Add a way to cancel previous push notification

[saschanaz]

We got a feature request in https://bugzilla.mozilla.org/show_bug.cgi?id=1957195, which I think is a valid one:

Chess.com is a the primary source for this issue, trigger many notifications by using the website on a secondary device (this can be as simple as playing a daily game) then launch firefox, the queued notifications will start one after the other with no way to dismiss all sometimes taking several minutes to finish.

Here, there are two independent push subscriptions managed by one app server, tied to one user account. The app server triggers two push messages for an event, but even if the user got one from either of the subscriptions, there's no way for the app server to cancel another push message for another subscription.

Canceling already delivered push notification may be more tricky (would it allow abusing e.g. for effectively no-notification push?) but it still sounds reasonable to cancel a push message that hasn't delivered yet.

[saschanaz]

The problem is that there's no unencrypted metadata field for push messages that push servers can read. 🤔 Add an HTTP header field instead?

[martinthomson]

The original idea for this was to allow a DELETE on the resource that was created by the push service when a message is posted.

See https://datatracker.ietf.org/doc/html/rfc8030#section-5.4 for an example of replacement. In addition to that, you might imagine that a push service could support deletion.

From a request:

POST /push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1
Host: push.example.net
TTL: 600
Topic: upd

... message contents ...

To which the server responds:

HTTP/1.1 201 Created
Date: Thu, 11 Dec 2014 23:57:02 GMT
Location: https://push.example.net/message/qDIYHNcfAIPP_5ITvURr-d6BGt

That example shows the message contents being replaced, but they could also be replaced with nothing (i.e., removed) as follows:

DELETE /message/qDIYHNcfAIPP_5ITvURr-d6BGt
Host: push.example.net

Using the "Topic" field to enable this ensures that the push service is able to know which messages to enable this treatment for, but it's equally possible that this could be done for all messages.