0.35.0 root_path handling breaks StaticFiles routes when calling an app with a root_path parameter

[wlang42]

It seems that PR #2400 breaks getting files from a mounted StaticFiles
app when started with a root_path parameter. It will result in 404
HTTP errors, even if the files exist in the specified StaticFiles
directory.

Looking at the starlette.staticfiles.StaticFiles.get_path() method
(which in turn uses starlette._utils.get_route_path()), which should
return the (file-) path relative the the StaticFiles.directory. Here is
a test app which overrides the get_path method and prints some
variables to see whats going on:

import logging
logger = logging.getLogger('uvicorn.error')

from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles

#app = FastAPI(root_path='/proxy/a')
app = FastAPI()

class MyStaticFiles(StaticFiles):

    def get_path(self, scope):

        logger.info('entering StaticFiles.get_path()')

        root_path = scope.get('root_path')
        logger.info(f'scope["root_path"]: {root_path!r}')

        app_root_path = scope.get('app_root_path')
        logger.info(f'scope["app_root_path"]: {app_root_path!r}')

        path = scope.get('path')
        logger.info(f'scope["path"]: {path!r}')

        route_path = super().get_path(scope)

        logger.info(f'<= route_path: {route_path!r}')

        return route_path

app.mount("/files", MyStaticFiles(directory='files'), name="files")

Without root_path, everthing works as expected:

INFO:     entering StaticFiles.get_path()
INFO:     scope["root_path"]: '/files'
INFO:     scope["app_root_path"]: ''
INFO:     scope["path"]: '/files/253912.png'
INFO:     <= route_path: '253912.png'
INFO:     127.0.0.1:56970 - "GET /files/253912.png HTTP/1.1" 200 OK

Ie, the root_path (/files) is removed from path
(/files/253912.png -> 253912.png) and that is found in the files
directory.

But when setting the root_path (eg /proxy/a), things go wrong:

INFO:     entering StaticFiles.get_path()
INFO:     scope["root_path"]: '/proxy/a/files'
INFO:     scope["app_root_path"]: '/proxy/a'
INFO:     scope["path"]: '/files/253912.png'
INFO:     <= route_path: 'files/253912.png'
INFO:     127.0.0.1:56886 - "GET /files/253912.png HTTP/1.1" 404 Not Found

With root_path set to /proxy/a/files, nothing (except the /) is
stripped from path (because root_path does not match the beginning
of path) and thus the file path (relative to the StaticFiles
configured directory is not found).

Note that the root_path in the StaticFiles mounted app (and thus in
the get_path method) is from the "child_scope" (see PR #2400) and
the original application root_path is stored in app_root_path.

Here is a workaround for StaticFiles to be used with (application)
root_path:

class MyStaticFiles(StaticFiles):

    def get_path(self, scope):
        """
        workaround PR #2400 changed root_path handling
        """
        root_path = scope.get('root_path')
        app_root_path = scope.get('app_root_path')
        if (root_path and app_root_path and
            root_path.startswith(app_root_path)):
            prefix = re.sub("^" + app_root_path, "", root_path)
            route_path = re.sub("^" + prefix, "", scope['path'])
            route_path = os.path.normpath(os.path.join(*route_path.split("/")))
        else:
            route_path = super().get_path(scope)

        return route_path

app.mount("/files", MyStaticFiles(directory='files'), name="files")

[wlang42]

This is related to discussion #2495 (because it was caused by PR #2400), but a
different issue. While #2495 is about APIRouter, this is about StaticFiles.