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Update next_funding spec requirements #4098

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Merged
merged 2 commits into from
Sep 25, 2025
+29 −8
Merged

Update next_funding spec requirements #4098

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9c2ead2
Update commitment_signed retransmission for next_funding
jkczyz Sep 19, 2025
e083a2d
Document some channel_reestablish spec requirements
jkczyz Sep 19, 2025
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37 changes: 29 additions & 8 deletions lightning/src/ln/channel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9380,6 +9380,10 @@ where
return Err(ChannelError::close("Peer sent a loose channel_reestablish not after reconnect".to_owned()));
}

// A node:
// - if `next_commitment_number` is zero:
// - MUST immediately fail the channel and broadcast any relevant latest commitment
// transaction.
if msg.next_local_commitment_number == 0
|| msg.next_local_commitment_number >= INITIAL_COMMITMENT_NUMBER
|| msg.next_remote_commitment_number >= INITIAL_COMMITMENT_NUMBER
Expand Down Expand Up @@ -9451,9 +9455,10 @@ where
let mut tx_signatures = None;
let mut tx_abort = None;

// if next_funding is set:
// A receiving node:
// - if the `next_funding` TLV is set:
if let Some(next_funding) = &msg.next_funding {
// - if `next_funding` matches the latest interactive funding transaction
// - if `next_funding_txid` matches the latest interactive funding transaction
// or the current channel funding transaction:
if let Some(session) = &self.context.interactive_tx_signing_session {
let our_next_funding_txid = session.unsigned_tx().compute_txid();
Expand All @@ -9468,8 +9473,12 @@ where
self.context.expecting_peer_commitment_signed = true;
}

// TODO(splicing): Add comment for spec requirements
if next_funding.should_retransmit(msgs::NextFundingFlag::CommitmentSigned) {
// - if it has not received `tx_signatures` for that funding transaction:
// - if the `commitment_signed` bit is set in `retransmit_flags`:
if !session.has_received_tx_signatures()
&& next_funding.should_retransmit(msgs::NextFundingFlag::CommitmentSigned)
{
// - MUST retransmit its `commitment_signed` for that funding transaction.
let funding = self
.pending_splice
.as_ref()
Expand Down Expand Up @@ -9605,6 +9614,13 @@ where
let is_awaiting_remote_revoke = self.context.channel_state.is_awaiting_remote_revoke();
let next_counterparty_commitment_number = INITIAL_COMMITMENT_NUMBER - self.context.counterparty_next_commitment_transaction_number + if is_awaiting_remote_revoke { 1 } else { 0 };

// A node:
// - if `next_commitment_number` is 1 in both the `channel_reestablish` it
// sent and received:
// - MUST retransmit `channel_ready`.
Comment on lines +9618 to +9620
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While testing, I noticed there's an overlap between this requirement and my_current_funding_locked when splicing. If we open a fresh channel and immediately splice it, we'll retransmit channel_ready after a reconnect since the commitments have not advanced even though we already locked the splice (either explicitly via splice_locked or implicitly via my_current_funding_locked).

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Mmm, probably worth skipping the channel_ready if we have a splice indicator in the reestablish.

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We can probably leave this for a follow-up? Relevant discussion in the spec: lightning/bolts#1160 (comment)

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Sure, will land this then, its otherwise trivial.

// - otherwise:
// - MUST NOT retransmit `channel_ready`, but MAY send `channel_ready` with
// a different `short_channel_id` `alias` field.
let channel_ready = if msg.next_local_commitment_number == 1 && INITIAL_COMMITMENT_NUMBER - self.holder_commitment_point.next_transaction_number() == 1 {
// We should never have to worry about MonitorUpdateInProgress resending ChannelReady
self.get_channel_ready(logger)
Expand Down Expand Up @@ -11307,26 +11323,31 @@ where
}

fn maybe_get_next_funding(&self) -> Option<msgs::NextFunding> {
// If we've sent `commtiment_signed` for an interactively constructed transaction
// during a signing session, but have not received `tx_signatures` we MUST set `next_funding`
// to the txid of that interactive transaction, else we MUST NOT set it.
// The sending node:
// - if it has sent `commitment_signed` for an interactive transaction construction but
// it has not received `tx_signatures`:
self.context
.interactive_tx_signing_session
.as_ref()
.filter(|session| !session.has_received_tx_signatures())
.map(|signing_session| {
// - MUST include the `next_funding` TLV.
// - MUST set `next_funding_txid` to the txid of that interactive transaction.
let mut next_funding = msgs::NextFunding {
txid: signing_session.unsigned_tx().compute_txid(),
retransmit_flags: 0,
};

// TODO(splicing): Add comment for spec requirements
// - if it has not received `commitment_signed` for this `next_funding_txid`:
// - MUST set the `commitment_signed` bit in `retransmit_flags`.
if !signing_session.has_received_commitment_signed() {
next_funding.retransmit(msgs::NextFundingFlag::CommitmentSigned);
}

next_funding
})
// - otherwise:
// - MUST NOT include the `next_funding` TLV.
}

fn maybe_get_my_current_funding_locked(&self) -> Option<msgs::FundingLocked> {
Expand Down
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