staticaddr: exclude close-to-expiry depositis from SelectDeposits

staticaddr/loopin/manager.go

@@ -862,8 +862,25 @@ func (m *Manager) GetAllSwaps(ctx context.Context) ([]*StaticAddressLoopIn,
 // are needed to cover the amount requested without leaving a dust change. It
 // returns an error if the sum of deposits minus dust is less than the requested
 // amount.
-func SelectDeposits(targetAmount btcutil.Amount, deposits []*deposit.Deposit,
-	csvExpiry uint32, blockHeight uint32) ([]*deposit.Deposit, error) {
+func SelectDeposits(targetAmount btcutil.Amount,
+	unfilteredDeposits []*deposit.Deposit, csvExpiry uint32,
+	blockHeight uint32) ([]*deposit.Deposit, error) {
+
+	// Filter out deposits that are too close to expiry to be swapped.
+	var deposits []*deposit.Deposit
+	for _, d := range unfilteredDeposits {
+		if !IsSwappable(
+			uint32(d.ConfirmationHeight), blockHeight, csvExpiry,
+		) {
+
+			log.Debugf("Skipping deposit %s as it expires before "+
+				"the htlc", d.OutPoint.String())
+
+			continue
+		}
+
+		deposits = append(deposits, d)
+	}
 
 	// Sort the deposits by amount in descending order, then by
 	// blocks-until-expiry in ascending order.
@@ -901,6 +918,25 @@ func SelectDeposits(targetAmount btcutil.Amount, deposits []*deposit.Deposit,
 		selectedAmount, targetAmount)
 }
 
+// IsSwappable checks if a deposit is swappable. It returns true if the deposit
+// is not expired and the htlc is not too close to expiry.
+func IsSwappable(confirmationHeight, blockHeight, csvExpiry uint32) bool {
+	// The deposit expiry height is the confirmation height plus the csv
+	// expiry.
+	depositExpiryHeight := confirmationHeight + csvExpiry
+
+	// The htlc expiry height is the current height plus the htlc
+	// cltv delta.
+	htlcExpiryHeight := blockHeight + DefaultLoopInOnChainCltvDelta
+
+	// Ensure that the deposit doesn't expire before the htlc.
+	if depositExpiryHeight < htlcExpiryHeight+DepositHtlcDelta {
+		return false
+	}
+
+	return true
+}
+
 // DeduceSwapAmount calculates the swap amount based on the selected amount and
 // the total deposit amount. It checks if the selected amount leaves a dust
 // change output or exceeds the total deposits value. Note that if the selected

staticaddr/loopin/manager_test.go

@@ -29,16 +29,16 @@ type testCase struct {
 func TestSelectDeposits(t *testing.T) {
 	d1, d2, d3, d4 := &deposit.Deposit{
 		Value:              1_000_000,
-		ConfirmationHeight: 1000,
+		ConfirmationHeight: 5_000,
 	}, &deposit.Deposit{
 		Value:              2_000_000,
-		ConfirmationHeight: 2000,
+		ConfirmationHeight: 5_001,
 	}, &deposit.Deposit{
 		Value:              3_000_000,
-		ConfirmationHeight: 3000,
+		ConfirmationHeight: 5_002,
 	}, &deposit.Deposit{
 		Value:              3_000_000,
-		ConfirmationHeight: 3001,
+		ConfirmationHeight: 5_003,
 	}
 	d1.Hash = chainhash.Hash{1}
 	d1.Index = 0
@@ -121,6 +121,43 @@ func TestSelectDeposits(t *testing.T) {
 			expected:    []*deposit.Deposit{d3},
 			expectedErr: "",
 		},
+		{
+			name: "prefilter filters deposits close to expiry",
+			deposits: func() []*deposit.Deposit {
+				// dClose expires before
+				// htlcExpiry+DepositHtlcDelta and must be
+				// filtered out. dOK expires exactly at the
+				// threshold and must be eligible.
+				dClose := &deposit.Deposit{
+					Value:              3_000_000,
+					ConfirmationHeight: 3000,
+				}
+				dClose.Hash = chainhash.Hash{5}
+				dClose.Index = 0
+				dOK := &deposit.Deposit{
+					Value:              2_000_000,
+					ConfirmationHeight: 3050,
+				}
+				dOK.Hash = chainhash.Hash{6}
+				dOK.Index = 0
+				return []*deposit.Deposit{dClose, dOK}
+			}(),
+			targetValue: 1_000_000,
+			csvExpiry:   1000,
+			blockHeight: 3000,
+			expected: func() []*deposit.Deposit {
+				// Only dOK should be considered.
+				// dClose is filtered.
+				dOK := &deposit.Deposit{
+					Value:              2_000_000,
+					ConfirmationHeight: 3050,
+				}
+				dOK.Hash = chainhash.Hash{6}
+				dOK.Index = 0
+				return []*deposit.Deposit{dOK}
+			}(),
+			expectedErr: "",
+		},
 	}
 
 	for _, tc := range testCases {