openingd: use a lower feerate for first commitment tx for anchors.

We need to know both, because in theory we could negotiate a
non-anchor channel even if they support it.

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

doc/lightning-fundchannel.7.md

@@ -36,8 +36,9 @@ limit, currently set to 546, nor more than 16777215 satoshi (unless large
 channels were negotiated with the peer).
 
 *feerate* is an optional feerate used for the opening transaction and
-as initial feerate for commitment and HTLC transactions (see NOTES in
-lightning-feerates(7)).  The default is *normal*.
+(unless *option\_anchors\_zero\_fee\_htlc\_tx* is negotiated), as initial feerate
+for commitment and HTLC transactions (see NOTES in lightning-feerates(7)).
+The default is *normal*.
 
 *announce* is an optional flag that triggers whether to announce this
 channel or not. Defaults to `true`. An unannounced channel is considered

doc/lightning-fundchannel_start.7.md

@@ -19,7 +19,9 @@ value MUST be accurate, otherwise the negotiated commitment transactions
 will not encompass the correct channel value.
 
 *feerate* is an optional field. Sets the feerate for subsequent
-commitment transactions: see **fundchannel**.
+commitment transactions: see **fundchannel**.  Note that this is ignored
+for channels with *option\_anchors\_zero\_fee\_htlc\_tx* (we always use a low
+commitment fee for these).
 
 *announce* whether or not to announce this channel.
 

lightningd/opening_control.c

@@ -1276,6 +1276,7 @@ static struct command_result *json_fundchannel_start(struct command *cmd,
 			fc->our_upfront_shutdown_script,
 			upfront_shutdown_script_wallet_index,
 			*feerate_per_kw,
+			unilateral_feerate(cmd->ld->topology, true),
 			&tmp_channel_id,
 			fc->channel_flags,
 			fc->uc->reserve);

openingd/openingd.c

@@ -326,7 +326,8 @@ static bool intuit_scid_alias_type(struct state *state, u8 channel_flags,
 
 /* We start the 'open a channel' negotation with the supplied peer, but
  * stop when we get to the part where we need the funding txid */
-static u8 *funder_channel_start(struct state *state, u8 channel_flags)
+static u8 *funder_channel_start(struct state *state, u8 channel_flags,
+				u32 nonanchor_feerate, u32 anchor_feerate)
 {
 	u8 *msg;
 	u8 *funding_output_script;
@@ -374,6 +375,14 @@ static u8 *funder_channel_start(struct state *state, u8 channel_flags)
 			channel_type_set_scid_alias(state->channel_type);
 	}
 
+	/* Which feerate do we use?  (We can lowball fees if using anchors!) */
+	if (channel_type_has(state->channel_type, OPT_ANCHOR_OUTPUTS)
+	    || channel_type_has(state->channel_type, OPT_ANCHORS_ZERO_FEE_HTLC_TX)) {
+		state->feerate_per_kw = anchor_feerate;
+	} else {
+		state->feerate_per_kw = nonanchor_feerate;
+	}
+
 	open_tlvs = tlv_open_channel_tlvs_new(tmpctx);
 	open_tlvs->upfront_shutdown_script
 		= state->upfront_shutdown_script[LOCAL];
@@ -1437,6 +1446,7 @@ static u8 *handle_master_in(struct state *state)
 	u8 channel_flags;
 	struct bitcoin_txid funding_txid;
 	u16 funding_txout;
+	u32 nonanchor_feerate, anchor_feerate;
 
 	switch (t) {
 	case WIRE_OPENINGD_FUNDER_START:
@@ -1445,12 +1455,13 @@ static u8 *handle_master_in(struct state *state)
 						    &state->push_msat,
 						    &state->upfront_shutdown_script[LOCAL],
 						    &state->local_upfront_shutdown_wallet_index,
-						    &state->feerate_per_kw,
+						    &nonanchor_feerate,
+						    &anchor_feerate,
 						    &state->channel_id,
 						    &channel_flags,
 						    &state->reserve))
 			master_badmsg(WIRE_OPENINGD_FUNDER_START, msg);
-		msg = funder_channel_start(state, channel_flags);
+		msg = funder_channel_start(state, channel_flags, nonanchor_feerate, anchor_feerate);
 
 		/* We want to keep openingd alive, since we're not done yet */
 		if (msg)

openingd/openingd_wire.csv

@@ -82,6 +82,7 @@ msgdata,openingd_funder_start,len_upfront,u16,
 msgdata,openingd_funder_start,upfront_shutdown_script,u8,len_upfront
 msgdata,openingd_funder_start,upfront_shutdown_wallet_index,?u32,
 msgdata,openingd_funder_start,feerate_per_kw,u32,
+msgdata,openingd_funder_start,anchor_feerate_per_kw,u32,
 msgdata,openingd_funder_start,temporary_channel_id,channel_id,
 msgdata,openingd_funder_start,channel_flags,u8,
 msgdata,openingd_funder_start,reserve,?amount_sat,

tests/test_opening.py

@@ -2182,7 +2182,13 @@ def test_no_anchor_liquidity_ads(node_factory, bitcoind):
 def test_commitment_feerate(bitcoind, node_factory):
     l1, l2 = node_factory.get_nodes(2)
 
-    opening_feerate = commitment_feerate = 2000
+    opening_feerate = 2000
+    if anchor_expected():
+        # anchors use lowball fees
+        commitment_feerate = 3750
+    else:
+        commitment_feerate = opening_feerate
+
     l1.fundwallet(10**8)
     l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
     l1.rpc.fundchannel(l2.info['id'], 10**6,