[bug]: sweep: AddWalletInputs modifies BudgetInputSet when error occurs

[morehouse]

BudgetInputSet.AddWalletInputs currently restores the original inputs if there aren't enough wallet inputs to meet the budget.

lnd/sweep/tx_input_set.go

Lines 297 to 327 in 9d358bc

	// 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 {
	input, err := createWalletTxInput(utxo)
	if err != nil {
	return err
	}
	pi := SweeperInput{
	Input: input,
	params: Params{
	DeadlineHeight: fn.Some(b.deadlineHeight),
	},
	}
	b.addInput(pi)
	// Return if we've reached the minimum output amount.
	if !b.NeedWalletInput() {
	return nil
	}
	}
	// The wallet doesn't have enough utxos to cover the budget. Revert the
	// input set to its original state.
	b.inputs = originalInputs
	return ErrNotEnoughInputs

But if createWalletTxInput returns an error, the original inputs are not restored.

Solution

Today createWalletTxInput shouldn't ever return an error, though it may in the future if a new witness type is added. We should unify all error paths in AddWalletInputs to ensure inputs are restored.