In FastAPI 0.71.0 and later, adding a middleware to the root app causes the system to swallow stack traces from sub-applications

[waweber]

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Example Code

from fastapi import FastAPI

# main and sub-app
main_app = FastAPI()
sub_app = FastAPI()

# Add a view to sub-app that raises an error
@sub_app.get("/")
async def fail():
    raise ValueError("oops")

# mount the sub app at /test
# note: doing this after the @app.middleware does not change the problem
main_app.mount("/test", sub_app)

# no-op middleware on main app
@main_app.middleware("http")
async def middleware(request, handler):
    return await handler(request)

# Now run me like `uvicorn example:main_app --reload --port 8000`
# GET /test/
# Notice that the server just prints 500 Internal Server Error (what was the error?)
# Now comment out the middleware
# Notice that the server prints the stack trace now

Description

When an unhandled exception occurs in a route handler, the server automatically responds with a 500 error response. It also prints the stack trace to the log, so the developer can tell what went wrong.

However, if you mount a sub-application AND add a middleware to the root application, exceptions raised in the sub-application will NOT print the stack trace, you will only see a "500 Internal Server Error" logged.

This behavior started in 0.71.0.

Operating System

Linux

Operating System Details

No response

FastAPI Version

Anything after 0.71.0

Python Version

3.9.10

Additional Context

I feel like this has something to do with Starlette's error handling middleware?

The answer might be "yeah don't set up your application that way" but if that's the case it would be helpful to warn about some middleware/subapplication quirks in the documentation.

[AlrasheedA]

Hi @waweber, faced the same issue and using the suggestion from @haomingyin on issue #1802 worked.

[waza-ari]

I can confirm this behaviour, having the same problem in my environment.

[devamin]

This issue still persists with latest versions of uvicorn==0.20.0 fastapi==0.89.1 starlette==0.22.0

[exxy]

This issue appears to be resolved with latest versions of uvicorn==0.24.0.post1 fastapi==0.104.1 starlette==0.27.0

[devamin]

I confirm

[mskrip]

This issue is once again present with latest versions of uvicorn==0.29.0 fastapi==0.110.0 starlette==0.36.3. Can confirm that versions mentioned above in #8577 (comment) fix the issue

[PiotrDabrowskey]

can confirm it still persists with

fastapi==0.110.2
starlette==0.37.2
uvicorn==0.29.0