feat(errors): default error serializer content negotiation

[copalco]

Summary of Changes

We would like to use choose from available content type handlers to return error response content in a best format basing on the accept header of the request.

Related Issues

Fixes https://github.com/falconry/falcon/issues/2023 Fixes #2349

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[CaselIT]

@copalco I've taken the liberty of improving the implementation.

@vytas7 should be ready for review. It should be mostly backward compatible, but I feel it's still better to have this for 4.0 instead of 4.x

[vytas7]

This is a simple test app that I used:

import falcon.asgi
import falcon.media

class AsyncHandler(falcon.media.BaseHandler):
    async def serialize_async(self, media, content_type):
        return f'{media}\n'.encode()

class Resource:
    async def on_get(self, req, resp):
        if not req.get_param('password'):
            raise falcon.HTTPForbidden(title='NotAllowed')

        resp.content_type = 'falcon/peregrine'
        resp.media = {'msg': 'Hello'}

app = falcon.asgi.App()
app.resp_options.media_handlers['falcon/peregrine'] = AsyncHandler()
app.add_route('/', Resource())

Rendering normal media using my new handler works:

$ xh http://localhost:8000/?password=secret Accept:falcon/peregrine

HTTP/1.1 200 OK
Content-Length: 17
Content-Type: falcon/peregrine
Date: Sun, 29 Sep 2024 10:44:42 GMT
Server: uvicorn

{'msg': 'Hello'}

Going via the exception route fails with an internal server error:

$ xh http://localhost:8000/ Accept:falcon/peregrine

HTTP/1.1 500 Internal Server Error
Connection: close
Content-Length: 21
Content-Type: text/plain; charset=utf-8
Date: Sun, 29 Sep 2024 10:45:39 GMT
Server: uvicorn

Internal Server Error

Traceback (most recent call last):
  File "/home/vytas/.virtualenvs/falcon-py312/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 401, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vytas/.virtualenvs/falcon-py312/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 70, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vytas/progs/tmp/copalco/falcon/falcon/asgi/app.py", line 529, in __call__
    if not await self._handle_exception(req, resp, ex, params):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vytas/progs/tmp/copalco/falcon/falcon/asgi/app.py", line 1299, in _handle_exception
    await err_handler(req, resp, ex, params, **kwargs)
  File "/home/vytas/progs/tmp/copalco/falcon/falcon/asgi/app.py", line 1207, in _http_error_handler
    self._compose_error_response(req, resp, error)
  File "/home/vytas/progs/tmp/copalco/falcon/falcon/app.py", line 1126, in _compose_error_response
    self._serialize_error(req, resp, error)
  File "/home/vytas/progs/tmp/copalco/falcon/falcon/app_helpers.py", line 328, in default_serialize_error
    resp.data = handler.serialize(exception.to_dict(), preferred)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vytas/progs/tmp/copalco/falcon/falcon/media/base.py", line 61, in serialize
    raise NotImplementedError()
NotImplementedError

[CaselIT]

good catch! fixed it

[CaselIT]

@vytas7 updated

[CaselIT]

Updated.

We also need to make a decision on being able to disable XML, and whether to make a breaking change, and disable XML by default.

Not too sure about this. Let's merge this one and think about it after

[vytas7]

As discussed on Gitter, we don't want to make a breaking change wrt XML right now.