Inconsistend handling of tailing slashes with prefix mounts.

[defnull]

Given the following example:

from starlette.applications import Starlette
from starlette.responses import PlainTextResponse
from starlette.routing import Route, Mount
from starlette.staticfiles import StaticFiles
from starlette.testclient import TestClient

app = Starlette(routes=[
    Mount("/prefix", routes=[Route("/", lambda r: PlainTextResponse('Hello, world!'))]),
    # Mount("/", app=StaticFiles(directory='/tmp')),
]) 

client = TestClient(app)
assert client.get('/prefix/').status_code == 200
assert client.get('/prefix').status_code == 200

The tests pass. Both GET /prefix and GET /prefix/ are are handled by the first Mount and return a 200 response. This is expected, because the default Router has redirect_slashes enabled.

When I now un-comment the second Mount, it stops working. A GET /prefix/ request still works as expected and is handled by the first Mount, but GET /prefix will no longer match the first Mount and instead be handled by the StaticFiles app.

This is unexpected. Mounts with a prefix should always handle requests that match their prefix. If there is no remainder after the prefix was removed from the requested path, they should either trigger the / sub-route, or allow sub-routes with an empty path. This logic should live in Mount and not in Router to avoid the inconsistent behavior of Router(redirect_slashes=True) described above. The logic in Router is only triggered if there is no match at all for the no-slash path, which is not the case here.

[defnull]

I noticed this issue because I want to have a 'catch all' route at the end of my routing table that handles all requests not handled by a mounted application. This 'catch all' route unfortunately breaks requests without a tailing slash. I had to add an explicit redirect for every single prefix mount.