tx: split 'compute fee' into two, with one that takes an input value

Transactions that we 'get' from bitciond don't have the input values
available on the transaction; for these cases we'll sum up the inputs
amounts using a different data source than the transaction's
`input_amounts`. So we need to expose it here.

bitcoin/tx.c

@@ -70,35 +70,46 @@ bool elements_tx_output_is_fee(const struct bitcoin_tx *tx, int outnum)
 	       tx->wtx->outputs[outnum].script_len == 0;
 }
 
-/**
- * Compute how much fee we are actually sending with this transaction.
- */
-static struct amount_sat bitcoin_tx_compute_fee(const struct bitcoin_tx *tx)
+struct amount_sat bitcoin_tx_compute_fee_w_inputs(const struct bitcoin_tx *tx,
+						  struct amount_sat input_val)
 {
-	struct amount_sat fee = AMOUNT_SAT(0), value;
 	struct amount_asset asset;
 	bool ok;
 
-	for (size_t i = 0; i < tal_count(tx->input_amounts); i++) {
-		value.satoshis = tx->input_amounts[i]->satoshis; /* Raw: fee computation */
-		ok = amount_sat_add(&fee, fee, value);
-		assert(ok);
-	}
-
 	for (size_t i = 0; i < tx->wtx->num_outputs; i++) {
 		asset = bitcoin_tx_output_get_amount(tx, i);
 		if (elements_tx_output_is_fee(tx, i) ||
 		    !amount_asset_is_main(&asset))
 			continue;
 
-		value = amount_asset_to_sat(&asset);
-		ok = amount_sat_sub(&fee, fee, value);
+		ok = amount_sat_sub(&input_val, input_val,
+				    amount_asset_to_sat(&asset));
 		assert(ok);
 	}
-	return fee;
+	return input_val;
 }
 
 /**
+ * Compute how much fee we are actually sending with this transaction.
+ * Note that using this with a transaction without the input_amounts
+ * initialized/populated is an error.
+ */
+struct amount_sat bitcoin_tx_compute_fee(const struct bitcoin_tx *tx)
+{
+	struct amount_sat input_total = AMOUNT_SAT(0);
+	bool ok;
+
+	for (size_t i = 0; i < tal_count(tx->input_amounts); i++) {
+		assert(tx->input_amounts[i]);
+		ok = amount_sat_add(&input_total, input_total,
+				    *tx->input_amounts[i]);
+		assert(ok);
+	}
+
+	return bitcoin_tx_compute_fee_w_inputs(tx, input_total);
+}
+
+/*
  * Add an explicit fee output if necessary.
  *
  * An explicit fee output is only necessary if we are using an elements

bitcoin/tx.h

@@ -177,4 +177,18 @@ void bitcoin_tx_finalize(struct bitcoin_tx *tx);
  */
 bool elements_tx_output_is_fee(const struct bitcoin_tx *tx, int outnum);
 
+/**
+ * Calculate the fees for this transaction
+ */
+struct amount_sat bitcoin_tx_compute_fee(const struct bitcoin_tx *tx);
+
+/*
+ * Calculate the fees for this transaction, given a pre-computed input balance.
+ *
+ * This is needed for cases where the input_amounts aren't properly initialized,
+ * typically due to being passed across the wire.
+ */
+struct amount_sat bitcoin_tx_compute_fee_w_inputs(const struct bitcoin_tx *tx,
+						  struct amount_sat input_val);
+
 #endif /* LIGHTNING_BITCOIN_TX_H */