Bug: mounted app path interferes with regular paths

[0xE111]

Description

According to "Mounting ASGI apps" documentation section, Litestar can mount ASGI apps in sub-paths. So it is expected that if ASGI app is mounted with path='/magic', every route starting with /magic will be handled by the ASGI app, and any other route will be handled by other handlers. However, it is not true.

Imagine this setup:

@asgi("/magic", is_mount=True)
async def xxx(...):
    print('Mounted')
    ...

@get("/{number:int}/magic/")
async def yyy() -> str:
    print('Parametrized')

@get("/static/magic/")
async def zzz() -> str:
    print('Static')

Here's "expectations VS reality" table:

Request path	Expected output	Real output
/magic	Mounted	Mounted
/123/magic/	Parametrized	Mounted
/static/magic/	Static	Static
/non-existent/magic/	404 error	Mounted

Why this happens?

litestar/_asgi/routing_trie/traversal.py:parse_path_to_route method has this line:

if mount_paths_regex and (match := mount_paths_regex.search(path)):

So instead of matching /magic to path, re.search is used which searches for occurrence of /magic anywhere in path, thus resulting in "false positives" for strings such as /123/magic/, /non-existent/magic/ and /non-existent/magic/something.

Possible solution

This cannot be solved by simply using regex:

@asgi("^/magic", is_mount=True)

since mount_paths_regex becomes re.compile('/^/magic'), so it not only doesn't solve the problem, but the /magic endpoint itself stops working.

I believe it may be solved by replacing mount_paths_regex.search(path) with mount_paths_regex.match(path) - I did manual tests and it solved the problem completely, but ofc such a change requires tests to ensure nothing else is broken.

I am ready to create a full-fledged pull request with tests once the issue is approved :)

URL to code causing the issue

No response

MCVE

from typing import Any

from litestar import Litestar, asgi, get
from litestar.response.base import ASGIResponse


@asgi("/magic", is_mount=True)
async def mounted_handler(scope: Any, receive: Any, send: Any) -> None:
    body = 'mounted!'
    response = ASGIResponse(body=body.encode("utf-8"))
    await response(scope, receive, send)


@get("/{number:int}/magic/")
async def parametrized_handler() -> str:
    return 'parametrized'


@get("/static/magic/")
async def static_handler() -> str:
    return 'static'


app = Litestar(route_handlers=[
    mounted_handler,
    parametrized_handler,
    static_handler,
])

Steps to reproduce

1. Use the source code from MCVE to run Litestar app
2. Run curl to see wrong handler invoked for parametrized path:
  > curl http://127.0.0.1:8000/123/magic
  mounted!     
3. Run curl to see wrong handler invoked for non-existent path:
  > curl http://127.0.0.1:8000/whatever/magic
  mounted!

Screenshots

No response

Logs

No response

Litestar Version

2.8.3

Platform

• Linux
• Mac
• Windows
• Other (Please specify in the description above)

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[peterschutt]

Issue confirmed @0xE111 - I've assigned the issue to you. Thanks!

[peterschutt]

@all-contributors add @0xE111 for bug

[allcontributors]

@peterschutt

I've put up a pull request to add @0xE111! 🎉

[provinzkraut]

Thanks @0xE111 for the very detailed report!

I believe it may be solved by replacing mount_paths_regex.search(path) with mount_paths_regex.match(path) - I did manual tests and it solved the problem completely, but ofc such a change requires tests to ensure nothing else is broken.

This sounds like the correct fix here to get the intended behaviour. A PR would be very welcome :)

[github-actions]

A fix for this issue has been released in v2.9.0