common/channel_type: add zero-fee-anchor type.

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

common/channel_type.c

@@ -55,6 +55,17 @@ void channel_type_set_scid_alias(struct channel_type *type)
 			COMPULSORY_FEATURE(OPT_SCID_ALIAS));
 }
 
+struct channel_type *channel_type_anchors_zero_fee_htlc(const tal_t *ctx)
+{
+	struct channel_type *type = channel_type_none(ctx);
+
+	set_feature_bit(&type->features,
+			COMPULSORY_FEATURE(OPT_ANCHORS_ZERO_FEE_HTLC_TX));
+	set_feature_bit(&type->features,
+			COMPULSORY_FEATURE(OPT_STATIC_REMOTEKEY));
+	return type;
+}
+
 struct channel_type *default_channel_type(const tal_t *ctx,
 					  const struct feature_set *our_features,
 					  const u8 *their_features)
@@ -64,12 +75,15 @@ struct channel_type *default_channel_type(const tal_t *ctx,
 	 *   - if `channel_type` was present in both `open_channel` and `accept_channel`:
 	 *     - This is the `channel_type` (they must be equal, required above)
 	 *   - otherwise:
+	 *     - if `option_anchors_zero_fee_htlc_tx` was negotiated:
+	 *       - the `channel_type` is `option_anchors_zero_fee_htlc_tx` and `option_static_remotekey` (bits 22 and 12)
+	 *   - otherwise, if `option_anchor_outputs` was negotiated:
+	 *     - the `channel_type` is `option_anchor_outputs` and
+	 *       `option_static_remotekey` (bits 20 and 12)
 	 */
-	/* BOLT #2:
-	 * - otherwise, if `option_anchor_outputs` was negotiated:
-	 *   - the `channel_type` is `option_anchor_outputs` and
-	 *     `option_static_remotekey` (bits 20 and 12)
-	 */
+	if (feature_negotiated(our_features, their_features,
+			       OPT_ANCHORS_ZERO_FEE_HTLC_TX))
+		return channel_type_anchors_zero_fee_htlc(ctx);
 	if (feature_negotiated(our_features, their_features,
 			       OPT_ANCHOR_OUTPUTS))
 		return channel_type_anchor_outputs(ctx);
@@ -137,6 +151,7 @@ struct channel_type *channel_type_accept(const tal_t *ctx,
 
 	static const size_t feats[] = {
 		OPT_ANCHOR_OUTPUTS,
+		OPT_ANCHORS_ZERO_FEE_HTLC_TX,
 		OPT_STATIC_REMOTEKEY,
 		OPT_SCID_ALIAS,
 		OPT_ZEROCONF,
@@ -185,6 +200,8 @@ struct channel_type *channel_type_accept(const tal_t *ctx,
 	if (channel_type_eq(&proposed, channel_type_none(tmpctx)) ||
 	    channel_type_eq(&proposed,
 			    channel_type_static_remotekey(tmpctx)) ||
+	    channel_type_eq(&proposed,
+			    channel_type_anchors_zero_fee_htlc(tmpctx)) ||
 	    channel_type_eq(&proposed, channel_type_anchor_outputs(tmpctx))) {
 		/* At this point we know it matches, and maybe has
 		 * a couple of extra options. So let's just reply

common/channel_type.h

@@ -9,6 +9,7 @@
 struct channel_type *channel_type_none(const tal_t *ctx);
 struct channel_type *channel_type_static_remotekey(const tal_t *ctx);
 struct channel_type *channel_type_anchor_outputs(const tal_t *ctx);
+struct channel_type *channel_type_anchors_zero_fee_htlc(const tal_t *ctx);
 
 /* channel_type variants */
 void channel_type_set_zeroconf(struct channel_type *channel_type);

common/initial_channel.c

@@ -169,6 +169,10 @@ struct channel_type *channel_upgradable_type(const tal_t *ctx,
 struct channel_type *channel_desired_type(const tal_t *ctx,
 					  const struct channel *channel)
 {
+	/* We don't actually want to downgrade anchors! */
+	if (channel_has(channel, OPT_ANCHORS_ZERO_FEE_HTLC_TX))
+		return channel_type_anchors_zero_fee_htlc(ctx);
+
 	/* We don't actually want to downgrade anchors! */
 	if (channel_has(channel, OPT_ANCHOR_OUTPUTS))
 		return channel_type_anchor_outputs(ctx);

doc/lightning-listpeerchannels.7.md

@@ -29,7 +29,7 @@ On success, an object containing **channels** is returned.  It is an array of ob
 - **state** (string): the channel state, in particular "CHANNELD\_NORMAL" means the channel can be used normally (one of "OPENINGD", "CHANNELD\_AWAITING\_LOCKIN", "CHANNELD\_NORMAL", "CHANNELD\_SHUTTING\_DOWN", "CLOSINGD\_SIGEXCHANGE", "CLOSINGD\_COMPLETE", "AWAITING\_UNILATERAL", "FUNDING\_SPEND\_SEEN", "ONCHAIN", "DUALOPEND\_OPEN\_INIT", "DUALOPEND\_AWAITING\_LOCKIN")
 - **opener** (string): Who initiated the channel (one of "local", "remote")
 - **features** (array of strings):
-  - BOLT #9 features which apply to this channel (one of "option\_static\_remotekey", "option\_anchor\_outputs", "option\_scid\_alias", "option\_zeroconf")
+  - BOLT #9 features which apply to this channel (one of "option\_static\_remotekey", "option\_anchor\_outputs", "option\_anchors\_zero\_fee\_htlc\_tx", "option\_scid\_alias", "option\_zeroconf")
 - **scratch\_txid** (txid, optional): The txid we would use if we went onchain now
 - **channel\_type** (object, optional): channel\_type as negotiated with peer *(added v23.05)*:
   - **bits** (array of u32s): Each bit set in this channel\_type:
@@ -194,4 +194,4 @@ Main web site: <https://github.com/ElementsProject/lightning> Lightning
 RFC site (BOLT \#9):
 <https://github.com/lightningnetwork/lightning-rfc/blob/master/09-features.md>
 
-[comment]: # ( SHA256STAMP:005c109a25f91eabf18e2003a4c83e305ca6b4a604999ceeef6ca3f0ee4b0006)
+[comment]: # ( SHA256STAMP:f5103dae3b20a7d2a4112a368e753f1dd0518a269bd34c2fd17657c9f90596d7)

doc/schemas/listpeerchannels.schema.json

@@ -212,6 +212,7 @@
               "enum": [
                 "option_static_remotekey",
                 "option_anchor_outputs",
+                "option_anchors_zero_fee_htlc_tx",
                 "option_scid_alias",
                 "option_zeroconf"
               ],