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

@annevk commented on this pull request.



> @@ -5960,18 +5977,24 @@ interface Headers {
   undefined append(ByteString name, ByteString value);
   undefined delete(ByteString name);
   ByteString? get(ByteString name);
+  sequence&ltByteString> getSetCookie();

```suggestion
  sequence<ByteString> getSetCookie();
```

> @@ -732,12 +732,29 @@ a <a for=/>header list</a> <var>list</var>, run these steps:
   <p><a for=list>For each</a> <var>name</var> in <var>names</var>:
 
   <ol>
-   <li><p>Let <var>value</var> be the result of <a for="header list">getting</a> <var>name</var>
-   from <var>list</var>.
+   <li><p>If <var>name</var> is "<code>set-cookie</code>", then:
+    
+    <ol>
+     <li><p>Let <var>values</var> be a list of all <a for=/>headers</a> in <var>list</var> whose
+      <a for=header>name</a> is a <a>byte-case-insensitive</a> match for <var>name</var>, in order.

be a list of all values of headers*?

> @@ -732,12 +732,29 @@ a <a for=/>header list</a> <var>list</var>, run these steps:
   <p><a for=list>For each</a> <var>name</var> in <var>names</var>:
 
   <ol>
-   <li><p>Let <var>value</var> be the result of <a for="header list">getting</a> <var>name</var>
-   from <var>list</var>.
+   <li><p>If <var>name</var> is "<code>set-cookie</code>", then:

```suggestion
   <li><p>If <var>name</var> is `<code>set-cookie</code>`, then:
```

> @@ -6018,6 +6041,9 @@ new Headers(meta2);
  <dd><p>Returns as a string the values of all headers whose name is <var>name</var>, separated by a
  comma and a space.
 
+ <dt><code><var>headers</var> . <a method for=Headers lt=getSetCookie()>getSetCookie</a>()</code>
+ <dd><p>Returns a list of the values for all headers whose name is <code>set-cookie</code>.

```suggestion
 <dd><p>Returns a list of the values for all headers whose name is `<code>Set-Cookie</code>`.
```

> @@ -6160,6 +6186,14 @@ method steps are to <a for=Headers>append</a> (<var>name</var>, <var>value</var>
  <a for=Headers>header list</a>.
 </ol>
 
+<p>The <dfn export for=Headers method><code>getSetCookie()</code></dfn> method steps are:
+
+<ol>
+ <li><p>Return <a lt=value for=header>values</a> of all <a for=/>headers</a> in <a>this</a>'s
+ <a for=Headers>header list</a> whose <a for=header>name</a> is a <a>byte-case-insensitive</a> match
+ for `<code>set-cookie</code>`, in order.
+</ol>

This needs a step that returns an empty list when there are no such headers.

The second step should say "Return the values ... Set-Cookie ..." (in particular "the" and casing).

> -`<code>Set-Cookie</code>` <a for=/>header</a>. In a way this is problematic as unlike all other
-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 can not be safely combined. For
+client-side JavaScript this is not very problematic, as the platform never exposes
+`<code>Set-Cookie</code>`. This can be problematic for server-side runtimes that do expose this
+header. Because of this there is some special handling for the `<code>Set-Cookie</code>` header in
+the implementation of the {{Headers}} interface: `<code>Set-Cookie</code>` headers are not
+concatenated when they are returned from the {{Headers}} iterator. This means that specifically for
+this header, multiple tuples of the same name can be returned. 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
 `<code>Set-Cookie</code>` headers.

It's not true that client-side JavaScript never exposes Set-Cookie. That's only true in the general case.

So I don't think we need the client-side vs server-side exposition here.

We should note that we special-case Set-Cookie, just like HTTP. And that this alternative representation could be used for header lists in theory.

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