Don't overwrite custom etag

[cristovao-trevisan]

Kit should not overwrite custom etag value.

Sometimes it's useful to provide a custom etag value which should not be overwritten by sveltekit.
For example, in my particular use case, when proxying an image from S3 it's possible to retrieve the ETAG with the metadata, without downloading it.
This is also useful when the etag is based on api requests, making it possible to skip rendering the page.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: c054545

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[Rich-Harris]

Thank you! Can you add a test and a changeset please? You could add a test to this section:

kit/packages/kit/test/apps/basics/test/test.js

Lines 690 to 735 in 9e482ac

	test.describe.parallel('ETags', () => {
	test.skip(({ javaScriptEnabled }) => !javaScriptEnabled);
	test('generates etag/304 for text body', async ({ request }) => {
	const r1 = await request.get('/etag/text');
	const etag = r1.headers()['etag'];
	expect(etag).toBeTruthy();
	const r2 = await request.get('/etag/text', {
	headers: {
	'if-none-match': etag
	}
	});
	expect(r2.status()).toBe(304);
	expect(r2.headers()['expires']).toBe('yesterday');
	});
	test('generates etag/304 for binary body', async ({ request }) => {
	const r1 = await request.get('/etag/binary');
	const etag = r1.headers()['etag'];
	expect(etag).toBeTruthy();
	const r2 = await request.get('/etag/binary', {
	headers: {
	'if-none-match': etag
	}
	});
	expect(r2.status()).toBe(304);
	});
	test('support W/ etag prefix', async ({ request }) => {
	const r1 = await request.get('/etag/text');
	const etag = r1.headers()['etag'];
	expect(etag).toBeTruthy();
	const r2 = await request.get('/etag/text', {
	headers: {
	'if-none-match': `W/${etag}`
	}
	});
	expect(r2.status()).toBe(304);
	});
	});

[Conduitry]

Should we also, then, trust an etag from the user's response object when we're checking whether we should return a 304?

[cristovao-trevisan]

Yes @Rich-Harris , I can add a test and the changeset, will do soon

@Conduitry , I don't think there's a stateless (or serveless) way to check the user provided etag, I assume that if the etag is being set manually the user know what he's doing and will check the if-none-matches equality himself

[Rich-Harris]

Thanks — response inline

[cristovao-trevisan]

Was going to apply your suggestion right now, sorry to be late but thanks for all the work!!