
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 157, in _create_span_call
    return await old_call(app, scope, new_receive, new_send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
    raise exc
  File "/usr/local/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
    await self.app(scope, receive, sender)
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 157, in _create_span_call
    return await old_call(app, scope, new_receive, new_send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/fastapi/middleware/asyncexitstack.py", line 20, in __call__
    raise e
  File "/usr/local/lib/python3.11/site-packages/fastapi/middleware/asyncexitstack.py", line 17, in __call__
    await self.app(scope, receive, send)
  File "/usr/local/lib/python3.11/site-packages/starlette/routing.py", line 718, in __call__
    await route.handle(scope, receive, send)
  File "/usr/local/lib/python3.11/site-packages/starlette/routing.py", line 515, in handle
    await self.app(scope, receive, send)
  File "/usr/local/lib/python3.11/site-packages/fastapi/applications.py", line 1106, in __call__
    await super().__call__(scope, receive, send)
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 364, in _sentry_patched_asgi_app
    return await middleware(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 146, in _run_asgi3
    return await self._run_app(scope, receive, send, asgi_version=3)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 161, in _run_app
    raise exc from None
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/asgi.py", line 157, in _run_app
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/starlette/applications.py", line 122, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 115, in _create_span_call
    name, source = _get_transaction_from_middleware(app, scope, integration)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 684, in _get_transaction_from_middleware
    name = _transaction_name_from_router(asgi_scope)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/sentry_sdk/integrations/starlette.py", line 647, in _transaction_name_from_router
    return route.path
           ^^^^^^^^^^
AttributeError: 'Host' object has no attribute 'path'

[sentrivana]

@OleksandrRakovets Thank you for reporting. We currently assume that all routes have a path attribute, but this is not the case for Host, so we'll need to fix that.

[szokeasaurusrex]

Hi @OleksandrRakovets, could you please provide more detail on how to reproduce this bug, including the exact steps you took?

[OleksandrRakovets]

Hi @szokeasaurusrex!

I cannot share the actual app code, tried to outline its setup in initial ticket.

The app purpose is to respond to different subdomains from a single container.

app = FastAPI()
sub_app_1 = FastAPI()
sub_app_2 = FastAPI()
# setup routes for each app

app.host('app.company.local', sub_app_1)
app.host('api.company.local', sub_app_2)

# alternatively, for local development, it can serve API sub app with a path prefix with
# app.mount('/api', sub_app_2)

I have updated sentry-sdk (1.11.1 -> 1.39.1) and according to the latest docs updated the package to sentry-sdk = {extras = ["fastapi"], version = "^1.39.1"}

This immediately crashed the app with the log above before I even tried to add a deliberate error to see in Sentry.

Then I followed the integration docs advice and changed transaction_style to endpoint.
The app stopped crashing, but every error in Sentry had been labelled from fastapi.middleware.asyncexitstack.AsyncExitStackMiddleware instead of actual function.

So I have pointed the issue to Starlette integration, managed to disable it from autoloading, and opened this ticket :)

[szokeasaurusrex]

Hi @OleksandrRakovets, are you also experiencing the same error when you use app.mount instead of app.host?

The part I was having difficulty with was accessing the app.host routes in my local development environment after starting the dev server. But, if you also observe the error with app.mount, I can try reproducing with app.mount, instead.