Add PushSubscription.expirationTime

The expiration time of a push subscription indicates the time, in
milliseconds since the UNIX epoch (UTC), at which the subscription will
be deactivated.

This is an important addition for user agents that refresh their
subscriptions, as it gives the developer knowledge of a final time after
which they can garbage collect the stored subscriptions if the user has
not communicated the details of the refreshed subscription.

Fixes #86

index.html

@@ -293,17 +293,19 @@ <h2>
         [[!FILEAPI]].
       </p>
       <p>
-        <code><a href="http://www.w3.org/TR/WebIDL/#idl-any"><dfn>Any</dfn></a></code>,
+        <code><a href="https://www.w3.org/TR/WebIDL/#idl-any"><dfn>Any</dfn></a></code>,
         <code><a href=
-        "http://heycam.github.io/webidl/#idl-ArrayBuffer"><dfn>ArrayBuffer</dfn></a></code>,
+        "https://heycam.github.io/webidl/#idl-ArrayBuffer"><dfn>ArrayBuffer</dfn></a></code>,
         <code><a href=
-        "http://heycam.github.io/webidl/#common-BufferSource"><dfn>BufferSource</dfn></a></code>,
+        "https://heycam.github.io/webidl/#common-BufferSource"><dfn>BufferSource</dfn></a></code>,
         <code><a href=
         "https://heycam.github.io/webidl/#idl-DOMString"><dfn>DOMString</dfn></a></code>,
         <code><a href=
-        "http://heycam.github.io/webidl/#notallowederror"><dfn>NotAllowedError</dfn></a></code>,
+        "https://heycam.github.io/webidl/#DOMTimeStamp"><dfn>DOMTimeStamp</dfn></a></code>,
+        <code><a href=
+        "https://heycam.github.io/webidl/#notallowederror"><dfn>NotAllowedError</dfn></a></code>,
         and <code><a href=
-        "http://heycam.github.io/webidl/#idl-USVString"><dfn>USVString</dfn></a></code> are defined
+        "https://heycam.github.io/webidl/#idl-USVString"><dfn>USVString</dfn></a></code> are defined
         in [[!WEBIDL]].
       </p>
       <p>
@@ -364,6 +366,12 @@ <h2>
           messages</a> to. A <a>push endpoint</a> MUST uniquely identify the <a>push
           subscription</a>.
         </p>
+        <p>
+          A <a>push subscription</a> MAY have an associated <dfn>subscription expiration time</dfn>.
+          When set, it MUST be the time, in milliseconds since 00:00:00 UTC on 1 January 1970,
+          at which the subscription will be <a>deactivated</a>. The <a>user agent</a> SHOULD attempt
+          to <a>refresh</a> the push subscription before the subscription expires.
+        </p>
         <p>
           A <a>push subscription</a> has internal slots for a P-256 <a>ECDH</a> key pair and an
           authentication secret in accordance with [[!WEBPUSH-ENCRYPTION]]. These slots MUST be
@@ -855,6 +863,7 @@ <h2>
       <pre class="idl">
         interface PushSubscription {
           readonly attribute USVString endpoint;
+          readonly attribute DOMTimeStamp? expirationTime;
           [SameObject] readonly attribute PushSubscriptionOptions options;
           ArrayBuffer? getKey(PushEncryptionKeyName name);
           Promise&lt;boolean&gt; unsubscribe();
@@ -866,6 +875,10 @@ <h2>
         attribute, the <a>user agent</a> MUST return the <a>push endpoint</a> associated with the
         <a>push subscription</a>.
       </p>
+      <p>
+        When getting the <dfn>expirationTime</dfn> attribute, the <a>user agent</a> MUST return the
+        <a>subscription expiration time</a> associated with the <a>push subscription</a> if there is
+        one, or <code>null</code> otherwise.
       <p>
         The <dfn>getKey</dfn> method retrieves keying material that
         can be used for encrypting and authenticating messages. When <a data-lt="PushSubscription.getKey">getKey</a> is
@@ -928,11 +941,14 @@ <h2>
         <li>Let <var>map</var> be an empty map.
         </li>
         <li>Add an entry to <var>map</var> whose key name is <code>endpoint</code> and whose value
-        is the result of converting the <code>endpoint</code> attribute of the
-        <a>PushSubscription</a> to a serialized value. The <a>user agent</a> MUST use
-        a serialization method that does not contain input-dependent branchs (that is, one that is
-        constant time). Note that a URL - as ASCII text - will not ordinarily require special
-        treatment.
+        is the result of converting the <code>endpoint</code> attribute to a serialized value. The
+        <a>user agent</a> MUST use a serialization method that does not contain input-dependent
+        branchs (that is, one that is constant time). Note that a URL - as ASCII text - will not
+        ordinarily require special treatment.
+        </li>
+        <li>Add an entry to <var>map</var> whose key name is <code>expirationTime</code> and whose
+        value is the result of converting the <code>expirationTime</code> attribute to a serialized
+        value.
         </li>
         <li>Let <var>keys</var> be an empty map.
         </li>