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 @carlaKC 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 80.11696% with 170 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.17%. Comparing base (d830f10) to head (32a67f8).
⚠️ Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	71.12%	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%   86.17%   -0.02%     
==========================================
  Files         160      160              
  Lines      107536   108046     +510     
  Branches   107536   108046     +510     
==========================================
+ Hits        92680    93105     +425     
- Misses      12231    12321      +90     
+ Partials     2625     2620       -5

Flag	Coverage Δ
tests	`86.17% <80.11%> (-0.02%)`	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

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

🔔 1st Reminder

Hey @TheBlueMatt @joostjager! 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.

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

🔔 1st Reminder

Hey @TheBlueMatt @joostjager! 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.

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

🔔 2nd Reminder

Hey @TheBlueMatt @joostjager! 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.

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

🔔 3rd 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.

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

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.

tankyleo · 2026-03-26T01:20:57Z

git diff bbe7a62 2967e6f, all significant changes in commit 5eb530d

diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index 8f8ab4a9b..fa394ee6e 100644
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -15586,6 +15586,10 @@
 		}
 		let is_manual_broadcast = Some(self.context.is_manual_broadcast);

+		let has_0reserve =
+			(self.funding.counterparty_selected_channel_reserve_satoshis.is_some_and(|r| r == 0)
+				|| self.funding.holder_selected_channel_reserve_satoshis == 0)
+				.then_some(());
 		let holder_commitment_point_previous_revoked =
 			self.holder_commitment_point.previous_revoked_point();
 		let holder_commitment_point_last_revoked =
@@ -15655,6 +15659,7 @@
 			// 65 was previously used for quiescent_action
 			(67, pending_outbound_held_htlc_flags, optional_vec), // Added in 0.2
 			(69, holding_cell_held_htlc_flags, optional_vec), // Added in 0.2
+			(70, has_0reserve, option), // Added in 0.3 to prevent downgrades
 			(71, holder_commitment_point_previous_revoked, option), // Added in 0.3
 			(73, holder_commitment_point_last_revoked, option), // Added in 0.3
 			(75, inbound_committed_update_adds, optional_vec),
@@ -16028,6 +16033,7 @@
 		let mut malformed_htlcs: Option<Vec<(u64, u16, [u8; 32])>> = None;
 		let mut monitor_pending_update_adds: Option<Vec<msgs::UpdateAddHTLC>> = None;

+		let mut _has_0reserve: Option<()> = None;
 		let mut holder_commitment_point_previous_revoked_opt: Option<PublicKey> = None;
 		let mut holder_commitment_point_last_revoked_opt: Option<PublicKey> = None;
 		let mut holder_commitment_point_current_opt: Option<PublicKey> = None;
@@ -16098,6 +16104,7 @@
 			// 65 quiescent_action: Added in 0.2; removed in 0.3
 			(67, pending_outbound_held_htlc_flags_opt, optional_vec), // Added in 0.2
 			(69, holding_cell_held_htlc_flags_opt, optional_vec), // Added in 0.2
+			(70, _has_0reserve, option), // Added in 0.3 to prevent downgrades
 			(71, holder_commitment_point_previous_revoked_opt, option), // Added in 0.3
 			(73, holder_commitment_point_last_revoked_opt, option), // Added in 0.3
 			(75, inbound_committed_update_adds_opt, optional_vec),
diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index 432070129..184bc4052 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -3555,7 +3555,7 @@
 	///
 	/// Note that there is no guarantee that the counterparty accepts such a channel themselves.
 	ZeroReserve,
-	/// Sets combination of [`TrustedChannelFeatures::ZeroConf`] and [`TrustedChannelFeatures::ZeroReserve`]
+	/// Sets the combination of [`TrustedChannelFeatures::ZeroConf`] and [`TrustedChannelFeatures::ZeroReserve`]
 	ZeroConfZeroReserve,
 }

ldk-reviews-bot · 2026-03-26T01:42:01Z

🔔 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

Happy to land this to get the API changes in now, but a few small tweaks we'll want to change/address before release.

TheBlueMatt · 2026-03-26T01:04:50Z

lightning/src/sign/tx_builder.rs

-	value_to_remote_after_htlcs: u64, channel_type: &ChannelTypeFeatures,
+fn saturating_sub_from_funder(
+	is_outbound_from_holder: bool, value_to_holder: u64, value_to_counterparty: u64,
+	subtrahend: u64,


I've never heard of a subtrahend 😂

haha yes I looked it up do you have a suggestion ? value_to_subtract comes to mind :)

TheBlueMatt · 2026-03-26T11:15:59Z

lightning/src/ln/msgs.rs

 			(0, self.common_fields.shutdown_scriptpubkey.as_ref().map(|s| WithoutLength(s)), option), // Don't encode length twice.
 			(1, self.common_fields.channel_type, option),
 			(2, self.require_confirmed_inputs, option),
+			(3, self.disable_channel_reserve, option),


I assume this needs a bLIP update? Given this is for a bLIP and not a BOLT (IIRC?) should we not be using a +1000 or whatever range?

This is a BOLT proposal. Using the staging bit makes sense to me yes.

TheBlueMatt · 2026-03-26T11:20:18Z

lightning/src/ln/channel.rs

 		let is_manual_broadcast = Some(self.context.is_manual_broadcast);

+		let has_0reserve =
+			(self.funding.counterparty_selected_channel_reserve_satoshis.is_some_and(|r| r == 0)


We've always supported counterparty-selected reserve of 0, so I don't think we want to break downgrade for that?

carlaKC

lgtm!

tankyleo · 2026-03-26T14:36:24Z

Thank you for the review ! Happy to hold off on the merge and get this PR just right :)

We prevent downgrades from 0.3 only in the case where the holder-selected reserve is 0, as we've had support for counterparty selected 0-reserves in prior releases. 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>

carlaKC

RIP subtrahend, word of the week ✊

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

View reviewed changes

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

View reviewed changes

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

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 2 times, most recently from 91565f1 to 2967e6f Compare March 25, 2026 23:23

TheBlueMatt previously approved these changes Mar 26, 2026

View reviewed changes

carlaKC previously approved these changes Mar 26, 2026

View reviewed changes

tankyleo and others added 10 commits March 26, 2026 15:15

Add 0-reserve to accept_inbound_channel_from_trusted_peer

954bf2d

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

Add ChannelManager::create_channel_to_trusted_peer_0reserve

ef7a0d1

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

Shakedown zero reserve channels

d6fc690

Update chanmon_consistency to include 0FC and 0-reserve channels

62c58b8

Co-Authored-By: HAL 9000

Format ChannelManager::create_channel_internal and...

2e86554

`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

670e5f8

Reduce line count and indentation

tankyleo dismissed stale reviews from carlaKC and TheBlueMatt via 32a67f8 March 26, 2026 15:32

tankyleo force-pushed the 2026-02-zero-reserve branch from 2967e6f to 32a67f8 Compare March 26, 2026 15:32

tankyleo requested review from TheBlueMatt and carlaKC March 26, 2026 16:28

TheBlueMatt approved these changes Mar 26, 2026

View reviewed changes

carlaKC approved these changes Mar 26, 2026

View reviewed changes

TheBlueMatt merged commit 688544d into lightningdevkit:main Mar 26, 2026
24 of 44 checks passed

github-project-automation bot moved this from Goal: Merge to Done in Weekly Goals Mar 26, 2026

tankyleo deleted the 2026-02-zero-reserve branch March 26, 2026 17:52

Conversation

tankyleo commented Feb 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

ldk-reviews-bot commented Feb 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

codecov bot commented Feb 26, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codecov Report

Uh oh!

TheBlueMatt left a comment

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

TheBlueMatt commented Mar 4, 2026

Uh oh!

tankyleo commented Mar 5, 2026

Uh oh!

TheBlueMatt commented Mar 5, 2026

Uh oh!

tankyleo commented Mar 5, 2026

Uh oh!

tankyleo commented Mar 5, 2026

Uh oh!

ldk-reviews-bot commented Mar 5, 2026

Uh oh!

ldk-reviews-bot commented Mar 7, 2026

Uh oh!

ldk-reviews-bot commented Mar 9, 2026

Uh oh!

ldk-reviews-bot commented Mar 9, 2026

Uh oh!

ldk-reviews-bot commented Mar 11, 2026

Uh oh!

carlaKC left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

TheBlueMatt commented Mar 25, 2026

Uh oh!

tankyleo commented Mar 26, 2026

Uh oh!

ldk-reviews-bot commented Mar 26, 2026

Uh oh!

TheBlueMatt left a comment

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

carlaKC left a comment

Choose a reason for hiding this comment

Uh oh!

tankyleo commented Mar 26, 2026

Uh oh!

carlaKC left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

tankyleo commented Feb 19, 2026 •

edited

Loading

ldk-reviews-bot commented Feb 19, 2026 •

edited

Loading

codecov bot commented Feb 26, 2026 •

edited

Loading