[BUG]: Fastapi page routes trigger 500 internal server error

[joschrag]

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Describe your context
Please provide us your environment, so we can easily reproduce the issue.

dash                 4.3.0

Describe the bug

When creating a simple dash app with fastapi backend like this:

app.py

import dash
from dash import Dash, html

app = Dash(__name__, backend="fastapi", use_pages=True)
app.layout = html.Div([html.H1("MWE"), dash.page_container])

if __name__ == "__main__":
    app.run()

pages/index.py

import dash
from dash import html

dash.register_page(__name__, path="/", layout=html.Div("home"))

pages/home.py

import dash
from dash import html

dash.register_page(__name__, path="/home", layout=html.Div("home"))

When browsing these pages or with curl we get the following:

curl -w "%{http_code}" -o NUL -s http://127.0.0.1:8050/ -> 200
curl -w "%{http_code}" -o NUL -s http://127.0.0.1:8050/home -> 500

with the traceback:

Traceback (most recent call last):
  File "...\uvicorn\protocols\http\httptools_impl.py", line 421, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...\uvicorn\middleware\proxy_headers.py", line 62, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...\fastapi\applications.py", line 1159, in __call__
    await super().__call__(scope, receive, send)
  File "...\starlette\applications.py", line 90, in __call__
    await self.middleware_stack(scope, receive, send)
  File "...\starlette\middleware\errors.py", line 186, in __call__
    raise exc
  File "...\starlette\middleware\errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "...\dash\backends\_fastapi.py", line 235, in __call__
    await self.app(scope, receive, send)
  File "...\starlette\middleware\exceptions.py", line 63, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "...\starlette\_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "...\starlette\_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "...\fastapi\middleware\asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "...\starlette\routing.py", line 660, in __call__
    await self.middleware_stack(scope, receive, send)
  File "...\starlette\routing.py", line 680, in app
    await route.handle(scope, receive, send)
  File "...\starlette\routing.py", line 276, in handle
    await self.app(scope, receive, send)
  File "...\fastapi\routing.py", line 134, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "...\starlette\_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "...\starlette\_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "...\fastapi\routing.py", line 120, in app
    response = await f(request)
               ^^^^^^^^^^^^^^^^
  File "...\fastapi\routing.py", line 674, in app
    raw_response = await run_endpoint_function(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...\fastapi\routing.py", line 328, in run_endpoint_function
    return await dependant.call(**values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...\dash\backends\_fastapi.py", line 338, in catchall
    return Response(content=dash_app.index(), media_type="text/html")
                            ^^^^^^^^^^^^^^^^
  File "...\dash\dash.py", line 1302, in index
    request = self.backend.request_adapter()
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...\dash\backends\_fastapi.py", line 842, in __init__
    self._request: Request = get_current_request()
                             ^^^^^^^^^^^^^^^^^^^^^
  File "...\dash\backends\_fastapi.py", line 109, in get_current_request
    raise RuntimeError("No active request in context")
RuntimeError: No active request in context

#3805 seems to be the cause as the /home route does not match any conditions and exits before the request context is set in dash/backends/_fastapi.py

async def __call_():
...

    if (
            not path.startswith(dash_prefix)
            and path != prefix
            and path != prefix.rstrip("/")
        ):
            await self.app(scope, receive, send)  # /home fails within here
            return
    
    request = Request(scope, receive=receive)
    token = set_current_request(request)
...

Expected behavior

The /home path should return a 200 Response and its content.

Screenshots