channeld: be more accurate with the weight of commitment txs.

We didn't add the weight of the two sigs!  The BOLT defines that to be a worst-case 73 byte sig,
but that turns out to be an overestimate (and this is not required for consensus) so we assume
everyone grinds.

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

Author: rustyrussell

channeld/channeld.c

@@ -1200,7 +1200,10 @@ static u8 *send_commit_part(const tal_t *ctx,
 		*anchor = tal(ctx, struct local_anchor_info);
 		bitcoin_txid(txs[0], &(*anchor)->anchor_point.txid);
 		(*anchor)->anchor_point.n = local_anchor_outnum;
-		(*anchor)->commitment_weight = bitcoin_tx_weight(txs[0]);
+		/* Actual weight will include witnesses!  Note: this assumes
+		 * 73 byte sigs (worst case as per BOLT), whereas we grind to
+		 * 71, and so does everyone else. */
+		(*anchor)->commitment_weight = bitcoin_tx_weight(txs[0]) + bitcoin_tx_2of2_input_witness_weight() - 4;
 		(*anchor)->commitment_fee = bitcoin_tx_compute_fee(txs[0]);
 	}