Add hooks for HTML

"legacy-clone a browsing session storage shed" can be used by HTML to define creation of auxiliary browsing contexts, as part of whatwg/html#5560.

"obtain a storage key" can be used by APIs that share keying logic with storage, such as BroadcastChannel and shared workers. See whatwg/html#3054. It's potentially also useful for Indexed DB as discussed in w3c/IndexedDB#334.

Closes #92.

storage.bs

@@ -185,13 +185,30 @@ anticipated that some APIs will be applicable to both <a>storage types</a> going
 <!-- If that does not happen by 2024 we probably ought to admit defeat and simplify this a bit. -->
 
 
-<h3 id=storage-sheds>Storage sheds</h3>
+<h3 id=storage-keys>Storage keys</h3>
 
 <p>A <dfn>storage key</dfn> is an <a for=/>origin</a>. [[HTML]]
 
 <p class=XXX>This is expected to change, see
 <a href="https://privacycg.github.io/storage-partitioning/">Client-Side Storage Partitioning</a>.
 
+<p>To <dfn export>obtain a storage key</dfn>, given an <a>environment settings object</a>
+<var>environment</var>, run these steps:
+
+<ol>
+ <li><p>Let <var>key</var> be <var>environment</var>'s
+ <a for="environment settings object">origin</a>.
+
+ <li><p>If <var>key</var> is an <a>opaque origin</a>, then return failure.
+
+ <li><p>If the user has disabled storage, then return failure.
+
+ <li><p>Return <var>key</var>.
+</ol>
+
+
+<h3 id=storage-sheds>Storage sheds</h3>
+
 <p>A <dfn>storage shed</dfn> is a <a for=/>map</a> of <a>storage keys</a> to <a>storage shelves</a>.
 It is initially empty.
 
@@ -211,6 +228,28 @@ It is initially empty.
 browsing session. It is roughly analogous to <a>top-level browsing context</a> except that it cannot
 be replaced due to <code>Cross-Origin-Opener-Policy</code> or navigation.
 
+<p>To <dfn export>legacy-clone a browsing session storage shed</dfn>, given a browsing session
+<var>A</var> and a browsing session <var>B</var>, run these steps:
+
+<ol>
+ <li>
+  <p><a for=map>For each</a> <var>key</var> → <var>shelf</var> of <var>A</var>'s
+  <a for="browsing session">storage shed</a>:
+
+  <ol>
+   <li><p>Let <var>newShelf</var> be the result of running <a>create a storage shelf</a> with
+   "<code>session</code>".
+
+   <li><p>Set <var>newShelf</var>'s <a>bucket map</a>["<code>default</code>"]'s
+   <a>bottle map</a>["<code>sessionStorage</code>"]'s <a for="storage bottle">map</a> to a
+   <a for=map>clone</a> of <var>shelf</var>'s <a>bucket map</a>["<code>default</code>"]'s
+   <a>bottle map</a>["<code>sessionStorage</code>"]'s <a for="storage bottle">map</a>.
+
+   <li><p>Set <var>B</var>'s <a for="browsing session">storage shed</a>[<var>key</var>] to
+   <var>newShelf</var>.
+  </ol>
+</ol>
+
 
 <h3 id=storage-shelves>Storage shelves</h3>
 
@@ -228,24 +267,14 @@ given a <a for=map>value</a> when a <a>storage shelf</a> is
 <var>type</var>, run these steps:
 
 <ol>
- <li><p>Let <var>key</var> be <var>environment</var>'s
- <a for="environment settings object">origin</a>.
+ <li><p>Let <var>key</var> be the result of running <a>obtain a storage key</a> with
+ <var>environment</var>.
 
- <li><p>If <var>key</var> is an <a>opaque origin</a>, then return failure.
+ <li><p>If <var>key</var> is failure, then return failure.
 
- <li><p>If the user has disabled storage, then return failure.
-
- <li>
-  <p>If <var>shed</var>[<var>key</var>] does not <a for=map>exist</a>, then:
-
-  <ol>
-   <li><p>Let <var>shelf</var> be a new <a>storage shelf</a>.
-
-   <li><p>Set <var>shelf</var>'s <a>bucket map</a>["<code>default</code>"] to the result of
-   <a>create a storage bucket</a> with <var>type</var>.
-
-   <li><p>Set <var>shed</var>[<var>key</var>] to <var>shelf</var>.
-  </ol>
+ <li><p>If <var>shed</var>[<var>key</var>] does not <a for=map>exist</a>, then set
+ <var>shed</var>[<var>key</var>] to the result of running <a>create a storage shelf</a> with
+ <var>type</var>.
 
  <li><p>Return <var>shed</var>[<var>key</var>].
 </ol>
@@ -254,6 +283,18 @@ given a <a for=map>value</a> when a <a>storage shelf</a> is
 <var>environment</var>, return the result of running <a>obtain a storage shelf</a> with the user
 agent's <a for="user agent">storage shed</a>, <var>environment</var>, and "<code>local</code>".
 
+<p>To <dfn>create a storage shelf</dfn>, given a <a>storage type</a> <var>type</var>, run these
+steps:
+
+<ol>
+ <li><p>Let <var>shelf</var> be a new <a>storage shelf</a>.
+
+ <li><p>Set <var>shelf</var>'s <a>bucket map</a>["<code>default</code>"] to the result of running
+ <a>create a storage bucket</a> with <var>type</var>.
+
+ <li><p>Return <var>shelf</var>.
+</ol>
+
 
 <h3 id=buckets oldids=boxes>Storage buckets</h3>