0.35.0 refactor of root_path handling is potentially returning incorrect route

[robbielaw]

After we applied the FastAPI 0.109.0 update released on Jan 11th, we began seeing certain endpoints in our API return a 404. This issue didn't appear when running the API locally, only when accessing via the API gateway proxy (which we supply a root_path to our FastAPI instance).

Part of the FastAPI 0.109.0 update included the move to Starlette 0.35.0. The introduction of get_route_path (#2400) in 0.35.0 attempts to strip the root_path from the beginning of the active route. However, if the active route also contains the string that is the root_path somewhere else within it, it results in it also being stripped out.

Example Scenario:
root_path for the API is /product. A router(s) has the prefix /products. That router has an endpoint of /detail. This should produce the route of product/products/detail. I believe the intention of get_route_path is to then return the route /products/detail, but its current implementation will return s/detail. This results in, when the endpoint is called, returning a 404.

[Pentusha]

Faced with the same issue.
Described an issue in the PR #2400. Attached an example project with tests on which regression can be reproduced.

[jaspersival]

Same problem, working with a root_path of /api and trying to mount static files at /prefix/static/my_static_file should return the static file at /api/prefix/static/my_static_file. However, I get a 404 error. I am slamming my head on this for days already.

[Pentusha]

The starting point for addressing issues should usually be a discussion... and issues raised during discussions will be ignored.
Sick rules. My next project will steer clear of using FastAPI and Starlette.

[smrtl]

We have the same issue. I wonder why Pentusha's comment in #2400 was ignored and the PR was merged nonetheless.

[DamJanCzarek]

Bump, we got the same problem in our project :(

[gabriel-f-santos]

We run in the same problem.

Its possible to fix this, making the regex look just for the root_path:

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

Just one parameter more concatenated in the regex. None test was broken with this.

+ r"(?=/|$)"

Cases:
scope = {"path": "/foo-123/bar", "root_path": "foo"}
print(get_route_path(scope)) # Output: '/foo-123/bar'

scope = {"path": "/foo/bar", "root_path": "foo"}
print(get_route_path(scope)) # Output: '/bar'

scope = {"path": "/foo", "root_path": "foo"}
print(get_route_path(scope)) # Output: ''