Refactor default views to use register_routes

[simonw]

It would be much cleaner if Datasette's default views were all registered using the new register_routes() plugin hook. Could dramatically reduce the code in datasette/app.py.

The ideal fix here would be to rework my BaseView subclass mechanism to work with register_routes() so that those views don't have any special privileges above plugin-provided views.
Originally posted by @simonw in #864 (comment)

[simonw]

This would be a lot easier if I had extracted out the hash logic to a plugin, see:

• Move hashed URL mode out to a plugin #647

[simonw]

There's a lot of complex logic in the DataView class, which handles conditionally returning content as .json or as HTML or as .csv.

That view subclasses AsgiView which is itself request-aware, so maybe I don't need to reconsider how those classes work - just figure out how to hook them up with register_routes.

[simonw]

The key to all of this may be the DatasetteRouter class. It deals with scope right now but if it internally dealt with request that could be enough to fix #864 by adding logic needed by the .add_message() mechanism.

datasette/datasette/app.py

Lines 904 to 938 in 0991ea7

	class DatasetteRouter(AsgiRouter):
	def __init__(self, datasette, routes):
	self.ds = datasette
	super().__init__(routes)
	async def route_path(self, scope, receive, send, path):
	# Strip off base_url if present before routing
	base_url = self.ds.config("base_url")
	if base_url != "/" and path.startswith(base_url):
	path = "/" + path[len(base_url) :]
	scope_modifications = {}
	# Apply force_https_urls, if set
	if (
	self.ds.config("force_https_urls")
	and scope["type"] == "http"
	and scope.get("scheme") != "https"
	):
	scope_modifications["scheme"] = "https"
	# Handle authentication
	default_actor = scope.get("actor") or None
	actor = None
	for actor in pm.hook.actor_from_request(
	datasette=self.ds, request=Request(scope, receive)
	):
	if callable(actor):
	actor = actor()
	if asyncio.iscoroutine(actor):
	actor = await actor
	if actor:
	break
	scope_modifications["actor"] = actor or default_actor
	return await super().route_path(
	dict(scope, **scope_modifications), receive, send, path
	)

[simonw]

Since AsgiRouter is only used as the super-class of the DatasetteRouter class maybe I should get rid of AsgiRouter entirely - no point in having a Datasette-specific subclass of it if the parent class isn't ever used by anything else.

I could also rename it to just Router which is a nicer name than DatasetteRouter.

[simonw]

Maybe I could add a as_request_view method as an alternative to as_asgi:

datasette/datasette/utils/asgi.py

Lines 150 to 174 in a8bcafc

	class AsgiView:
	async def dispatch_request(self, request, *args, **kwargs):
	handler = getattr(self, request.method.lower(), None)
	return await handler(request, *args, **kwargs)
	@classmethod
	def as_asgi(cls, *class_args, **class_kwargs):
	async def view(scope, receive, send):
	# Uses scope to create a request object, then dispatches that to
	# self.get(...) or self.options(...) along with keyword arguments
	# that were already tucked into scope["url_route"]["kwargs"] by
	# the router, similar to how Django Channels works:
	# https://channels.readthedocs.io/en/latest/topics/routing.html#urlrouter
	request = Request(scope, receive)
	self = view.view_class(*class_args, **class_kwargs)
	response = await self.dispatch_request(
	request, **scope["url_route"]["kwargs"]
	)
	await response.asgi_send(send)
	view.view_class = cls
	view.__doc__ = cls.__doc__
	view.__module__ = cls.__module__
	view.__name__ = cls.__name__
	return view

Or I could teach the Router to spot the dispatch_request method and call it directly.

[simonw]

I'm going to ditch that AsgiView class too, by combining it into BaseView.

[simonw]

So now the problem is simpler: I need to get BaseView to a state where it can accept a shared request object and it can be used in conjunction with register_routes().

[simonw]

This code is interesting:

datasette/datasette/app.py

Lines 948 to 955 in 3bc2461

	scope_modifications["actor"] = actor or default_actor
	scope = dict(scope, **scope_modifications)
	for regex, view in self.routes:
	match = regex.match(path)
	if match is not None:
	new_scope = dict(scope, url_route={"kwargs": match.groupdict()})
	try:
	return await view(new_scope, receive, send)

I want to change the signature of that return await view(new_scope, receive, send) method to instead take (request, send) - so I can have a single shared request object that's created just once per HTTP request.

The problem is the scope modification: I have code that modifies the scope, but how should that impact a shared Request instance? Should its .scope be replaced with alternative scopes as it travels through the codebase?

[simonw]

I'm going to create the single Request() instance in the DatasetteRouter class - at the beginning of the route_path method:

datasette/datasette/app.py

Lines 905 to 925 in 3bc2461

	class DatasetteRouter:
	def __init__(self, datasette, routes):
	self.ds = datasette
	routes = routes or []
	self.routes = [
	# Compile any strings to regular expressions
	((re.compile(pattern) if isinstance(pattern, str) else pattern), view)
	for pattern, view in routes
	]
	async def __call__(self, scope, receive, send):
	# Because we care about "foo/bar" v.s. "foo%2Fbar" we decode raw_path ourselves
	path = scope["path"]
	raw_path = scope.get("raw_path")
	if raw_path:
	path = raw_path.decode("ascii")
	return await self.route_path(scope, receive, send, path)
	async def route_path(self, scope, receive, send, path):
	# Strip off base_url if present before routing
	base_url = self.ds.config("base_url")