issue_comments: 706740250

Building this plugin reminded me of an oddity of the register_output_renderer() plugin hook: one of the arguments that can be passed to it is data, which is the default internal data structure created by Datasette - but I deliberately avoided documenting that on https://docs.datasette.io/en/stable/plugin_hooks.html#register-output-renderer-datasette because it's not a stable interface.

That's not ideal. I'd like custom renderers to be able to access this data to get at things like suggested facets, on an opt-in basis.

So maybe that kind of stuff is re-implemented as "extras" which are awaitable callables - then renderer plugins can call the extras that they need to as part of their execution.

To illustrate the problem (in this case the need to access next_url) here's my first prototype of the plugin: ```python from datasette import hookimpl from datasette.utils.asgi import Response

@hookimpl def register_output_renderer(datasette): return { "extension": "json-preview", "render": json_preview, }

def json_preview(data, columns, rows): next_url = data.get("next_url") headers = {} if next_url: headers["link"] = '<{}>; rel="next"'.format(next_url) return Response.json([dict(zip(columns, row)) for row in rows], headers=headers) ```