lightningd: turn on experimental support for zero-fee htlc anchors.

We disabled experimental support for opening non-zero-fee anchor
channels (though old nodes may still have such channels if they turned
that on!).

So we simply call this `experimental-anchors`, since this is the variant
which we expect to be used widely.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-EXPERIMENTAL: protocol: added support for zero-fee-htlc anchors (`option_anchors_zero_fee_htlc_tx`), using `--experimental-anchors`.

doc/lightning-listconfigs.7.md

@@ -135,6 +135,9 @@ On success, an object is returned, containing:
   - **experimental-peer-storage** (object, optional) *(added v23.02)*:
     - **set** (boolean): `true` if set in config or cmdline
     - **source** (string): source of configuration setting
+  - **experimental-anchors** (object, optional) *(added v23.08)*:
+    - **set** (boolean): `true` if set in config or cmdline
+    - **source** (string): source of configuration setting
   - **database-upgrade** (object, optional):
     - **value\_bool** (boolean): field from config or cmdline, or default
     - **source** (string): source of configuration setting
@@ -463,4 +466,4 @@ RESOURCES
 
 Main web site: <https://github.com/ElementsProject/lightning>
 
-[comment]: # ( SHA256STAMP:0440e4634e4a28681323f891307c7bb61143aacad4824f952f24f027a7543835)
+[comment]: # ( SHA256STAMP:d24d8d540253bbe3cf9113b265fd3128e351d4615c28f6054730d1b03b363155)

doc/lightningd-config.5.md

@@ -728,6 +728,16 @@ protocol to update channel types.  At the moment, we only support setting
 this option.
 
 
+* **experimental-anchors**
+
+  Specifying this option turns on the `option_anchors_zero_fee_htlc_tx`
+feature, meaning we can open anchor-based channels.  This will become
+the default for new channels in future, after more testing.  Anchor-based
+channels use larger commitment transactions, with the trade-off that they
+don't have to use a worst-case fee, but can bump the commitment transaction
+if it's needed.  Note that this means that we need to keep
+some funds aside: see `min-emergency-msat`.
+
 BUGS
 ----
 

doc/schemas/listconfigs.schema.json

@@ -493,6 +493,25 @@
             }
           }
         },
+        "experimental-anchors": {
+          "type": "object",
+          "added": "v23.08",
+          "additionalProperties": false,
+          "required": [
+            "set",
+            "source"
+          ],
+          "properties": {
+            "set": {
+              "type": "boolean",
+              "description": "`true` if set in config or cmdline"
+            },
+            "source": {
+              "type": "string",
+              "description": "source of configuration setting"
+            }
+          }
+        },
         "database-upgrade": {
           "type": "object",
           "additionalProperties": false,

lightningd/hsm_control.c

@@ -160,6 +160,12 @@ struct ext_key *hsm_init(struct lightningd *ld)
 			  hsmd_wire_name(ld->hsm_capabilities[i]));
 	}
 
+	if (feature_offered(ld->our_features->bits[INIT_FEATURE],
+			    OPT_ANCHORS_ZERO_FEE_HTLC_TX)
+	    && !hsm_capable(ld, WIRE_HSMD_SIGN_ANCHORSPEND)) {
+		fatal("--experimental-anchors needs HSM capable of signing anchors!");
+	}
+
 	/* This is equivalent to makesecret("bolt12-invoice-base") */
 	msg = towire_hsmd_derive_secret(NULL, tal_dup_arr(tmpctx, u8,
 							  (const u8 *)INVOICE_PATH_BASE_STRING,

lightningd/options.c

@@ -20,7 +20,9 @@
 #include <common/wireaddr.h>
 #include <dirent.h>
 #include <errno.h>
+#include <hsmd/hsmd_wiregen.h>
 #include <lightningd/chaintopology.h>
+#include <lightningd/hsm_control.h>
 #include <lightningd/options.h>
 #include <lightningd/plugin.h>
 #include <lightningd/subd.h>
@@ -1187,6 +1189,15 @@ static char *opt_set_quiesce(struct lightningd *ld)
 	return NULL;
 }
 
+static char *opt_set_anchor_zero_fee_htlc_tx(struct lightningd *ld)
+{
+	/* Requires static_remotekey, but we always set that */
+	feature_set_or(ld->our_features,
+		       take(feature_set_for_feature(NULL,
+						    OPTIONAL_FEATURE(OPT_ANCHORS_ZERO_FEE_HTLC_TX))));
+	return NULL;
+}
+
 static char *opt_set_offers(struct lightningd *ld)
 {
 	ld->config.exp_offers = true;
@@ -1284,6 +1295,10 @@ static void register_opts(struct lightningd *ld)
 				 opt_set_quiesce, ld,
 				 "experimental: Advertise ability to quiesce"
 				 " channels.");
+	opt_register_early_noarg("--experimental-anchors",
+				 opt_set_anchor_zero_fee_htlc_tx, ld,
+				 "EXPERIMENTAL: enable option_anchors_zero_fee_htlc_tx"
+				 " to open zero-fee-anchor channels");
 	clnopt_witharg("--announce-addr-dns", OPT_EARLY|OPT_SHOWBOOL,
 		       opt_set_bool_arg, opt_show_bool,
 		       &ld->announce_dns,