Introduce the concept of an associated session

An intermediary node needs to route commands to an upstream
node rather than acting upon them directly. A later PR will
include the mechanism for doing this, but for now track the
session that's been opened upstream and ensure that it will
be closed properly at the end of the session.

Author: shs96c

webdriver-spec.html

@@ -2423,6 +2423,14 @@ <h2>Sessions</h2>
  or implicitly when <a>Close Window</a> is called
  at the last remaining <a>top-level browsing context</a>.
 
+<p>An <a>intermediary node</a> will maintain an <dfn>associated
+ session</dfn> for each active <a>session</a>. This is
+ the <a>session</a> on the <a>upstream</a> neighbor that is created
+ when the <a>intermediary node</a> executes the <a>New
+ Session</a> <a>command</a>. Closing a <a>session</a> on
+ an <a>intermediary node</a> will also <a>close the session</a> of
+ the <a>associated session</a>.
+
 <p>All <a>commands</a>, except <a>New Session</a> and <a>Status</a>,
  have an associated <dfn>current session</dfn>,
  which is the <a>session</a> in which that <a>command</a> will run.
@@ -2496,15 +2504,34 @@ <h2>Sessions</h2>
  a <a>remote end</a> must take the following steps:
 
 <ol>
- <li><p>Set the <a>webdriver-active flag</a> to false.
+ <li><p>Perform the folowing substeps based on the <a>remote end</a>'s
+  type:
+  <dl class=switch>
+   <dt><a>Remote end</a> is an <a>endpoint node</a>
+   <dd>
+    <ol>
+     <li><p>Set the <a>webdriver-active flag</a> to false.
 
- <li><p><a>Close</a> any <a>top-level browsing contexts</a>
-  associated with the <a>session</a>,
-  without <a>prompting to unload</a>.
+     <li><p>An <a>endpoint node</a> must <a>close</a> any <a>top-level
+      browsing contexts</a> associated with the <a>session</a>,
+      without <a>prompting to unload</a>.
+    </ol>
+  <dt><a>Remote end</a> is an <a>intermediary node</a>
+  <dd>
+   <ol>
+    <li><p><a>Close</a> the <a>associated session</a>. If this causes
+     an <a>error</a> to occur, complete the remainder of this
+     algorithm before returning the <a>error</a>.
+   </ol>
+  </dl>
 
  <li><p>Remove the <a>current session</a> from <a>active sessions</a>.
 
  <li><p>Perform any implementation-specific cleanup steps.
+
+ <li><p>If an <a>error</a> has occurred in any of the steps above,
+  return the <a>error</a>, otherwise return <a>success</a> with
+  data <a><code>null</code></a>.
 </ol>
 
 <p>Closing a <a>session</a> might cause the associated browser process to be killed.
@@ -2565,12 +2592,21 @@ <h3>New Session</h3>
 <p>The <a>remote end steps</a> are:
 
 <ol>
- <li><p>If the <a>remote end</a> is an <a>intermediary node</a>,
-  take implementation-defined steps that either result in
-  returning an <a>error</a> with <a>error code</a> <a>session not created</a>,
-  or in returning a <a>success</a> with data
-  that is isomorphic to that returned by <a>remote ends</a>
-  according to the rest of this algorithm.
+ <li><p>If the <a>remote end</a> is an <a>intermediary node</a>, take
+  implementation-defined steps that either result in returning
+  an <a>error</a> with <a>error code</a> <a>session not created</a>,
+  or in returning a <a>success</a> with data that is isomorphic to
+  that returned by <a>remote ends</a> according to the rest of this
+  algorithm. If an <a>error</a> is not returned, the <a>intermediary
+  node</a> must retain a reference to the <a>session</a> created on
+  the <a>upstream</a> node as the <a>associated session</a> such that
+  commands may be forwarded to this <a>associated session</a> on
+  subsequent commands.
+
+  <p class=note>How this is done is entirely up to the implementation,
+   but typically the <code>sessionId</code>, and <a>URL</a> and
+   <a>URL prefix</a> of the <a>upstream</a> <a>remote end</a> will need
+   to be tracked.
 
  <li><p>If the <a>maximum active sessions</a> is equal to
   the length of the list of <a>active sessions</a>,
@@ -2685,7 +2721,7 @@ <h3>Delete Session</h3>
 <p>The <a>remote end steps</a> are:
 
 <ol>
- <li><p><a>Close the session</a>.
+ <li><p><a>Try</a> to <a>close the session</a>.
 
  <li><p>Return <a>success</a> with data <a><code>null</code></a>.
 </ol>
@@ -3439,7 +3475,7 @@ <h3>Close Window</h3>
  <li><p><a>Close</a> the <a>current top-level browsing context</a>.
 
  <li><p>If there are no more open <a>top-level browsing contexts</a>,
-  then <a>close the session</a>.
+  then <a>try</a> to <a>close the session</a>.
 
  <li>Return the result of running the <a>remote end steps</a>
   for the <a>Get Window Handles</a> <a>command</a>.