channeld: implement and refine fee-related functions.

We had some in the header, now implement them, and add a channel_feerate()
accessor.

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

channeld/channel.c

@@ -893,6 +893,12 @@ static struct io_plan *handle_peer_commit_sig(struct io_conn *conn,
 			    "commit_sig with no changes");
 	}
 
+	/* We were supposed to check this was affordable as we go. */
+	if (peer->channel->funder == REMOTE)
+		assert(can_funder_afford_feerate(peer->channel,
+						 peer->channel->view[LOCAL]
+						 .feerate_per_kw));
+
 	if (!fromwire_commitment_signed(tmpctx, msg, NULL,
 					&channel_id, &commit_sig, &htlc_sigs))
 		peer_failed(io_conn_fd(peer->peer_conn),

channeld/full_channel.c

@@ -678,6 +678,53 @@ static int change_htlcs(struct channel *channel,
 	return cflags;
 }
 
+bool can_funder_afford_feerate(const struct channel *channel, u32 feerate_per_kw)
+{
+	u64 fee_msat, dust = dust_limit_satoshis(channel, !channel->funder);
+	size_t untrimmed;
+	const struct htlc **committed, **adding, **removing;
+	const tal_t *tmpctx = tal_tmpctx(channel);
+
+	gather_htlcs(tmpctx, channel, !channel->funder,
+		     &committed, &removing, &adding);
+
+	untrimmed = commit_tx_num_untrimmed(committed, feerate_per_kw, dust,
+					    !channel->funder)
+			+ commit_tx_num_untrimmed(adding, feerate_per_kw, dust,
+						  !channel->funder)
+			- commit_tx_num_untrimmed(removing, feerate_per_kw, dust,
+						  !channel->funder);
+
+	fee_msat = commit_tx_base_fee(feerate_per_kw, untrimmed);
+
+	tal_free(tmpctx);
+
+	/* BOLT #2:
+	 *
+	 * A receiving node SHOULD fail the channel if the sender cannot afford
+	 * the new fee rate on the receiving node's current commitment
+	 * transaction */
+	/* Note: sender == funder */
+
+	/* How much does it think it has?  Must be >= reserve + fee */
+	return channel->view[!channel->funder].owed_msat[channel->funder]
+		>= channel_reserve_msat(channel, channel->funder) + fee_msat;
+}
+
+bool channel_update_feerate(struct channel *channel, u32 feerate_per_kw)
+{
+	if (!can_funder_afford_feerate(channel, feerate_per_kw))
+		return false;
+
+	channel->view[!channel->funder].feerate_per_kw = feerate_per_kw;
+	return true;
+}
+
+u32 channel_feerate(const struct channel *channel, enum side side)
+{
+	return channel->view[side].feerate_per_kw;
+}
+
 /* FIXME: Handle fee changes too. */
 bool channel_sending_commit(struct channel *channel,
 			    const struct htlc ***htlcs)

channeld/full_channel.h

@@ -173,24 +173,33 @@ enum channel_remove_err channel_fulfill_htlc(struct channel *channel,
  * approx_max_feerate: what's the we (initiator) could raise fee rate to?
  * @channel: The channel state
  *
- * This is not exact!  To check if their offer is valid, use can_afford_feerate.
+ * This is not exact!  To check if their offer is valid, try
+ * channel_update_feerate.
  */
 u32 approx_max_feerate(const struct channel *channel);
 
 /**
- * can_afford_feerate: could the initiator pay for the fee at fee_rate?
+ * can_funder_afford_feerate: could the funder pay the fee?
  * @channel: The channel state
- * @feerate_per_kw: the new fee rate proposed
+ * @feerate: The feerate in satoshi per 1000 bytes.
  */
-bool can_afford_feerate(const struct channel *channel, u32 feerate_per_kw);
+bool can_funder_afford_feerate(const struct channel *channel, u32 feerate);
 
 /**
- * adjust_fee: Change fee rate.
+ * channel_update_feerate: Change fee rate on non-funder side.
- * @channel: The channel state
+ * @channel: The channel
  * @feerate_per_kw: fee in satoshi per 1000 bytes.
- * @side: which side to adjust.
+ *
+ * Returns true if it's affordable, otherwise does nothing.
  */
-void adjust_fee(struct channel *channel, u32 feerate_per_kw, enum side side);
+bool channel_update_feerate(struct channel *channel, u32 feerate_per_kw);
+
+/**
+ * channel_feerate: Get fee rate for this side of channel.
+ * @channel: The channel
+ * @side: the side
+ */
+u32 channel_feerate(const struct channel *channel, enum side side);
 
 /**
  * channel_sending_commit: commit all remote outstanding changes.