## What
This fix serves to address issues where multiple `Set-Cookie` headers
were combined in some runtimes.
## Why
This is because `set-cookie` behaves differently than other headers in
some cases.
Eg. when iterating on a `Headers` instance, multiple set-cookie headers
are folded. To set them correctly, we need to split them. But it'd not
be enough to naively split on the first occurrence, because `,` is a
valid cookie value when for example it's used in `Expires` in a date
string.
So we use a method to correctly detect where to split the cookie.
This should fix all runtimes.
Note, the spec now has `Headers#getSetCookie` which should be preferred
if it's present. https://github.com/whatwg/fetch/pull/1346. We are using
the [`edge-runtime`](https://github.com/vercel/edge-runtime), so this
should be fixed upstream and then reused in Next.js in the future.
## How
Wherever we can, we reuse the `fromNodeHeaders` and `toNodeHeaders`
methods that have the correct implementation. This should be preferred
in the future in other parts of the codebase. We fixed some related TS
issues as well.
Fixes#46579, supersedes #40579
fix NEXT-735 ([link](https://linear.app/vercel/issue/NEXT-735))
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Co-authored-by: Balázs Orbán <info@balazsorban.com>
