Mechanism for register_output_renderer hooks to access full count

[simonw]

The cause of this bug:

• 500 errors in Datasette 1.0 alpha datasette-export-notebook#17

Is that datasette-export-notebook was consulting data["filtered_table_rows_count"] in the render output plugin function in order to show the total number of rows that would be exported.

That field is no longer available by default - the "count" field is only available if ?_extra=count was passed.

It would be useful if plugins like this could access the total count on demand, should they need to.

[simonw]

Here's where it's called at the moment:

datasette/datasette/views/base.py

Lines 297 to 313 in 16f0b6d

	result = call_with_supported_arguments(
	self.ds.renderers[_format][0],
	datasette=self.ds,
	columns=data.get("columns") or [],
	rows=data.get("rows") or [],
	sql=data.get("query", {}).get("sql", None),
	query_name=data.get("query_name"),
	database=database,
	table=data.get("table"),
	request=request,
	view_name=self.name,
	truncated=False, # TODO: support this
	error=data.get("error"),
	# These will be deprecated in Datasette 1.0:
	args=request.args,
	data=data,
	)

And the docs: https://github.com/simonw/datasette/blob/1.0a6/docs/plugin_hooks.rst#register-output-renderer-datasette

I'm tempted to add a get_count argument which, when awaited, returns the full count. Then plugins could do this:

async def render_notebook(datasette, request, get_count, rows):
    count = await get_count()
    # ...

[simonw]

This feels like it might be quite a nice pattern generally - providing optional arguments to plugins and views that can be await get_x() called to run an extra calculation.