lightningd/commit_tx: support for printing out actual tx fees.

This was included in the lightningnetwork/lightning-rfc#105 update
to the test vectors, and it's a good idea.  Takes a bit of work to
calculate (particularly, being aware of rounding issues).

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

lightningd/commit_tx.c

@@ -231,6 +231,23 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 	subtract_fee(funder, side, base_fee_msat,
 		     &self_pay_msat, &other_pay_msat);
 
+#ifdef PRINT_ACTUAL_FEE
+	{
+		u64 satoshis_out = 0;
+		for (i = n = 0; i < tal_count(htlcs); i++) {
+			if (!trim(htlcs[i], feerate_per_kw, dust_limit_satoshis,
+				  side))
+				satoshis_out += htlcs[i]->msatoshi / 1000;
+		}
+		if (self_pay_msat / 1000 >= dust_limit_satoshis)
+			satoshis_out += self_pay_msat / 1000;
+		if (other_pay_msat / 1000 >= dust_limit_satoshis)
+			satoshis_out += other_pay_msat / 1000;
+		SUPERVERBOSE("# actual commitment transaction fee = %"PRIu64"\n",
+			     funding_satoshis - satoshis_out);
+	}
+#endif
+
 	/* Worst-case sizing: both to-local and to-remote outputs. */
 	tx = bitcoin_tx(ctx, 1, untrimmed + 2);
 

lightningd/test/run-commit_tx.c

@@ -5,6 +5,7 @@
 static bool print_superverbose;
 #define SUPERVERBOSE(...)					\
 	do { if (print_superverbose) printf(__VA_ARGS__); } while(0)
+#define PRINT_ACTUAL_FEE
 #include "../commit_tx.c"
 #include "../htlc_tx.c"
 #include <bitcoin/preimage.h>
@@ -15,6 +16,9 @@ static bool print_superverbose;
 #include <ccan/str/hex/hex.h>
 #include <type_to_string.h>
 
+/* Turn this on to brute-force fee values */
+/*#define DEBUG */
+
 static struct sha256 sha256_from_hex(const char *hex)
 {
 	struct sha256 sha256;