commit_tx: make fee msat vs sat explicit.

Suggested-by: @niftynei
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

channeld/commit_tx.c

@@ -125,7 +125,7 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 	 * 2. Calculate the base [commitment transaction
 	 * fee](#fee-calculation).
 	 */
-	base_fee_msat = commit_tx_base_fee(feerate_per_kw, untrimmed) * 1000;
+	base_fee_msat = commit_tx_base_fee_msat(feerate_per_kw, untrimmed);
 
 	SUPERVERBOSE("# base commitment transaction fee = %"PRIu64"\n",
 		     base_fee_msat / 1000);

channeld/full_channel.c

@@ -414,7 +414,7 @@ static enum channel_add_err add_htlc(struct channel *channel,
 			- commit_tx_num_untrimmed(removing, feerate, dust,
 						  recipient);
 
-		fee_msat = commit_tx_base_fee(feerate, untrimmed) * 1000;
+		fee_msat = commit_tx_base_fee_msat(feerate, untrimmed);
 	} else
 		fee_msat = 0;
 
@@ -718,7 +718,7 @@ bool can_funder_afford_feerate(const struct channel *channel, u32 feerate_per_kw
 			- commit_tx_num_untrimmed(removing, feerate_per_kw, dust,
 						  !channel->funder);
 
-	fee_msat = commit_tx_base_fee(feerate_per_kw, untrimmed) * 1000;
+	fee_msat = commit_tx_base_fee_msat(feerate_per_kw, untrimmed);
 
 	/* BOLT #2:
 	 *

channeld/test/run-commit_tx.c

@@ -916,8 +916,7 @@ int main(void)
 	/* Now make sure we cover case where funder can't afford the fee;
 	 * its output cannot go negative! */
 	for (;;) {
-		u64 base_fee_msat = commit_tx_base_fee(feerate_per_kw, 0)
-			* 1000;
+		u64 base_fee_msat = commit_tx_base_fee_msat(feerate_per_kw, 0);
 
 		if (base_fee_msat <= to_local_msat) {
 			feerate_per_kw++;

common/initial_commit_tx.c

@@ -92,7 +92,7 @@ struct bitcoin_tx *initial_commit_tx(const tal_t *ctx,
 	 * 2. Calculate the base [commitment transaction
 	 * fee](#fee-calculation).
 	 */
-	base_fee_msat = commit_tx_base_fee(feerate_per_kw, untrimmed) * 1000;
+	base_fee_msat = commit_tx_base_fee_msat(feerate_per_kw, untrimmed);
 
 	/* BOLT #3:
 	 *

common/initial_commit_tx.h

@@ -18,7 +18,7 @@ u64 commit_number_obscurer(const struct pubkey *opener_payment_basepoint,
 			   const struct pubkey *accepter_payment_basepoint);
 
 /* Helper to calculate the base fee if we have this many htlc outputs */
-static inline u64 commit_tx_base_fee(u32 feerate_per_kw,
+static inline u64 commit_tx_base_fee_sat(u32 feerate_per_kw,
 					 size_t num_untrimmed_htlcs)
 {
 	u64 weight;
@@ -44,7 +44,14 @@ static inline u64 commit_tx_base_fee(u32 feerate_per_kw,
 	 *    3. Multiply `feerate_per_kw` by `weight`, divide by 1000 (rounding
 	 *    down).
 	 */
-	return feerate_per_kw * weight / 1000;
+	return (feerate_per_kw * weight / 1000);
+}
+
+static inline u64 commit_tx_base_fee_msat(u32 feerate_per_kw,
+					  size_t num_untrimmed_htlcs)
+{
+	return commit_tx_base_fee_sat(feerate_per_kw, num_untrimmed_htlcs)
+		* 1000;
 }
 
 /**

lightningd/closing_control.c

@@ -184,11 +184,11 @@ void peer_start_closingd(struct channel *channel,
 	 *    fee of the final commitment transaction, as calculated in
 	 *    [BOLT #3](03-transactions.md#fee-calculation).
 	 */
-	feelimit = commit_tx_base_fee(channel->channel_info.feerate_per_kw[LOCAL],
+	feelimit = commit_tx_base_fee_sat(channel->channel_info.feerate_per_kw[LOCAL],
 					  0);
 
 	/* Pick some value above slow feerate (or min possible if unknown) */
-	minfee = commit_tx_base_fee(feerate_min(ld, NULL), 0);
+	minfee = commit_tx_base_fee_sat(feerate_min(ld, NULL), 0);
 
 	/* If we can't determine feerate, start at half unilateral feerate. */
 	feerate = mutual_close_feerate(ld->topology);
@@ -197,7 +197,7 @@ void peer_start_closingd(struct channel *channel,
 		if (feerate < feerate_floor())
 			feerate = feerate_floor();
 	}
-	startfee = commit_tx_base_fee(feerate, 0);
+	startfee = commit_tx_base_fee_sat(feerate, 0);
 
 	if (startfee > feelimit)
 		startfee = feelimit;