Parachain heads are synced without demand in parachain <> parachain bridge

[svyatonik]

It seems that when we bridge two parachains and run a complex relayer, parachains heads are relayed even when this isn't required by the message relay. It needs to be investigated and fixed.

[svyatonik]

The code responsible for handling this situation:

parity-bridges-common/relays/lib-substrate-relay/src/parachains/source.rs

Lines 118 to 133 in c209bb9

	let mut para_head_id = AvailableHeader::Missing;
	if let Some(on_chain_para_head_id) = self.on_chain_para_head_id(at_block, para_id).await? {
	// Never return head that is larger than requested. This way we'll never sync
	// headers past `max_header_id`.
	para_head_id = match *self.max_head_id.lock().await {
	AvailableHeader::Unavailable => AvailableHeader::Unavailable,
	AvailableHeader::Missing => {
	// `max_header_id` is not set. There is no limit.
	AvailableHeader::Available(on_chain_para_head_id)
	},
	AvailableHeader::Available(max_head_id) => {
	// We report at most `max_header_id`.
	AvailableHeader::Available(std::cmp::min(on_chain_para_head_id, max_head_id))
	},
	}
	}

So either it isn't working, or something is misconfigured