.db downloads should be served with an ETag

[simonw]

I noticed that my Pyodide Datasette prototype is downloading the same database file every single time rather than browser caching it:

[simonw]

Here's how Starlette does it: https://github.com/encode/starlette/blob/830f3486537916bae6b46948ff922adc14a22b7c/starlette/staticfiles.py#L213

[simonw]

Reminded myself how this works by reviewing conditional-get: https://github.com/simonw/conditional-get/blob/db6dfec0a296080aaf68fcd80e55fb3f0714e738/conditional_get/cli.py#L33-L52

Simply add a If-None-Match: last-known-etag header to the request and check that the response is a status 304 with an empty body.

[simonw]

Database downloads are served here:

datasette/datasette/views/database.py

Lines 186 to 192 in d60f163

	headers["Transfer-Encoding"] = "chunked"
	return AsgiFileDownload(
	filepath,
	filename=os.path.basename(filepath),
	content_type="application/octet-stream",
	headers=headers,
	)

Here's AsgiFileDownload:

datasette/datasette/utils/asgi.py

Lines 410 to 430 in d60f163

	class AsgiFileDownload:
	def __init__(
	self,
	filepath,
	filename=None,
	content_type="application/octet-stream",
	headers=None,
	):
	self.headers = headers or {}
	self.filepath = filepath
	self.filename = filename
	self.content_type = content_type
	async def asgi_send(self, send):
	return await asgi_send_file(
	send,
	self.filepath,
	filename=self.filename,
	content_type=self.content_type,
	headers=self.headers,
	)

I can add an etag= parameter to that and populate it with db.hash, if it is populated (which it always should be for immutable databases that can be downloaded).

[simonw]

Relevant tests start here:

datasette/tests/test_html.py

Line 395 in d60f163

def test_database_download_for_immutable():

[simonw]

That worked - both DBs are 304 for me now on a subsequent load of the page:

[simonw]

That prototype is now public: https://github.com/simonw/datasette-lite