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Clarify EventSource behavior with the back/forward cache #12381

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Clarify EventSource behavior with the back/forward cache #12381

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Original file line number Diff line number Diff line change
Expand Up @@ -111623,6 +111623,19 @@ location.href = '#foo';</code></pre>
</ol>
</li>

<li><p>For each <code>EventSource</code> object <var>eventSource</var> whose
<span>relevant global object</span>'s <span
data-x="concept-document-window">associated <code>Document</code></span> is
<var>document</var>, if <var>eventSource</var>'s <span
data-x="concept-event-stream-reconnect-after-bfcache">reconnect after back/forward cache
restoration</span> flag is true, then set it to false and <span>reestablish the
connection</span> for <var>eventSource</var>.</p>

<p class="note">This step ensures that any <code>EventSource</code> whose fetch was aborted
while the document was in the <a href="#note-bfcache">back/forward cache</a> will attempt to
reconnect now that the document has been restored and is <span>fully active</span>
again.</p></li>

<li><p><span>Update the navigation API entries for reactivation</span> given
<var>document</var>'s <span>relevant global object</span>'s <span
data-x="window-navigation-api">navigation API</span>, <var>entriesForNavigationAPI</var>, and
Expand Down Expand Up @@ -113221,6 +113234,36 @@ new PaymentRequest(&hellip;); // Allowed to use
<span>map of active timers</span>.</p></li>
</ol>
</li>

<li>
<p>Otherwise, for each <code>EventSource</code> object <var>eventSource</var> whose
<span>relevant global object</span> is equal to <var>window</var>, if there are any instances
of the <span data-x="concept-fetch">fetch</span> algorithm started for
<var>eventSource</var>:</p>

<ol>
<li><p>Abort any instances of the <span data-x="concept-fetch">fetch</span> algorithm started
for <var>eventSource</var>.</p></li>
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@smaug---- smaug---- May 4, 2026

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Are there tests for the events this might fire?

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@zp1996 zp1996 May 5, 2026

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Good point. This could be observable through EventSource events. I'll add tests
for the expected error/open behavior around BFCache entry/restore.


<li><p>Set <var>eventSource</var>'s <code
data-x="dom-EventSource-readyState">readyState</code> attribute to <code
data-x="dom-EventSource-CONNECTING">CONNECTING</code>.</p></li>

<li><p>Set <var>eventSource</var>'s <span
data-x="concept-event-stream-reconnect-after-bfcache">reconnect after back/forward cache
restoration</span> flag to true.</p></li>
</ol>

<p class="note">This allows user agents to interrupt an ongoing <code>EventSource</code>
fetch when a <code>Document</code> is being stored in the <a href="#note-bfcache">back/forward
cache</a>, without <span data-x="fail the connection">failing the connection</span> or <span
data-x="concept-EventSource-forcibly-close">forcibly closing</span> the
<code>EventSource</code>. Events that would have arrived while the document is in the
back/forward cache are not queued for later delivery. When the document is later
<span data-x="reactivate a document">reactivated</span>, any affected <code>EventSource</code>
will attempt to reconnect via its normal <span>reestablish the connection</span>
algorithm.</p>
</li>
</ol>
</div>

Expand Down Expand Up @@ -128863,6 +128906,9 @@ dictionary <dfn dictionary>EventSourceInit</dfn> {

<li><p>A <dfn data-x="concept-event-stream-last-event-id">last event ID string</dfn>. This must
initially be the empty string.</p></li>

<li><p>A <dfn data-x="concept-event-stream-reconnect-after-bfcache">reconnect after
back/forward cache restoration</dfn> flag, which must initially be false.</p></li>
</ul>

<p>Apart from <span data-x="concept-EventSource-url">url</span> these are not currently exposed on
Expand Down Expand Up @@ -129168,6 +129214,18 @@ dictionary <dfn dictionary>EventSourceInit</dfn> {
data-x="queue a task">queued</span> by <code>EventSource</code> objects is the <dfn>remote event
task source</dfn>.</p>

<div class="note">
<p>When a <code>Document</code> becomes non-<span>fully active</span> because it is stored in
the <a href="#note-bfcache">back/forward cache</a>, its <code>EventSource</code> connections
stop delivering events. The user agent can abort an ongoing instance of the <span
data-x="concept-fetch">fetch</span> algorithm for an <code>EventSource</code> at that point;
doing so does not make the document unsalvageable. Events that would have arrived while the
document is in the back/forward cache are not queued for later delivery. When the document is
<span data-x="reactivate a document">reactivated</span>, any affected <code>EventSource</code>
can reconnect via the normal <span>reestablish the connection</span> algorithm (sending a
`<code>Last-Event-ID</code>` header if applicable).</p>
</div>

</div>


Expand Down Expand Up @@ -129595,7 +129653,8 @@ data:&nbsp;test
<div algorithm>
<p>If a user agent is to <dfn data-x="concept-EventSource-forcibly-close">forcibly close</dfn> an
<code>EventSource</code> object (this happens when a <code>Document</code> object goes away
permanently), the user agent must abort any instances of the <span
permanently, and not when a <code>Document</code> enters the <a href="#note-bfcache">back/forward
cache</a>), the user agent must abort any instances of the <span
data-x="concept-fetch">fetch</span> algorithm started for this <code>EventSource</code> object,
and must set the <code data-x="dom-EventSource-readyState">readyState</code> attribute to <code
data-x="dom-EventSource-CLOSED">CLOSED</code>.</p> <!-- same as calling close() -->
Expand Down