Repository: encode/starlette

Issue:
`StaticFiles` causes Internal Server Error when user accesses existing files as directory
### Checklist

<!-- Please make sure you check all these items before submitting your bug report. -->

- [x] The bug is reproducible against the latest release and/or `master`.
- [x] There are no similar issues or pull requests to fix it yet.

### Describe the bug

<!-- A clear and concise description of what the bug is. -->
`StaticFiles` causes Internal Server Error when user accesses existing files as directory (e.g. `/static/somefile.txt/foobar`)

### To reproduce

<!-- Provide a *minimal* example with steps to reproduce the bug locally.

NOTE: try to keep any external dependencies *at an absolute minimum*
(middleware, servers, proxies, certificates...).
In other words, remove anything that doesn't make the bug go away.
-->

1. create virtual env and activate it with `python -m venv venv && source venv/bin/activate`
2. install dependencies with `pip install starlette uvicorn aiofiles`
3. setup application as follows:

directory structure:
```
.
├── poc.py
├── static
│   └── sample.txt
└── venv
```

poc.py
```py
# code from https://www.starlette.io/staticfiles/
from starlette.applications import Starlette
from starlette.routing import Mount
from starlette.staticfiles import StaticFiles


routes = [
    Mount("/static", app=StaticFiles(directory="static"), name="static"),
]

app = Starlette(routes=routes)
```

4. run application with `uvicorn poc:app`
5. access `http://127.0.0.1:8000/static/sample.txt/foo`

### Expected behavior

<!-- A clear and concise description of what you expected to happen. -->
`StaticFiles` returns "404 Not Found" HTTP response (as Apache HTTP Server and Nginx does).

### Actual behavior

<!-- A clear and concise description of what actually happens. -->
`StaticFiles` returns "500 Internal Server Error" HTTP response.

### Debugging material

<!-- Any tracebacks, screenshots, etc. that can help understanding the problem.

NOTE:
- Please list tracebacks in full (don't truncate them).
- Consider using `<details>` to make tracebacks/logs collapsible if they're very large (see https://gist.github.com/ericclemmons/b146fe5da72ca1f706b2ef72a20ac39d).
-->
console log with tracebacks:
```
INFO:     Started server process [13052]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:64288 - "GET /static/sample.txt/foo HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/uvicorn/protocols/http/h11_impl.py", line 394, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/middleware/errors.py", line 181, in __call__
    raise exc from None
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/exceptions.py", line 82, in __call__
    raise exc from None
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/routing.py", line 582, in __call__
    await route.handle(scope, receive, send)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/routing.py", line 392, in handle
    await self.app(scope, receive, send)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/staticfiles.py", line 97, in __call__
    response = await self.get_response(path, scope)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/staticfiles.py", line 114, in get_response
    full_path, stat_result = await self.lookup_path(path)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/starlette/staticfiles.py", line 154, in lookup_path
    stat_result = await aio_stat(full_path)
  File "/Users/xkhorasan/programs/test_starlette/venv/lib/python3.9/site-packages/aiofiles/os.py", line 13, in run
    return await loop.run_in_executor(executor, pfunc)
  File "/usr/local/Cellar/python@3.9/3.9.1_2/Frameworks/Python.framework/Versions/3.9/lib/python3.9/concurrent/futures/thread.py", line 52, in run
    result = self.fn(*self.args, **self.kwargs)
NotADirectoryError: [Errno 20] Not a directory: '/Users/xkhorasan/programs/test_starlette/static/sample.txt/foo'
```

### Environment

- OS: macOS
- Python version: 3.9.1
- Starlette version:  0.14.1

### Additional context

<!-- Any additional information that can help understanding the problem.

Eg. linked issues, or a description of what you were trying to achieve. -->

Apache HTTP Server and Nginx treat this case (access existing file as directory) as "404 Not Found".
Samples:
* Apache HTTP Server: https://httpd.apache.org/images/httpd_logo_wide_new.png/foo
* Nginx: https://nginx.org/download/nginx-1.18.0.tar.gz/foo

Discussion:
aminalaee: @Kludex I'm not sure if this is related to this PR. It was working alright, and the tests are from before this PR.

Patch:
diff --git a/starlette/staticfiles.py b/starlette/staticfiles.py
index 33ea0b0..39a6972 100644
--- a/starlette/staticfiles.py
+++ b/starlette/staticfiles.py
@@ -7,12 +7,8 @@ from email.utils import parsedate
 import anyio
 
 from starlette.datastructures import URL, Headers
-from starlette.responses import (
-    FileResponse,
-    PlainTextResponse,
-    RedirectResponse,
-    Response,
-)
+from starlette.exceptions import HTTPException
+from starlette.responses import FileResponse, RedirectResponse, Response
 from starlette.types import Receive, Scope, Send
 
 PathLike = typing.Union[str, "os.PathLike[str]"]
@@ -109,9 +105,30 @@ class StaticFiles:
         Returns an HTTP response, given the incoming path, method and request headers.
         """
         if scope["method"] not in ("GET", "HEAD"):
-            return PlainTextResponse("Method Not Allowed", status_code=405)
+            raise HTTPException(status_code=405)
 
-        full_path, stat_result = await self.lookup_path(path)
+        try:
+            full_path, stat_result = await anyio.to_thread.run_sync(
+                self.lookup_path, path
+            )
+        except (FileNotFoundError, NotADirectoryError):
+            if self.html:
+                # Check for '404.html' if we're in HTML mode.
+                full_path, stat_result = await anyio.to_thread.run_sync(
+                    self.lookup_path, "404.html"
+                )
+                if stat_result and stat.S_ISREG(stat_result.st_mode):
+                    return FileResponse(
+                        full_path,
+                        stat_result=stat_result,
+                        method=scope["method"],
+                        status_code=404,
+                    )
+            raise HTTPException(status_code=404)
+        except PermissionError:
+            raise HTTPException(status_code=401)
+        except OSError:
+            raise
 
         if stat_result and stat.S_ISREG(stat_result.st_mode):
             # We have a static file to serve.
@@ -121,7 +138,9 @@ class StaticFiles:
             # We're in HTML mode, and have got a directory URL.
             # Check if we have 'index.html' file to serve.
             index_path = os.path.join(path, "index.html")
-            full_path, stat_result = await self.lookup_path(index_path)
+            full_path, stat_result = await anyio.to_thread.run_sync(
+                self.lookup_path, index_path
+            )
             if stat_result is not None and stat.S_ISREG(stat_result.st_mode):
                 if not scope["path"].endswith("/"):
                     # Directory URLs should redirect to always end in "/".
@@ -130,20 +149,9 @@ class StaticFiles:
                     return RedirectResponse(url=url)
                 return self.file_response(full_path, stat_result, scope)
 
-        if self.html:
-            # Check for '404.html' if we're in HTML mode.
-            full_path, stat_result = await self.lookup_path("404.html")
-            if stat_result is not None and stat.S_ISREG(stat_result.st_mode):
-                return FileResponse(
-                    full_path,
-                    stat_result=stat_result,
-                    method=scope["method"],
-                    status_code=404,
-                )
-
-        return PlainTextResponse("Not Found", status_code=404)
+        raise HTTPException(status_code=404)
 
-    async def lookup_path(
+    def lookup_path(
         self, path: str
     ) -> typing.Tuple[str, typing.Optional[os.stat_result]]:
         for directory in self.all_directories:
@@ -153,11 +161,7 @@ class StaticFiles:
                 # Don't allow misbehaving clients to break out of the static files
                 # directory.
                 continue
-            try:
-                stat_result = await anyio.to_thread.run_sync(os.stat, full_path)
-                return full_path, stat_result
-            except FileNotFoundError:
-                pass
+            return full_path, os.stat(full_path)
         return "", None
 
     def file_response(


Test patch:
diff --git a/tests/test_staticfiles.py b/tests/test_staticfiles.py
index d5ec1af..48fdaf1 100644
--- a/tests/test_staticfiles.py
+++ b/tests/test_staticfiles.py
@@ -1,11 +1,13 @@
 import os
 import pathlib
+import stat
 import time
 
 import anyio
 import pytest
 
 from starlette.applications import Starlette
+from starlette.exceptions import HTTPException
 from starlette.requests import Request
 from starlette.routing import Mount
 from starlette.staticfiles import StaticFiles
@@ -71,8 +73,10 @@ def test_staticfiles_post(tmpdir, test_client_factory):
     with open(path, "w") as file:
         file.write("<file content>")
 
-    app = StaticFiles(directory=tmpdir)
+    routes = [Mount("/", app=StaticFiles(directory=tmpdir), name="static")]
+    app = Starlette(routes=routes)
     client = test_client_factory(app)
+
     response = client.post("/example.txt")
     assert response.status_code == 405
     assert response.text == "Method Not Allowed"
@@ -83,8 +87,10 @@ def test_staticfiles_with_directory_returns_404(tmpdir, test_client_factory):
     with open(path, "w") as file:
         file.write("<file content>")
 
-    app = StaticFiles(directory=tmpdir)
+    routes = [Mount("/", app=StaticFiles(directory=tmpdir), name="static")]
+    app = Starlette(routes=routes)
     client = test_client_factory(app)
+
     response = client.get("/")
     assert response.status_code == 404
     assert response.text == "Not Found"
@@ -95,8 +101,10 @@ def test_staticfiles_with_missing_file_returns_404(tmpdir, test_client_factory):
     with open(path, "w") as file:
         file.write("<file content>")
 
-    app = StaticFiles(directory=tmpdir)
+    routes = [Mount("/", app=StaticFiles(directory=tmpdir), name="static")]
+    app = Starlette(routes=routes)
     client = test_client_factory(app)
+
     response = client.get("/404.txt")
     assert response.status_code == 404
     assert response.text == "Not Found"
@@ -136,11 +144,15 @@ def test_staticfiles_config_check_occurs_only_once(tmpdir, test_client_factory):
     app = StaticFiles(directory=tmpdir)
     client = test_client_factory(app)
     assert not app.config_checked
-    client.get("/")
-    assert app.config_checked
-    client.get("/")
+
+    with pytest.raises(HTTPException):
+        client.get("/")
+
     assert app.config_checked
 
+    with pytest.raises(HTTPException):
+        client.get("/")
+
 
 def test_staticfiles_prevents_breaking_out_of_directory(tmpdir):
     directory = os.path.join(tmpdir, "foo")
@@ -154,9 +166,12 @@ def test_staticfiles_prevents_breaking_out_of_directory(tmpdir):
     # We can't test this with 'requests', so we test the app directly here.
     path = app.get_path({"path": "/../example.txt"})
     scope = {"method": "GET"}
-    response = anyio.run(app.get_response, path, scope)
-    assert response.status_code == 404
-    assert response.body == b"Not Found"
+
+    with pytest.raises(HTTPException) as exc_info:
+        anyio.run(app.get_response, path, scope)
+
+    assert exc_info.value.status_code == 404
+    assert exc_info.value.detail == "Not Found"
 
 
 def test_staticfiles_never_read_file_for_head_method(tmpdir, test_client_factory):
@@ -284,3 +299,70 @@ def test_staticfiles_cache_invalidation_for_deleted_file_html_mode(
     )
     assert resp_deleted.status_code == 404
     assert resp_deleted.text == "<p>404 file</p>"
+
+
+def test_staticfiles_with_invalid_dir_permissions_returns_401(
+    tmpdir, test_client_factory
+):
+    path = os.path.join(tmpdir, "example.txt")
+    with open(path, "w") as file:
+        file.write("<file content>")
+
+    os.chmod(tmpdir, stat.S_IRWXO)
+
+    routes = [Mount("/", app=StaticFiles(directory=tmpdir), name="static")]
+    app = Starlette(routes=routes)
+    client = test_client_factory(app)
+
+    response = client.get("/example.txt")
+    assert response.status_code == 401
+    assert response.text == "Unauthorized"
+
+
+def test_staticfiles_with_missing_dir_returns_404(tmpdir, test_client_factory):
+    path = os.path.join(tmpdir, "example.txt")
+    with open(path, "w") as file:
+        file.write("<file content>")
+
+    routes = [Mount("/", app=StaticFiles(directory=tmpdir), name="static")]
+    app = Starlette(routes=routes)
+    client = test_client_factory(app)
+
+    response = client.get("/foo/example.txt")
+    assert response.status_code == 404
+    assert response.text == "Not Found"
+
+
+def test_staticfiles_access_file_as_dir_returns_404(tmpdir, test_client_factory):
+    path = os.path.join(tmpdir, "example.txt")
+    with open(path, "w") as file:
+        file.write("<file content>")
+
+    routes = [Mount("/", app=StaticFiles(directory=tmpdir), name="static")]
+    app = Starlette(routes=routes)
+    client = test_client_factory(app)
+
+    response = client.get("/example.txt/foo")
+    assert response.status_code == 404
+    assert response.text == "Not Found"
+
+
+def test_staticfiles_unhandled_os_error_returns_500(
+    tmpdir, test_client_factory, monkeypatch
+):
+    def mock_timeout(*args, **kwargs):
+        raise TimeoutError
+
+    path = os.path.join(tmpdir, "example.txt")
+    with open(path, "w") as file:
+        file.write("<file content>")
+
+    routes = [Mount("/", app=StaticFiles(directory=tmpdir), name="static")]
+    app = Starlette(routes=routes)
+    client = test_client_factory(app, raise_server_exceptions=False)
+
+    monkeypatch.setattr("starlette.staticfiles.StaticFiles.lookup_path", mock_timeout)
+
+    response = client.get("/example.txt")
+    assert response.status_code == 500
+    assert response.text == "Internal Server Error"