Mechanism for register_output_renderer to suggest extension or not

[simonw]

datasette-atom only works if the user constructs a SQL query with specific output columns (atom_id ,atom_updated etc).

It would be good if the .atom link wasn't shown on the query/table page unless those columns were present. Right now you get a link which results in a 400 error:

See also #581.

[simonw]

Plugins are currently expected to return this:

@hookimpl
def register_output_renderer(datasette):
    return {
        "extension": "test",
        "callback": render_test
    }

We can add an optional third argument, "should_suggest", which takes the same arguments as the callback but simply returns True or False depending on if the plugin can work for the current set of data.

If that dictionary key is omitted, Datasette will treat this test as returning True.

[simonw]

It should be optionally awaitable - as should the existing "callback". Can use the same pattern as this one:

datasette/datasette/views/base.py

Lines 124 to 135 in 8c642f0

	for extra_vars in pm.hook.extra_template_vars(
	template=template.name,
	database=context.get("database"),
	table=context.get("table"),
	view_name=self.name,
	request=request,
	datasette=self.ds,
	):
	if callable(extra_vars):
	extra_vars = extra_vars()
	if asyncio.iscoroutine(extra_vars):
	extra_vars = await extra_vars

[simonw]

I'm going to add unit tests for the hook, and as part of that I'll fix the weird thing at the moment where the plugins for the unit tests are defined inside a quoted string as opposed to their own separate file.

[simonw]

This is a duplicate of a more recent, more developed issue: #770