commit 99dfa43640d8312375fd9836d0c0b00af021fffd
parent d39df6fe47e0d793b85471ad1baedc630bc6cc3e
Author: Jared Tobin <jared@jtobin.io>
Date: Sat, 10 Oct 2026 13:35:27 +0400
lib, test: don't create dust outputs in sweeps
The sweep builders only checked that the fee was below the input
value, so a small output could leave e.g. a 264-sat P2WPKH output,
which is non-standard and won't relay. They now also return Nothing
when the output would be below the dust threshold of the destination
script, computed as Bitcoin Core does at the default dust relay
feerate (546 for P2PKH, 540 for P2SH, 294 for P2WPKH, 330 for P2WSH
and P2TR). Signatures are unchanged.
Affects spend_to_local (and its alias spend_htlc_output),
spend_remote_htlc_timeout, spend_remote_htlc_preimage,
spend_revoked_to_local, spend_revoked_htlc and
spend_revoked_htlc_output.
Diffstat:
2 files changed, 144 insertions(+), 53 deletions(-)
diff --git a/lib/Lightning/Protocol/BOLT5/Spend.hs b/lib/Lightning/Protocol/BOLT5/Spend.hs
@@ -39,7 +39,7 @@ module Lightning.Protocol.BOLT5.Spend (
import Bitcoin.Prim.Tx (TxOut(..))
import Bitcoin.Prim.Tx.Sighash (SighashType(..))
import Data.List.NonEmpty (NonEmpty(..))
-import Data.Word (Word32)
+import Data.Word (Word32, Word64)
import qualified Data.ByteString as BS
import Lightning.Protocol.BOLT3 hiding
(txout_value, txout_script)
@@ -53,7 +53,8 @@ import Lightning.Protocol.BOLT5.Types
-- broadcasting. The caller signs with the local delayed privkey
-- and uses 'to_local_witness_spend' from bolt3.
--
--- Returns 'Nothing' if the fee would exceed the output value.
+-- Returns 'Nothing' if the fee would exceed the output value,
+-- or leave an output below the destination's dust threshold.
--
-- The input nSequence is set to the to_self_delay value.
spend_to_local
@@ -75,12 +76,10 @@ spend_to_local !op !value !revpk !delay !delayedpk
!weight = to_local_penalty_input_weight
+ penalty_tx_base_weight
!fee = spending_fee feerate weight
- in if unSatoshi fee >= unSatoshi value
- then Nothing
- else
- let !outputValue =
- Satoshi (unSatoshi value - unSatoshi fee)
- !tx = mk_spending_tx op
+ in case sweep_value value fee destScript of
+ Nothing -> Nothing
+ Just !outputValue ->
+ let !tx = mk_spending_tx op
(fromIntegral (unToSelfDelay delay))
destScript outputValue 0
in Just (SpendingTx tx witnessScript value
@@ -156,7 +155,8 @@ spend_htlc_success !ctx !keys =
-- same to_local script. The caller signs with the local
-- delayed privkey and uses 'htlc_output_witness_spend'.
--
--- Returns 'Nothing' if the fee would exceed the output value.
+-- Returns 'Nothing' if the fee would exceed the output value,
+-- or leave an output below the destination's dust threshold.
spend_htlc_output
:: OutPoint
-- ^ Outpoint of the second-stage output.
@@ -179,7 +179,8 @@ spend_htlc_output = spend_to_local
-- On the remote commitment, their received HTLCs (our offered)
-- have timed out and we can sweep them directly.
--
--- Returns 'Nothing' if the fee would exceed the output value.
+-- Returns 'Nothing' if the fee would exceed the output value,
+-- or leave an output below the destination's dust threshold.
spend_remote_htlc_timeout
:: OutPoint
-- ^ Outpoint of the HTLC output.
@@ -206,12 +207,10 @@ spend_remote_htlc_timeout !op !value !htlc !keys
!weight = accepted_htlc_penalty_input_weight
+ penalty_tx_base_weight
!fee = spending_fee feerate weight
- in if unSatoshi fee >= unSatoshi value
- then Nothing
- else
- let !outputValue =
- Satoshi (unSatoshi value - unSatoshi fee)
- !locktime =
+ in case sweep_value value fee destScript of
+ Nothing -> Nothing
+ Just !outputValue ->
+ let !locktime =
unCltvExpiry (htlc_cltv_expiry htlc)
!seqNo =
if has_anchors features then 1 else 0
@@ -226,7 +225,8 @@ spend_remote_htlc_timeout !op !value !htlc !keys
-- On the remote commitment, their offered HTLCs (our received)
-- can be claimed with the payment preimage.
--
--- Returns 'Nothing' if the fee would exceed the output value.
+-- Returns 'Nothing' if the fee would exceed the output value,
+-- or leave an output below the destination's dust threshold.
spend_remote_htlc_preimage
:: OutPoint
-- ^ Outpoint of the HTLC output.
@@ -252,12 +252,10 @@ spend_remote_htlc_preimage !op !value !htlc !keys
!weight = offered_htlc_penalty_input_weight
+ penalty_tx_base_weight
!fee = spending_fee feerate weight
- in if unSatoshi fee >= unSatoshi value
- then Nothing
- else
- let !outputValue =
- Satoshi (unSatoshi value - unSatoshi fee)
- !seqNo =
+ in case sweep_value value fee destScript of
+ Nothing -> Nothing
+ Just !outputValue ->
+ let !seqNo =
if has_anchors features then 1 else 0
!tx = mk_spending_tx op seqNo destScript
outputValue 0
@@ -271,7 +269,8 @@ spend_remote_htlc_preimage !op !value !htlc !keys
-- The caller signs with the revocation privkey and uses
-- 'to_local_witness_revoke' from bolt3.
--
--- Returns 'Nothing' if the fee would exceed the output value.
+-- Returns 'Nothing' if the fee would exceed the output value,
+-- or leave an output below the destination's dust threshold.
spend_revoked_to_local
:: OutPoint
-- ^ Outpoint of the to_local output.
@@ -291,12 +290,10 @@ spend_revoked_to_local !op !value !revpk !delay
!weight = to_local_penalty_input_weight
+ penalty_tx_base_weight
!fee = spending_fee feerate weight
- in if unSatoshi fee >= unSatoshi value
- then Nothing
- else
- let !outputValue =
- Satoshi (unSatoshi value - unSatoshi fee)
- !tx = mk_spending_tx op 0xFFFFFFFF destScript
+ in case sweep_value value fee destScript of
+ Nothing -> Nothing
+ Just !outputValue ->
+ let !tx = mk_spending_tx op 0xFFFFFFFF destScript
outputValue 0
in Just (SpendingTx tx witnessScript value
SIGHASH_ALL)
@@ -308,8 +305,8 @@ spend_revoked_to_local !op !value !revpk !delay
-- 'received_htlc_witness_revoke' from bolt3, depending on
-- the output type.
--
--- Returns 'Nothing' if the fee would exceed the output
--- value.
+-- Returns 'Nothing' if the fee would exceed the output value,
+-- or leave an output below the destination's dust threshold.
spend_revoked_htlc
:: OutPoint
-- ^ Outpoint of the HTLC output.
@@ -338,13 +335,10 @@ spend_revoked_htlc !op !value !htype !revpk !keys
!weight = offered_htlc_penalty_input_weight
+ penalty_tx_base_weight
!fee = spending_fee feerate weight
- in if unSatoshi fee >= unSatoshi value
- then Nothing
- else
- let !outputValue =
- Satoshi
- (unSatoshi value - unSatoshi fee)
- !tx = mk_spending_tx op 0xFFFFFFFF
+ in case sweep_value value fee destScript of
+ Nothing -> Nothing
+ Just !outputValue ->
+ let !tx = mk_spending_tx op 0xFFFFFFFF
destScript outputValue 0
in Just (SpendingTx tx witnessScript value
SIGHASH_ALL)
@@ -359,13 +353,10 @@ spend_revoked_htlc !op !value !htype !revpk !keys
!weight = accepted_htlc_penalty_input_weight
+ penalty_tx_base_weight
!fee = spending_fee feerate weight
- in if unSatoshi fee >= unSatoshi value
- then Nothing
- else
- let !outputValue =
- Satoshi
- (unSatoshi value - unSatoshi fee)
- !tx = mk_spending_tx op 0xFFFFFFFF
+ in case sweep_value value fee destScript of
+ Nothing -> Nothing
+ Just !outputValue ->
+ let !tx = mk_spending_tx op 0xFFFFFFFF
destScript outputValue 0
in Just (SpendingTx tx witnessScript value
SIGHASH_ALL)
@@ -377,7 +368,8 @@ spend_revoked_htlc !op !value !htype !revpk !keys
-- to_local script. The caller signs with the revocation privkey
-- and uses 'htlc_output_witness_revoke'.
--
--- Returns 'Nothing' if the fee would exceed the output value.
+-- Returns 'Nothing' if the fee would exceed the output value,
+-- or leave an output below the destination's dust threshold.
spend_revoked_htlc_output
:: OutPoint
-- ^ Outpoint of the second-stage output.
@@ -397,12 +389,10 @@ spend_revoked_htlc_output !op !value !revpk !delay
!weight = to_local_penalty_input_weight
+ penalty_tx_base_weight
!fee = spending_fee feerate weight
- in if unSatoshi fee >= unSatoshi value
- then Nothing
- else
- let !outputValue =
- Satoshi (unSatoshi value - unSatoshi fee)
- !tx = mk_spending_tx op 0xFFFFFFFF destScript
+ in case sweep_value value fee destScript of
+ Nothing -> Nothing
+ Just !outputValue ->
+ let !tx = mk_spending_tx op 0xFFFFFFFF destScript
outputValue 0
in Just (SpendingTx tx witnessScript value
SIGHASH_ALL)
@@ -447,6 +437,44 @@ spend_anchor_anyone !op !value !fundpk !destScript =
-- internal helpers ---------------------------------------------------
+-- | Value of a sweep's single output: the input value less the fee.
+--
+-- 'Nothing' if the fee meets or exceeds the input value, or if the
+-- output would be below the dust threshold of its script.
+sweep_value :: Satoshi -> Satoshi -> Script -> Maybe Satoshi
+sweep_value !value !fee !destScript
+ | unSatoshi fee >= unSatoshi value = Nothing
+ | unSatoshi out < unSatoshi (dust_threshold destScript) = Nothing
+ | otherwise = Just out
+ where
+ out = Satoshi (unSatoshi value - unSatoshi fee)
+
+-- | Dust threshold for an output paying to a script.
+--
+-- As Bitcoin Core computes it for spendable outputs at the default
+-- dust relay feerate of 3 sat/vbyte: three times the size of the
+-- output plus that of an input spending it (67 vbytes for a
+-- witness program, 148 otherwise). This gives 546 sats for P2PKH,
+-- 540 for P2SH, 294 for P2WPKH, and 330 for P2WSH and P2TR.
+dust_threshold :: Script -> Satoshi
+dust_threshold (Script s) =
+ let !len = fromIntegral (BS.length s) :: Word64
+ !lenSize = if len < 0xfd then 1
+ else if len <= 0xffff then 3
+ else 5
+ !spendSize = if is_witness_program s then 67 else 148
+ in Satoshi (3 * (8 + lenSize + len + spendSize))
+
+-- | Whether a script is a segwit witness program: a version opcode
+-- (OP_0 or OP_1 to OP_16) followed by a single 2-40 byte push.
+is_witness_program :: BS.ByteString -> Bool
+is_witness_program s = case BS.unpack (BS.take 2 s) of
+ [version, pushLen] ->
+ (version == 0x00 || (version >= 0x51 && version <= 0x60))
+ && pushLen >= 2 && pushLen <= 40
+ && fromIntegral pushLen + 2 == BS.length s
+ _ -> False
+
-- | Build a simple single-input single-output spending tx.
mk_spending_tx
:: OutPoint -- ^ Input outpoint
diff --git a/test/Main.hs b/test/Main.hs
@@ -981,6 +981,33 @@ spend_tests = testGroup "Spend" [
dummyDestScript dummyFeerate
@?= Nothing
+ , testCase "spend_to_local dust threshold by script" $ do
+ let fee = unSatoshi $ B5.spending_fee dummyFeerate
+ (B5.to_local_penalty_input_weight
+ + B5.penalty_tx_base_weight)
+ sweep dest v = B5.spend_to_local
+ dummyOutPoint (Satoshi v)
+ dummyRevocationPubkey dummyDelay
+ dummyLocalDelayedPubkey dest dummyFeerate
+ outValue = fmap
+ (txout_value . head' . tx_outputs . B5.stx_tx)
+ check (name, dest, dust) = do
+ assertEqual (name <> " at threshold") (Just dust)
+ (outValue (sweep dest (fee + dust)))
+ assertEqual (name <> " below threshold") Nothing
+ (sweep dest (fee + dust - 1))
+ mapM_ check
+ [ ("p2pkh", Script $ BS.pack ([0x76, 0xa9, 0x14]
+ <> replicate 20 0xCC <> [0x88, 0xac]), 546)
+ , ("p2sh", Script $ BS.pack ([0xa9, 0x14]
+ <> replicate 20 0xCC <> [0x87]), 540)
+ , ("p2wpkh", dummyDestScript, 294)
+ , ("p2wsh", Script $ BS.pack ([0x00, 0x20]
+ <> replicate 32 0xCC), 330)
+ , ("p2tr", Script $ BS.pack ([0x51, 0x20]
+ <> replicate 32 0xCC), 330)
+ ]
+
]
-- htlc spend tests ---------------------------------------------------
@@ -1241,6 +1268,42 @@ property_tests = testGroup "Properties" [
Satoshi (unSatoshi value
- unSatoshi expectedFee)
+ , testProperty "sweeps never create dust outputs" $
+ forAll (choose (0, 3000 :: Int)) $ \v ->
+ let value = Satoshi (fromIntegral v)
+ sweeps =
+ [ B5.spend_to_local dummyOutPoint value
+ dummyRevocationPubkey dummyDelay
+ dummyLocalDelayedPubkey dummyDestScript
+ dummyFeerate
+ , B5.spend_remote_htlc_timeout dummyOutPoint value
+ dummyHTLC dummyKeys dummyFeatures
+ dummyDestScript dummyFeerate
+ , B5.spend_remote_htlc_preimage dummyOutPoint value
+ dummyReceivedHTLC dummyKeys dummyFeatures
+ dummyDestScript dummyFeerate
+ , B5.spend_revoked_to_local dummyOutPoint value
+ dummyRevocationPubkey dummyDelay
+ dummyLocalDelayedPubkey dummyDestScript
+ dummyFeerate
+ , B5.spend_revoked_htlc dummyOutPoint value
+ (B5.HTLCOfferedOutput (CltvExpiry 500000))
+ dummyRevocationPubkey dummyKeys dummyFeatures
+ dummyPaymentHash dummyDestScript dummyFeerate
+ , B5.spend_revoked_htlc dummyOutPoint value
+ (B5.HTLCReceivedOutput (CltvExpiry 500000))
+ dummyRevocationPubkey dummyKeys dummyFeatures
+ dummyPaymentHash dummyDestScript dummyFeerate
+ , B5.spend_revoked_htlc_output dummyOutPoint value
+ dummyRevocationPubkey dummyDelay
+ dummyLocalDelayedPubkey dummyDestScript
+ dummyFeerate
+ ]
+ notDust = maybe True $ \stx ->
+ txout_value (head' (tx_outputs (B5.stx_tx stx)))
+ >= 294
+ in all notDust sweeps
+
, testProperty "htlc_timed_out monotonic" $
\(NonNegative h1) (NonNegative h2) ->
let height1 = fromIntegral (h1 :: Int)
