Re: [whatwg/fetch] Add special handling of `set-cookie` to Headers (PR #1346)

@andreubotella commented on this pull request.



> -headers `<code>Set-Cookie</code>` headers cannot be combined, but since `<code>Set-Cookie</code>`
-headers are not exposed to client-side JavaScript this is deemed an acceptable compromise.
-Implementations could choose the more efficient {{Headers}} object representation even for a
-<a for=/>header list</a>, as long as they also support an associated data structure for
+<p class=note>Unlike a <a for=/>header list</a>, a {{Headers}} object combines the values of
+multiple headers of the same name into a single header value. This is problematic for the
+`<code>Set-Cookie</code>` header, as unlike all other headers, it cannot be safely combined. Because
+of this, there is some special handling for the `<code>Set-Cookie</code>` header in
+the implementation of the {{Headers}} interface, just like in the HTTP spec.
+`<code>Set-Cookie</code>` headers are not concatenated when they are returned from the {{Headers}}
+iterator, and there is a
+<code><var>headers</var> . <a method for=Headers lt=getSetCookie()>getSetCookie</a>()</code> method
+to retrieve the list of all `<code>Set-Cookie</code>`, ordered and uncombined. This means that
+specifically for this header, multiple tuples of the same name can be returned from the {{Headers}}
+iterator. Implementations could choose the more efficient {{Headers}} object representation even for
+a <a for=/>header list</a>, as long as they also support an associated data structure for

Before this PR, header lists had to store `Set-Cookie` header values separately, since cookie parsing is a step in the HTTP-network fetch algorithm which uses header lists, but `Headers` objects could combine them. Now there is no difference in the handling of `Set-Cookie` between header lists and `Headers` objects.

This note should therefore be rewritten, since now there is no "more efficient" representation for `Headers` that doesn't work for header lists. It should also probably be moved to the definition of header list.

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