XCM Deposit is not Unreserved after Closing XCM Channel

[albertov19]

It is clear that when a team inits open or accepts an XCM channel, there is a deposit (currently 10 DOT) for each action:

• Init Open:

  polkadot-sdk/polkadot/runtime/parachains/src/hrmp.rs

  Lines 1470 to 1478 in d5b96e9

  	// Do not require deposits for channels with or amongst the system.
  	let is_system = origin.is_system() || recipient.is_system();
  	let deposit = if is_system { 0 } else { config.hrmp_sender_deposit };
  	if !deposit.is_zero() {
  	T::Currency::reserve(
  	&origin.into_account_truncating(),
  	deposit.unique_saturated_into(),
  	)?;
  	}

• Accept:

  polkadot-sdk/polkadot/runtime/parachains/src/hrmp.rs

  Lines 1534 to 1542 in d5b96e9

  	// Do not require deposits for channels with or amongst the system.
  	let is_system = origin.is_system() || sender.is_system();
  	let deposit = if is_system { 0 } else { config.hrmp_recipient_deposit };
  	if !deposit.is_zero() {
  	T::Currency::reserve(
  	&origin.into_account_truncating(),
  	deposit.unique_saturated_into(),
  	)?;
  	}

Nevertheless, I don't see this balance being unreserved when the XCM channel is closed ->

polkadot-sdk/polkadot/runtime/parachains/src/hrmp.rs

Line 1593 in d5b96e9

fn close_channel(origin: ParaId, channel_id: HrmpChannelId) -> Result<(), Error<T>> {

Any reasons?

[bkontur]

close_channel does not close channel :), it just creates a request for closing HrmpCloseChannelRequests,
and the real closing is done on new session or something like that here:

polkadot-sdk/polkadot/runtime/parachains/src/hrmp.rs

Lines 1123 to 1137 in d5b96e9

	fn process_hrmp_close_channel_requests() {
	let close_reqs = HrmpCloseChannelRequestsList::<T>::take();
	for condemned_ch_id in close_reqs {
	HrmpCloseChannelRequests::<T>::remove(&condemned_ch_id);
	Self::close_hrmp_channel(&condemned_ch_id);
	}
	}
	/// Close and remove the designated HRMP channel.
	///
	/// This includes returning the deposits.
	///
	/// This function is idempotent, meaning that after the first application it should have no
	/// effect (i.e. it won't return the deposits twice).
	fn close_hrmp_channel(channel_id: &HrmpChannelId) {

and there in the close_hrmp_channel is also unreserve calls.

So the real closing is done here: https://github.com/paritytech/polkadot-sdk/blob/d5b96e9e7f24adc1799f8e426c5cb69b4f2dbf8a/polkadot/runtime/parachains/src/hrmp.rs#L1137C5-L1137C23

[bkontur]

so, iiuc, when a team closes an HRMP channel, they receive their funds back in the new session

[albertov19]

Thanks for clarifying! That makes sense, maybe it would also make sense to reserve the balance when the actual channel is established then.

But it is clear! Thanks!

[bkontur]

maybe it would also make sense to reserve the balance when the actual channel is established then.

I don't think so. The establishment is done asynchronously in the new session in process_hrmp_open_channel_requests. What if the relevant parachains don't have enough balance at that time? Potentially, those requests could hang indefinitely, and we need to handle that case (maybe with a timeout). I guess, it would complicate things, and perhaps there are other reasons why it isn't done this way.
Additionally, I think it's also a form of spam protection that para teams must pay a deposit on the hrmp_init_open_channel / hrmp_accept_open_channel extrinsics.