0.35.0 refactor of root_path handling is potentially returning incorrect route

[gabriel-f-santos]

Discussed in #2495

^{Originally posted by robbielaw February 8, 2024}

When running starlette/fastapi behing a proxy we got some errors when calling the app directly.

when calling curl http://127.0.0.1:8000/products/, we get:
root_path: /product
scope['path']: /products
route_path: s (after apply the regex - https://github.com/encode/starlette/blob/master/starlette/_utils.py#L96)
because the the root_path to be removed if calling directly the app is "product" and endpoint path is /products

Example Scenario:
libs:

starlette==0.37.2
uvicorn==0.29.0

gist: https://gist.github.com/gabriel-f-santos/df75213fe6860e369a1e096611a767e5
run: uvicorn main:app --root-path /product

Code causing the error:
https://github.com/encode/starlette/blob/master/starlette/_utils.py#L96

def get_route_path(scope: Scope) -> str:
    root_path = scope.get("root_path", "")
    route_path = re.sub(r"^" + root_path, "", scope["path"])
    return route_path

Fix suggestion:

def get_route_path(scope: Scope) -> str:
    root_path = scope.get("root_path", "")
    route_path = re.sub(r"^" + root_path + r"(?=/|$)", "", scope["path"])
    return route_path

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