Open and accept zero reserve channels by tankyleo · Pull Request #4428 · lightningdevkit/rust-lightning

tankyleo · 2026-02-19T17:43:43Z

ldk-reviews-bot · 2026-02-19T17:43:46Z

👋 Thanks for assigning @TheBlueMatt as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

codecov · 2026-02-26T11:44:38Z

Codecov Report

❌ Patch coverage is 80.04695% with 170 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.15%. Comparing base (d830f10) to head (91565f1).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	70.96%	148 Missing and 3 partials ⚠️
lightning/src/ln/channelmanager.rs	84.78%	14 Missing ⚠️
lightning/src/ln/msgs.rs	94.87%	4 Missing ⚠️
lightning/src/sign/tx_builder.rs	99.37%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4428      +/-   ##
==========================================
- Coverage   86.18%   86.15%   -0.04%     
==========================================
  Files         160      160              
  Lines      107536   108043     +507     
  Branches   107536   108043     +507     
==========================================
+ Hits        92680    93084     +404     
- Misses      12231    12338     +107     
+ Partials     2625     2621       -4

Flag	Coverage Δ
tests	`86.15% <80.04%> (-0.04%)`	⬇️

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TheBlueMatt

chanmon_consistency needs to be updated to have a 0-reserve channel or two (I believe we now have three channels between each pair of peers, so we can just do it on a subset of them, in fact we could have three separate channel types for better coverage).

TheBlueMatt · 2026-02-27T01:58:37Z

lightning/src/ln/channelmanager.rs

+	/// Creates a new outbound channel to the given remote node and with the given value.
+	///
+	/// The only difference between this method and [`ChannelManager::create_channel`] is that this method sets
+	/// the reserve the counterparty must keep at all times in the channel to zero. This allows the counterparty to


nit: If that's the only difference let's say create_channel_to_trusted_peer_0_reserve? Nice to be explicit, imo.

TheBlueMatt · 2026-02-27T02:10:13Z

lightning/src/ln/channel.rs


 		let channel_value_satoshis =
 			our_funding_contribution_sats.saturating_add(msg.common_fields.funding_satoshis);
+		// TODO(zero_reserve): support reading and writing the `disable_channel_reserve` field


Two questions. Shouldn't we check that if a channel has the 0-reserve feature bit and if it is fail if the user isn't accepting 0-reserve? Also why shouldn't we just set it now? I'm not sure we need to bother with a staging bit, really, honestly...

TheBlueMatt · 2026-03-04T18:36:41Z

Needs rebase now :/

tankyleo · 2026-03-05T10:57:51Z

Needs rebase now :/

Let me know if you prefer I rebase first

TheBlueMatt · 2026-03-05T14:25:02Z

Feel free to go ahead and rebase and squash, yea.

tankyleo · 2026-03-05T17:24:05Z

Squash diff (do not click compare just above, I pushed the wrong branch, and later corrected it):

https://github.com/lightningdevkit/rust-lightning/compare/253db4d9a05cda43f74c2835448126d3205b27b8..43be438f70ba28ad7918e407026a78763ac2b368

tankyleo · 2026-03-05T17:44:48Z

git range-diff main 43be438 7fde002

1:  bfba6e993 ! 1:  b916c3596 Add inbound and outbound checks for zero reserve channels
    @@ lightning/src/ln/channel.rs: impl FundingScope {

                // New reserve values are based on the new channel value and are v2-specific
     -          let counterparty_selected_channel_reserve_satoshis =
    --                  Some(get_v2_channel_reserve_satoshis(post_channel_value, MIN_CHAN_DUST_LIMIT_SATOSHIS));
    +-                  get_v2_channel_reserve_satoshis(post_channel_value, MIN_CHAN_DUST_LIMIT_SATOSHIS);
     -          let holder_selected_channel_reserve_satoshis = get_v2_channel_reserve_satoshis(
     -                  post_channel_value,
     -                  context.counterparty_dust_limit_satoshis,
    @@ lightning/src/ln/channel.rs: impl FundingScope {
     +                  == 0
     +          {
     +                  // If we previously had a 0-value reserve, continue with the same reserve
    -+                  Some(0)
    ++                  0
     +          } else {
    -+                  Some(get_v2_channel_reserve_satoshis(post_channel_value, MIN_CHAN_DUST_LIMIT_SATOSHIS))
    ++                  get_v2_channel_reserve_satoshis(post_channel_value, MIN_CHAN_DUST_LIMIT_SATOSHIS)
     +          };
     +          let holder_selected_channel_reserve_satoshis =
     +                  if prev_funding.holder_selected_channel_reserve_satoshis == 0 {
    @@ lightning/src/sign/tx_builder.rs: fn get_next_commitment_stats(
        );

     @@ lightning/src/sign/tx_builder.rs: fn get_available_balances(
    -   let fee_spike_buffer_htlc =
                if channel_type.supports_anchor_zero_fee_commitments() { 0 } else { 1 };

    +   // Note that the feerate is 0 in zero-fee commitment channels, so this statement is a noop
     -  let local_feerate = feerate_per_kw
     +  let spiked_feerate = feerate_per_kw
                * if is_outbound_from_holder && !channel_type.supports_anchors_zero_fee_htlc_tx() {
2:  1f8ca6c66 = 2:  53fc9a354 Add 0-reserve to `accept_inbound_channel_from_trusted_peer`
3:  1b94f7cc3 = 3:  da0520818 Add `ChannelManager::create_channel_to_trusted_peer_0reserve`
4:  19b8d7943 = 4:  1905f94e7 Shakedown zero reserve channels
5:  de2366da4 = 5:  4e6a7527a Format `ChannelManager::create_channel_internal` and...
6:  43be438f7 = 6:  7fde002e7 Update `chanmon_consistency` to include 0FC and 0-reserve channels

ldk-reviews-bot · 2026-03-05T18:44:59Z

✅ Added second reviewer: @joostjager

ldk-reviews-bot · 2026-03-07T10:57:40Z

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Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

ldk-reviews-bot · 2026-03-09T00:00:12Z

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ldk-reviews-bot · 2026-03-09T10:58:39Z

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ldk-reviews-bot · 2026-03-11T10:59:40Z

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carlaKC

First pass - haven't gone through the tests yet.

lightning/src/sign/tx_builder.rs

lightning/src/ln/channel.rs

carlaKC · 2026-03-11T14:49:53Z

lightning/src/ln/channelmanager.rs

 	/// If it does not confirm before we decide to close the channel, or if the funding transaction
 	/// does not pay to the correct script the correct amount, *you will lose funds*.
 	///
+	/// # Zero-reserve


Shouldn't we be setting the option_zero_reserve feature in lightning/bolts#1140?

Discussed offline, I'll hold off on signaling for now pending further spec discussions

lightning/src/ln/channel.rs

tankyleo · 2026-03-23T07:25:12Z

fixups: https://github.com/tankyleo/rust-lightning/releases/tag/2026-03-23-zero-reserve-fixups
a. make chan type part of the fuzz bytes
b. allow the fundee to spend dust HTLCs up to the exact dust limit satoshis
squashed (no changes from 1): https://github.com/tankyleo/rust-lightning/releases/tag/2026-03-23-zero-reserve-squashed
then rebase, new HEAD is now 6a49cf6

TheBlueMatt · 2026-03-23T15:33:05Z

lightning/src/sign/tx_builder.rs

+	counterparty_balance_before_fee_msat: u64, feerate_per_kw: u32, nondust_htlc_count: usize,
+	broadcaster_dust_limit_satoshis: u64, channel_type: &ChannelTypeFeatures,
+) -> bool {
+	let commit_tx_fee_sat = commit_tx_fee_sat(feerate_per_kw, nondust_htlc_count, channel_type);


We have this logic repeated in get_next_commitment_stats given we have the same block to calculate to-self/remote outputs.

See the commit below, I created better helpers.

TheBlueMatt · 2026-03-23T18:06:28Z

lightning/src/ln/msgs.rs

 			(0, shutdown_scriptpubkey, (option, encoding: (ScriptBuf, WithoutLength))),
 			(1, channel_type, option),
 			(2, require_confirmed_inputs, option),
+			(4, disable_channel_reserve, option),


lol why is this an even TLV :(

Hmm if receiver silently ignores, the two parties disagree on what HTLCs they can send right ?

Isn't it unidirectional? If A sends B disable_channel_reserve then B is allowed to send more HTLCs because A doesn't care if B has a reserve. B can ignore that and decide to send less.

Discussed offline, see commit below where I make the TLV odd.

TheBlueMatt · 2026-03-23T18:10:59Z

lightning/src/ln/channel.rs

+						} else {
+							if !script::is_bolt2_compliant(&script, their_features) {
+								return Err(ChannelError::close(format!(
+								"Peer is signaling upfront_shutdown but has provided an unacceptable scriptpubkey format: {}",


nit, if you feel so inspired: might be a good opportunity to inline the variables in the {} -

Suggested change

"Peer is signaling upfront_shutdown but has provided an unacceptable scriptpubkey format: {}",

"Peer is signaling upfront_shutdown but has provided an unacceptable scriptpubkey format: {script}"

See commit below, I add a separate commit

TheBlueMatt · 2026-03-23T18:13:30Z

lightning/src/ln/channel.rs

+		let counterparty_selected_channel_reserve_satoshis = get_v2_channel_reserve_satoshis(
+			post_channel_value,
+			MIN_CHAN_DUST_LIMIT_SATOSHIS,
+			prev_funding


Hmm, does this need a spec change/feature? If a peer gave us zero reserve prior to a splice and then we spliced, I'd expect it to stay zero, but I imagine the splice spec doesn't say that and theoretically this could cause us to have issues.

Yes ! Now that splicing is merged, I'll propose an addition to the spec once this PR lands.

fuzz/src/chanmon_consistency.rs

ldk-claude-review-bot · 2026-03-24T19:59:07Z

lightning/src/ln/channelmanager.rs

 			debug_assert!(peer_state.is_connected);
 			peer_state.pending_msg_events.push(send_msg_err_event);
-			let err_str = "Please use accept_inbound_channel_from_trusted_peer_0conf to accept channels with zero confirmations.".to_owned();
+			let err_str = "Please use accept_inbound_channel_from_trusted_peer to accept channels with zero confirmations.".to_owned();


Misleading error message: If a user calls accept_inbound_channel_from_trusted_peer(id, node, 0, TrustedChannelFeatures::ZeroReserve, None) on a channel whose type requires zero-conf, they hit this error saying "Please use accept_inbound_channel_from_trusted_peer..." — but they already ARE calling that method. The issue is they passed ZeroReserve instead of ZeroConf or ZeroConfZeroReserve.

Consider updating the message to indicate the actual problem, e.g.:

"Channel requires zero confirmations. Please use TrustedChannelFeatures::ZeroConf or ZeroConfZeroReserve when calling accept_inbound_channel_from_trusted_peer."

I prefer to leave as is

lightning/src/ln/msgs.rs

ldk-reviews-bot · 2026-03-25T07:41:15Z

🔔 1st Reminder

Hey @TheBlueMatt @carlaKC! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

TheBlueMatt · 2026-03-25T14:39:27Z

I think the changes here LGTM. Feel free to squash from my side, once @carlaKC says good I think we're good.

The floor for *our* selected reserve is *their* dust limit.

The goal is to prevent any commitments with no outputs, since these are not broadcastable.

There is no need for this sentinel in `FundingScope` serialization code as this would only apply to pending `FundingScope`'s. Also, if the current scope has some zero-reserve, that reserve is carried over to all pending scopes automatically. Therefore it is not possible for a pending scope to have some 0-reserve without the current one also having it.

This new flag sets 0-reserve for the channel opener.

This new method sets 0-reserve for the channel accepter.

Co-Authored-By: HAL 9000

We do not care if our balance drops below the counterparty-selected reserve upon an inbound `update_add_htlc`. This is the counterparty's problem. Hence, we drop the assumption that once our balance rises above the counterparty-selected reserve, it will always remain above this reserve for the lifetime of a funding scope. In the following commit, we make the assumption that the counterparty does not complain if we push them below our selected reserve when adding a HTLC, so we accommodate this assumption here.

Note that this currently does not match the spec as we use an odd TLV for the `disable_channel_reserve` field in `open_channel2` and `accept_channel2` msgs. If the counterparty does not understand this field, that's ok as it just means that the counterparty will not send some HTLCs we would have accepted. We make the assumption that the counterparty will not complain if we send a HTLC that pushes their balance below our selected reserve; this could happen if the counterparty is the funder of the channel. They should not complain because if we push them below our selected reserve, this is our problem.

`ChannelContext::do_accept_channel_checks`, `ChannelContext::new_for_outbound_channel`, `ChannelContext::new_for_inbound_channel`, `InboundV1Channel::new`, `OutboundV1Channel::new`.

Reduce line count and indentation

Convert format string arguments to inline `{var}` captures where the argument is a simple identifier (variable or constant). Field accesses, method calls, and expressions remain as positional args. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tankyleo added this to Weekly Goals Feb 19, 2026

tankyleo moved this to Goal: Merge in Weekly Goals Feb 19, 2026

tankyleo self-assigned this Feb 19, 2026

tankyleo force-pushed the 2026-02-zero-reserve branch from ffa1657 to 5fa3a7c Compare February 26, 2026 09:56

tankyleo marked this pull request as ready for review February 26, 2026 10:45

tankyleo requested a review from TheBlueMatt February 26, 2026 10:46

tankyleo added this to the 0.3 milestone Feb 26, 2026

TheBlueMatt reviewed Feb 27, 2026

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tankyleo force-pushed the 2026-02-zero-reserve branch from 471ba8f to 253db4d Compare March 5, 2026 10:55

tankyleo requested a review from TheBlueMatt March 5, 2026 10:57

tankyleo force-pushed the 2026-02-zero-reserve branch 2 times, most recently from 5fa3a7c to 43be438 Compare March 5, 2026 17:21

tankyleo force-pushed the 2026-02-zero-reserve branch from 43be438 to 7fde002 Compare March 5, 2026 17:36

ldk-reviews-bot requested a review from joostjager March 5, 2026 18:44

tankyleo requested review from carlaKC and removed request for joostjager March 10, 2026 14:33

carlaKC reviewed Mar 11, 2026

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tankyleo requested a review from carlaKC March 13, 2026 12:50

tankyleo force-pushed the 2026-02-zero-reserve branch 2 times, most recently from 5fa3a7c to 6a49cf6 Compare March 23, 2026 06:51

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tankyleo requested review from TheBlueMatt and carlaKC March 23, 2026 07:40

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tankyleo force-pushed the 2026-02-zero-reserve branch from 71a772e to bbe7a62 Compare March 24, 2026 21:00

tankyleo added 2 commits March 25, 2026 17:51

Set the correct floor for the reserves in inbound V2 channels

a3dded1

The floor for *our* selected reserve is *their* dust limit.

Add inbound and outbound checks for zero reserve channels

98b71c8

The goal is to prevent any commitments with no outputs, since these are not broadcastable.

tankyleo force-pushed the 2026-02-zero-reserve branch from bbe7a62 to 91565f1 Compare March 25, 2026 19:01

tankyleo and others added 10 commits March 25, 2026 23:19

Add 0-reserve to accept_inbound_channel_from_trusted_peer

04f5cae

This new flag sets 0-reserve for the channel opener.

Add ChannelManager::create_channel_to_trusted_peer_0reserve

8ebbc43

This new method sets 0-reserve for the channel accepter.

Shakedown zero reserve channels

47b1043

Update chanmon_consistency to include 0FC and 0-reserve channels

f396a46

Co-Authored-By: HAL 9000

Format ChannelManager::create_channel_internal and...

766a0fc

`ChannelContext::do_accept_channel_checks`, `ChannelContext::new_for_outbound_channel`, `ChannelContext::new_for_inbound_channel`, `InboundV1Channel::new`, `OutboundV1Channel::new`.

Create better helper functions in tx_builder

7df573c

Reduce line count and indentation

tankyleo force-pushed the 2026-02-zero-reserve branch from 91565f1 to 2967e6f Compare March 25, 2026 23:23

	"Peer is signaling upfront_shutdown but has provided an unacceptable scriptpubkey format: {}",
	"Peer is signaling upfront_shutdown but has provided an unacceptable scriptpubkey format: {script}"

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tankyleo commented Feb 19, 2026 •

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