fix: correctly restore trailing slash in url pathname for data requests

[GingerAdonis]

Fixes #9595 and #10125.

Because DATA_SUFFIX is set to /__data.json an existing trailing slash gets omitted in a data request. This makes it impossible to know if the request URL pathname has a trailing slash. This issue normally applies only to routes with trailingSlash set to ignore in combination with a server load function.

This PR fixes this behaviour by adding a x-sveltekit-trailing-slash searchparam to data requests with a trailing slash so the server can restore the omitted trailing slash. This approach keeps the DATA_SUFFIX the same to ensure backwards compatibility. This was the least intrusive way I found to fix the issue.

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[dummdidumm]

Thank you! I think the idea makes sense to signal the server that the trailing slash should be kept. I'm not 100% sure we need a query param for this, or if we could just do .../data.json/ instead.

Could you also add a test for this?

[GingerAdonis]

I'd say that adding a trailing slash to the data request itself might unnecessarily complicate the structure of the data requests URLs.

Another option would be to simply change DATA_SUFFIX from /__data.json to __data.json. Though I'm unsure if that affects compatibility in the various environments. What do you prefer?

I'll try to implement a test. :)

[GingerAdonis]

I'm unsure how to approach a test for this. Could anyone point me in the right direction?

[eltigerchino]

I'm unsure how to approach a test for this. Could anyone point me in the right direction?

Navigate to /packages/kit/test/apps/basics to spin up the test pages with pnpm dev.
The route /packages/kit/test/apps/basics/src/routes/routing/trailing-slash provides some sample pages to use when testing trailing slash conditions. You could add a load function to a new page under that directory to run tests on.

You can explore /packages/kit/test/apps/basics/test to find a existing tests to add the test next to (or maybe create a new suite?). I found reading the other tests as examples very helpful.

An example test could be fetching from a trailing slash endpoint and checking if it returns successfully using the expected URL structure.

[GingerAdonis]

Thank you @s3812497 for the thorough explanation.

I've added tests for this PR. However, these tests will fail until #10531 is merged.

[GingerAdonis]

I've included the changes from #10531 as requested. @dummdidumm

This should be good to go.

[dummdidumm]

Thank you!