UTD Hook | Add support for late decryption grace period for parity with Web

[BillCarsonFr]

On web we have a configurable grace period for late key delivery
https://github.com/element-hq/element-web/blob/9a126795a81d13aba2d331b38df75878682d54a1/src/DecryptionFailureTracker.ts#L122

    /** If the event is successfully decrypted in less than 4s, we don't report. */
    public static GRACE_PERIOD_MS = 4000;

There is no similar mecanism with the rust UTD hook, the late decryption are never graced

matrix-rust-sdk/crates/matrix-sdk-ui/src/unable_to_decrypt_hook.rs

Lines 268 to 283 in 9b6de4e

	// Only let the parent hook know about the late decryption if the event is
	// a pending UTD. If so, remove the event from the pending list —
	// doing so will cause the reporting task to no-op if it runs.
	let Some(pending_utd_report) = self.pending_delayed.lock().unwrap().remove(event_id) else {
	return;
	};
	// We can also cancel the reporting task.
	pending_utd_report.report_task.abort();
	// Now we can report the late decryption.
	let info = UnableToDecryptInfo {
	event_id: event_id.to_owned(),
	time_to_decrypt: Some(pending_utd_report.marked_utd_at.elapsed()),
	cause,
	};

[bnjbvr]

For what it's worth: it was simple enough that we implemented it at the FFI layer, in the UTD hook trait impl:

matrix-rust-sdk/bindings/matrix-sdk-ffi/src/sync_service.rs

Lines 171 to 186 in 02c7c2c

	impl UnableToDecryptHook for UtdHook {
	fn on_utd(&self, info: SdkUnableToDecryptInfo) {
	const IGNORE_UTD_PERIOD: Duration = Duration::from_secs(4);
	// UTDs that have been decrypted in the `IGNORE_UTD_PERIOD` are just ignored and
	// not considered UTDs.
	if let Some(duration) = &info.time_to_decrypt {
	if *duration < IGNORE_UTD_PERIOD {
	return;
	}
	}
	// Report the UTD to the client.
	self.delegate.on_utd(info.into());
	}
	}

Might be fine to move it over to the SDK, if that's what you implied here, but probably lower priority.

[andybalaam]

@BillCarsonFr will check over this and close this item.