[Webkit-unassigned] [Bug 201169] New: Add support for postMessage buffering between the service worker and window

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=201169

            Bug ID: 201169
           Summary: Add support for postMessage buffering between the
                    service worker and window
           Product: WebKit
           Version: Safari 12
          Hardware: Macintosh
                OS: macOS 10.14
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Service Workers
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: philip at philipwalton.com

Safari 12 implements the `ServiceWorkerContainer.startMessages()` API [1], but it does not actually implement the underlying buffering of messages sent from the service worker to the window in the `fetch` event for navigation requests [2].

Firefox has supported buffering for a while, and Chrome added support in M74 [3]. Prior to Chrome supporting this feature, we (on the Chrome team) recommended feature detecting support via the presence of the `startMessages()` method (since its purpose is to allow the buffer to start clearing before the DOMContentLoaded event):

```
if (navigator.serviceWorker && navigator.serviceWorker.startMessages) {
  // Assuming the service worker supports message buffering
}
```

But given that Safari has the `startMessages()` method but does not implement the underlying buffering behavior, the above will result in a false positive, and it will likely mean lots of missed message from the service worker after navigation requests.

The ability to reliably send a message from the service worker to the window after new navigations is important, as developers commonly rely on postMessage to inform the window that a newer version of the page exists [4]. If these messages are being dropped in Safari, then Safari users may be seeing outdated content and not receiving the message that new content is available.


[1] https://w3c.github.io/ServiceWorker/#dom-serviceworkercontainer-startmessages

[2] https://twitter.com/chris_dumez/status/1166170761125490689

[3] https://www.chromestatus.com/features/5751307839209472

[4] https://developers.google.com/web/tools/workbox/modules/workbox-broadcast-cache-update

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