[FastAPI] exception handler always has span_id is 0

[sdg9670f]

Describe your environment

OS: Ubuntu
Python version: 3.12

What happened?

When an exception is raised and handled by the exception_handler, the span_id printed is always 0. However, the span_idprinted in the middleware is correct.

Steps to Reproduce

from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
from fastapi.responses import JSONResponse
from fastapi import FastAPI, Request
from opentelemetry.trace import get_current_span
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import BatchSpanProcessor
from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import OTLPSpanExporter

otlp_url = 'http://localhost:4317'

trace.set_tracer_provider(TracerProvider())
span_exporter = OTLPSpanExporter(endpoint=otlp_url)
span_processor = BatchSpanProcessor(span_exporter)
trace.get_tracer_provider().add_span_processor(span_processor)


app = FastAPI()


@app.exception_handler(Exception)
def exception_handler(request: Request, exception: Exception):
    print('exception_handler', get_current_span().get_span_context().span_id)

    return JSONResponse(
        status_code=500,
        content={"message": "Internal server error"},
    )


@app.middleware("http")
async def middleware(request: Request, call_next):
    print('middleware', get_current_span().get_span_context().span_id)

    response = await call_next(request)
    return response


@app.get("/")
def read_root():
    raise Exception("error")


FastAPIInstrumentor.instrument_app(app)

Expected Result

middleware 1234
exception_handler 1234

Actual Result

middleware 1234
exception_handler 0

Additional context

No response

Would you like to implement a fix?

None

[niehongxu116]

You can pay attention to the registration order of fastApi's middleware and the way it captures exceptions. The source code is as above. Although you have registered the handling method for Exception exceptions, the source code has processed them. Capture and processing occur in ServerError Middleware, not in the user_midddware you expected. Therefore, trace and span IDs cannot be obtained in the processing function you defined.

    def build_middleware_stack(self) -> ASGIApp:
        debug = self.debug
        error_handler = None
        exception_handlers: dict[typing.Any, typing.Callable[[Request, Exception], Response]] = {}

        for key, value in self.exception_handlers.items():
            if key in (500, Exception):
                error_handler = value
            else:
                exception_handlers[key] = value

        middleware = (
            [Middleware(ServerErrorMiddleware, handler=error_handler, debug=debug)]
            + self.user_middleware
            + [Middleware(ExceptionMiddleware, handlers=exception_handlers, debug=debug)]
        )

        app = self.router
        for cls, args, kwargs in reversed(middleware):
            app = cls(app, *args, **kwargs)
        return app

To solve the above problems, you can use

from opentelemetry.instrumentation.asgi import OpenTelemetryMiddleware
asgi_app = OpenTelemetryMiddleware(
    app,
    tracer_provider=tracer,
    excluded_urls=parse_excluded_urls("metrics,health")
)

The above is my personal opinion, hoping it will be helpful to you！