Update MSC4028 implementation: Enable the unstable push rule by default

[giomfo]

Since MSC4028 has been implemented, I updated the spec about the unstable push rule with this commit.

Indeed the unstable push rule org.matrix.msc4028.encrypted_event used during development should be enabled by default like the stable push rule.

This change has been triggered by a comment from the mobile dev team here. I realized that it was not risky to enabled this unstable push rule by default because we are using the experimental mode. Apply the same enabling value on the unstable and the stable push rules would make more relevant the current tests of this feature. See here the current test plan.

I would add here a question for the Backend team: How the new push rule is handled when the MSC has successfully passed a merge FCP? I mean when the feature becomes stable.
Is the unstable push rule (stored in the existing account data) renamed/replaced automatically with the stable push rule?

[Velin92]

Hello!
Some users are actually starting to experiment on their homeserver to these settings with Element X, and most of them by simply reading the MSC spec, think that when enabling the MSC in the server configuration, will automatically have the push rule also enabled for all their users, but as of right now that's not the case.

Recently discussed here:
element-hq/element-x-ios#2411

[anoadragon453]

For context, the unstable org.matrix.msc4028.encrypted_event is included in every user's default base append override rules. However, the push rule will only be evaluated if the experimental_features. config option is set to true:

synapse/rust/src/push/mod.rs

Lines 593 to 597 in d4e3ad0

	if !self.msc4028_push_encrypted_events
	&& rule.rule_id == "global/override/.org.matrix.msc4028.encrypted_event"
	{
	return false;
	}

How the new push rule is handled when the MSC has successfully passed a merge FCP? I mean when the feature becomes stable.

Ideally we would just rename it in the codebase and provide a database migration to do the same for users automatically. This would result in push notifications being sent to mobile clients with the new rules immediately.

As for clients that compute push rules locally... do clients regularly pull down their push rules? If so, then we don't need to worry about telling clients proactively and can wait until they update themselves.

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I apologise for such a slow response on this ticket. What would you currently like to see done about this issue? I see that the MSC hasn't moved forward much in the meantime, and is still not accepted.