sweepbatcher: batch change outputs #976

hieblmi · 2025-07-09T13:39:12Z

The batcher receives the ability to sign and publish batches with change outputs.

Change is created when the clients choose fractional swap amounts from the input values to be swapped. Usually the change is sent back to client's addresses.

If change is provided to ensurePresigned, presign and publishPresigned, it is expected to be placed within the passed []sweep.
Each sweep optionally holds a pointer to a change output. Only the first sweep in a sweep group called primary deposit will hold a pointer to the respective swap's change output.

unit test coverage for change outputs

sweepbatcher/presigned.go

hieblmi · 2025-07-10T07:29:21Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces the ability to handle change outputs in sweep batches, which is a significant feature. I've found a critical bug in fee calculation within CheckSignedTx that needs to be addressed. I've also pointed out a minor code simplification and a gap in test coverage for the new logic.

sweepbatcher/presigned.go

sweepbatcher/sweep_batch_test.go

sweepbatcher/sweep_batch.go

gemini-code-assist

Code Review

The pull request introduces functionality to handle change outputs in sweep batcher, specifically for presigned transactions. The changes involve modifying the sweep struct, updating transaction construction to include change outputs, and adding a function to retrieve and de-duplicate change outputs. The code also includes unit tests for the new functionality. The changes look good overall, but there are a few areas that could be improved for robustness and clarity.

sweepbatcher/sweep_batch.go

sweepbatcher/sweep_batcher.go

sweepbatcher/presigned.go

sweepbatcher/sweep_batch_test.go

starius · 2025-07-14T18:43:42Z

Multiple sweeps holding the same change reference will produce a single change output in the batch transaction.

This seems to be outdated.

sweepbatcher/greedy_batch_selection.go

sweepbatcher/presigned.go

sweepbatcher/presigned_test.go

sweepbatcher/sweep_batch.go

sweepbatcher/sweep_batch_test.go

sweepbatcher/sweep_batch.go

sweepbatcher/sweep_batcher.go

sweepbatcher/sweep_batcher_presigned_test.go

sweepbatcher/sweep_batch.go

sweepbatcher/sweep_batch_test.go

sweepbatcher/sweep_batcher_presigned_test.go

starius

LGTM! 🥇

Added a couple of proposals.

sweepbatcher/sweep_batch.go

starius

LGTM! 💾

I sent PR hieblmi#1 with some changes that I propose, mostly for tests, but also most importantly to use minRelayFeeRate in presign and ensurePresigned as start value for presigning and when passing to SignTx. Please review the changes. You can squash the changes into proper commits or keep them as a separate commit.

sweepbatcher/presigned.go

starius

LGTM! 🍇
I think, it is ready. Great work!

sputn1ck

Left some comments re multiple change outputs

sputn1ck · 2025-07-24T07:46:45Z

sweepbatcher/greedy_batch_selection.go

@@ -210,6 +210,13 @@ func estimateBatchWeight(batch *batch) (feeDetails, error) {
 			err)
 	}

+	// Add change output weights.
+	for _, s := range batch.sweeps {
+		if s.change != nil {


Would it make sense to add a check for if a change pkscript repeats that we add to the other outputs value instead of adding a new output with the same pkscript? Is that something that can happen (e.g. on static loop in if change is the static addr change would be the same?)

Very good suggestion!

sputn1ck · 2025-07-24T07:52:29Z

sweepbatcher/sweep_batch.go

@@ -1255,6 +1260,13 @@ func constructUnsignedTx(sweeps []sweep, address btcutil.Address,
 		LockTime: uint32(currentHeight),
 	}

+	var changeOutputs []*wire.TxOut
+	for _, sweep := range sweeps {


I think here a similiar comment to my other comment, what happens if the pkscript is the same between multiple sweeps?

It totally makes sense to consolidate values of change outputs with identical pkscript. I've changed the workings to
do so here and in the greedy batch selection. I will also add unit tests for this.

sputn1ck · 2025-07-24T07:53:37Z

sweepbatcher/sweep_batch_test.go

+		},
+
+		{
+			name: "all sweeps different change outputs",


I think a testcase "all sweeps same change output" makes sense and the result should be a single change output right?

added "all sweeps identical change pkscripts"

hieblmi · 2025-07-28T10:21:20Z

I've added commit sweepbatcher: consolidate identical change pkscripts to tally up the values of identical change pkscripts.

sputn1ck

LGTM! thanks for the additional change consolidation 🙏

starius

LGTM! 🌴
The code of last commit is great. I propose to add few tests to validate it.

starius · 2025-07-29T04:48:28Z

sweepbatcher/greedy_batch_selection.go

+		}
+
+		weight.AddOutput(s.change.PkScript)
+		changeOutputs[pkScriptString] = struct{}{}


👍

Please add a test case to TestEstimateBatchWeight.

Added "two sweeps regular batch with identical change".

starius · 2025-07-29T04:50:03Z

sweepbatcher/sweep_batch_test.go

@@ -395,6 +395,57 @@ func TestConstructUnsignedTx(t *testing.T) {
 			wantFee:          1248,
 		},

+		{
+			name: "all sweeps identical change pkscripts",


s/all sweeps identical change pkscripts/identical change pkscripts/

starius · 2025-07-29T04:51:21Z

sweepbatcher/sweep_batch_test.go

+			wantWeight:       1076,
+			wantFeeForWeight: 1076,
+			wantFee:          1076,
+		},


Please add another test case with equal change pkscripts, but different satoshi values.

added "identical change pkscripts different values".

starius · 2025-07-29T05:04:35Z

sweepbatcher/sweep_batcher_presigned_test.go

+
+	// Mine a blocks to trigger republishing.
+	require.NoError(t, lnd.NotifyHeight(601))
+}


I propose to add a unit test with two swaps sharing the same change output pkscript.

added testPresigned_presigned_group_with_identical_change_pkscript.

This commit adds an optional change output to the sweep struct.

Presigning sweeps takes change outputs into account. Each primary deposit id of a sweep group points to an optional change output. sweepbatcher.presign scans all passed sweeps for change outputs and passes them to constructUnsignedTx. Optional change of a swap is encoded in its sweeps as a pointer to the same change output. This change is taken into account when constructing the unsigned batch transaction when it comes to tx weight and outputs.

if constructUnsignedTx constructs a batch transaction that is below the minimum relay fee, an error is returned.

- ensurePresigned: use passed minRelayFeeRate instead of chainfee.FeePerKwFloor - presign: use minRelayFeeRate for start and minRelayFee - presign: make sure minRelayFeeRate is set - add tests for presign to test this new behavior; make sure the number of transactions is lower if minRelayFeeRate is higher - update error message in constructUnsignedTx: use <, not <= (more accurate) - use utils.DustLimitForPkScript instead of lnwallet.DustLimitForSize in tests - in tests adjust amounts to edge values, add controls

hieblmi · 2025-07-29T13:02:06Z

@starius I had to sort the change outputs in constructUnsignedTx after they've been deduplicated in a map in order to make the order deterministic for the tests, see 29161bb#diff-355a81f8dab9aa0faaaa04740ca44f425ff0dd18dea7240e6a28e1697f222dd1R1441

starius

LGTM! 🥇
nit: I propose to use bip-0069 for outputs sorting.

starius · 2025-07-29T14:50:27Z

sweepbatcher/sweep_batch.go

+	}
+
+	// Sort the keys
+	sort.Strings(keys)


Let's use BIP-0069 for outputs sorting (change only, so main output is 0).
We have it implemented for inputs in utils/get_prevout_info.go

I've added a bip69 compatible utils method to sort tx outputs. I've added it in a separate commit 245a666.

batch separate change outputs with identical pkscripts are tallied up and consolidated to a single change output.

starius

LGTM! 🏆

hieblmi self-assigned this Jul 9, 2025

hieblmi marked this pull request as draft July 9, 2025 13:39

hieblmi requested a review from starius July 9, 2025 13:39

hieblmi force-pushed the batcher-change-outputs branch from 19b13cd to c7091de Compare July 9, 2025 13:46

hieblmi commented Jul 9, 2025

View reviewed changes

sweepbatcher/presigned.go Outdated Show resolved Hide resolved

gemini-code-assist bot reviewed Jul 10, 2025

View reviewed changes

sweepbatcher/presigned.go Show resolved Hide resolved

sweepbatcher/sweep_batch_test.go Outdated Show resolved Hide resolved

sweepbatcher/sweep_batch.go Outdated Show resolved Hide resolved

hieblmi force-pushed the batcher-change-outputs branch 4 times, most recently from c20504a to 0567051 Compare July 10, 2025 09:31

gemini-code-assist bot reviewed Jul 10, 2025

View reviewed changes

sweepbatcher/sweep_batch.go Outdated Show resolved Hide resolved

sweepbatcher/sweep_batcher.go Show resolved Hide resolved

sweepbatcher/presigned.go Outdated Show resolved Hide resolved

sweepbatcher/sweep_batch_test.go Outdated Show resolved Hide resolved

hieblmi force-pushed the batcher-change-outputs branch from 0567051 to 950fd4e Compare July 10, 2025 13:33

hieblmi marked this pull request as ready for review July 10, 2025 13:41

hieblmi force-pushed the batcher-change-outputs branch 4 times, most recently from 576270f to 35f5810 Compare July 14, 2025 07:04

starius reviewed Jul 14, 2025

View reviewed changes

starius reviewed Jul 15, 2025

View reviewed changes

hieblmi force-pushed the batcher-change-outputs branch from 35f5810 to 4c58f85 Compare July 15, 2025 18:34

starius reviewed Jul 16, 2025

View reviewed changes

hieblmi force-pushed the batcher-change-outputs branch 4 times, most recently from fe4ec68 to c0418e3 Compare July 16, 2025 10:14

hieblmi requested a review from starius July 16, 2025 11:54

starius approved these changes Jul 16, 2025

View reviewed changes

sweepbatcher/sweep_batch.go Outdated Show resolved Hide resolved

sweepbatcher/sweep_batch.go Outdated Show resolved Hide resolved

starius reviewed Jul 17, 2025

View reviewed changes

sweepbatcher/sweep_batch.go Outdated Show resolved Hide resolved

hieblmi force-pushed the batcher-change-outputs branch from a728c80 to 1b31b88 Compare July 22, 2025 12:31

hieblmi requested a review from starius July 22, 2025 12:41

starius approved these changes Jul 23, 2025

View reviewed changes

sweepbatcher/presigned.go Show resolved Hide resolved

starius approved these changes Jul 24, 2025

View reviewed changes

sputn1ck reviewed Jul 24, 2025

View reviewed changes

hieblmi requested review from starius and sputn1ck July 28, 2025 10:20

hieblmi force-pushed the batcher-change-outputs branch from d2eee3f to fd4ee81 Compare July 28, 2025 10:32

sputn1ck approved these changes Jul 28, 2025

View reviewed changes

starius approved these changes Jul 29, 2025

View reviewed changes

hieblmi force-pushed the batcher-change-outputs branch 2 times, most recently from 16ebc11 to c7b0428 Compare July 29, 2025 10:14

hieblmi and others added 6 commits July 29, 2025 12:33

utils: function to get dust limit for a pkscript.

94168c3

sweepbatcher: optional change in sweep struct

f30db74

This commit adds an optional change output to the sweep struct.

sweepbatcher: consider optional change in greedy selection

8399a63

sweepbatcher: consider min relay fee when constructing batch tx

29348a9

if constructUnsignedTx constructs a batch transaction that is below the minimum relay fee, an error is returned.

hieblmi force-pushed the batcher-change-outputs branch 2 times, most recently from 6679a5d to 29161bb Compare July 29, 2025 12:43

hieblmi requested a review from starius July 29, 2025 12:59

starius approved these changes Jul 29, 2025

View reviewed changes

hieblmi added 2 commits July 29, 2025 18:03

utils: bip69 output sorting

245a666

sweepbatcher: consolidate identical change pkscripts

d00b78e

batch separate change outputs with identical pkscripts are tallied up and consolidated to a single change output.

hieblmi force-pushed the batcher-change-outputs branch from 29161bb to d00b78e Compare July 29, 2025 19:22

hieblmi requested a review from starius July 29, 2025 19:42

starius approved these changes Jul 29, 2025

View reviewed changes

hieblmi merged commit dd85a61 into lightninglabs:master Jul 29, 2025
4 checks passed

sweepbatcher: batch change outputs #976

sweepbatcher: batch change outputs #976

Uh oh!

Conversation

hieblmi commented Jul 9, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

hieblmi commented Jul 10, 2025

Uh oh!

gemini-code-assist bot left a comment

Choose a reason for hiding this comment

Code Review

Uh oh!

Uh oh!

Uh oh!

Uh oh!

gemini-code-assist bot left a comment

Choose a reason for hiding this comment

Code Review

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

starius commented Jul 14, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

starius left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

starius left a comment • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

starius left a comment

Choose a reason for hiding this comment

Uh oh!

sputn1ck 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

hieblmi commented Jul 9, 2025 •

edited

Loading

starius left a comment •

edited

Loading