Use shared input's txOut in shouldSignFirst
#724
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When deciding who must send `tx_signatures` first for a splice, we compute how much each node contributes to the splice transaction by adding the amount of the `tx_add_input` they have sent. For the shared input (current channel output), instead of using the `txOut` amount, we previously summed the balance of each node in the channel. The result is the same when there are no pending HTLCs, but when there are pending HTLCs, it underestimates the amount of this input. This is hard to reproduce (and test) because it requires both nodes to contribute to the splice while having large pending HTLCs that tilt the balance in favor of the wrong side.
t-bast
commented
Oct 24, 2024
| @@ -1147,7 +1147,7 @@ data class InteractiveTxSigningSession( | |||
| } | |||
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| fun shouldSignFirst(isInitiator: Boolean, channelParams: ChannelParams, tx: SharedTransaction): Boolean { | |||
| val sharedAmountIn = tx.sharedInput?.let { it.localAmount + it.remoteAmount } ?: 0.msat | |||
| val sharedAmountIn = tx.sharedInput?.txOut?.amount?.toMilliSatoshi() ?: 0.msat | |||
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Note that the definition of txOut in InteractiveTxInput.Shared includes the pending HTLCs.
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When deciding who must send
tx_signaturesfirst for a splice, we compute how much each node contributes to the splice transaction by adding the amount of thetx_add_inputthey have sent.For the shared input (current channel output), instead of using the
txOutamount, we previously summed the balance of each node in the channel. The result is the same when there are no pending HTLCs, but when there are pending HTLCs, it underestimates the amount of this input.This is hard to reproduce (and test) because it requires both nodes to contribute to the splice while having large pending HTLCs that tilt the balance in favor of the wrong side.