fastapi: fix wrapping of middlewares

[adriangb]

MRE:

import anyio
import fastapi
import httpx
import uvicorn
from opentelemetry import trace
from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
from opentelemetry.sdk.resources import SERVICE_NAME, Resource
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import BatchSpanProcessor, ConsoleSpanExporter

resource = Resource(attributes={SERVICE_NAME: '🐛'})

traceProvider = TracerProvider(resource=resource)
processor = BatchSpanProcessor(ConsoleSpanExporter())
traceProvider.add_span_processor(processor)
trace.set_tracer_provider(traceProvider)

app = fastapi.FastAPI()

@app.get('/ok')
async def ok() -> int:
    return 123

@app.get('/error')
async def error() -> int:
    raise Exception('An error occurred!')

FastAPIInstrumentor.instrument_app(app)


async def main() -> None:
    config = uvicorn.Config(app=app, port=9999, log_config=None)
    server = uvicorn.Server(config)

    async with anyio.create_task_group() as tg:
        tg.start_soon(server.serve)
        await anyio.sleep(1)  # Wait for the server to start
        async with httpx.AsyncClient(base_url='http://localhost:9999') as client:
            response = await client.get('/ok')
            assert response.status_code == 200, f'{response.status_code}: {response.text}'
            response = await client.get('/error')
            assert response.status_code == 500,  f'{response.status_code}: {response.text}'
            tg.cancel_scope.cancel()


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

With this change it shows up properly

Run this and you'll see that there is no "http.status_code": 500 in the logs.

[codefromthecrypt]

nice comment and test!

[xrmx]

@adriangb Please take a look at the failing tests

[lzchen]

Just to clarify, is the purpose of this pr to handle exceptions generated from the OpenTelemetryMiddleware itself (if there is a bug) or unhandled exceptions thrown from the request?

[adriangb]

Yep! If there is a bug in OpenTelemetryMiddleware without this double wrapping then the server might not send a 500 to the client and instead abruptly disconnect. In practice I believe every ASGI server I know of does also have it's own handling of unhandled exceptions such that this would not be the case, but I'm not sure we want to rely on that + there would still be a noticeable change (different response content at least, maybe different headers, etc.)

[Kludex]

fixes #795