sweep: create sweep tx even the budget cannot be met

We now always create the sweeping tx even though the budget cannot be
covered so we don't miss the deadline. Note that the fee bump will fail
once the provided wallet input cannot cover the increase fees, which is
fine as these inputs will be marked as failed and be retried again in
the next block. When that happens, if there are new wallet UTXOs, a new
batch will be created to perform the fee bump.

sweep/tx_input_set.go

@@ -296,7 +296,9 @@ func (b *BudgetInputSet) NeedWalletInput() bool {
 		}
 
 		// Get the amount left after covering the input's own budget.
-		// This amount can then be lent to the above input.
+		// This amount can then be lent to the above input. For a wallet
+		// input, its `Budget` is set to zero, which means the whole
+		// input can be borrowed to cover the budget.
 		budget := inp.params.Budget
 		output := btcutil.Amount(inp.SignDesc().Output.Value)
 		budgetBorrowable += output - budget
@@ -350,7 +352,7 @@ func (b *BudgetInputSet) AddWalletInputs(wallet Wallet) error {
 
 	// Exit early if there are no wallet UTXOs.
 	if len(utxos) == 0 {
-		return ErrNotEnoughInputs
+		return fmt.Errorf("%w: empty wallet", ErrNotEnoughInputs)
 	}
 
 	// Sort the UTXOs by putting smaller values at the start of the slice
@@ -362,11 +364,6 @@ func (b *BudgetInputSet) AddWalletInputs(wallet Wallet) error {
 		return utxos[i].Value < utxos[j].Value
 	})
 
-	// Make a copy of the current inputs. If the wallet doesn't have enough
-	// utxos to cover the budget, we will revert the current set to its
-	// original state by removing the added wallet inputs.
-	originalInputs := b.copyInputs()
-
 	// Add wallet inputs to the set until the specified budget is covered.
 	for _, utxo := range utxos {
 		err := b.addWalletInput(utxo)
@@ -380,11 +377,10 @@ func (b *BudgetInputSet) AddWalletInputs(wallet Wallet) error {
 		}
 	}
 
-	// The wallet doesn't have enough utxos to cover the budget. Revert the
-	// input set to its original state.
-	b.inputs = originalInputs
+	log.Warn("Not enough wallet UTXOs to cover the budget, sweeping " +
+		"anyway...")
 
-	return ErrNotEnoughInputs
+	return nil
 }
 
 // Budget returns the total budget of the set.