openingd: Add reserve to fundchannel and multifundchannel

Changelog-Added: JSON-RPC: `fundchannel`, `multifundchannel` and `fundchannel_start` now accept a `reserve` parameter to indicate the absolute reserve to impose on the peer.

.msggen.json

@@ -393,6 +393,7 @@
             "FundChannel.mindepth": 12,
             "FundChannel.push_msat": 5,
             "FundChannel.request_amt": 7,
+            "FundChannel.reserve": 13,
             "FundChannel.utxos[]": 11
         },
         "FundchannelResponse": {

contrib/pyln-client/pyln/client/lightning.py

@@ -730,7 +730,8 @@ class LightningRpc(UnixDomainSocketRpc):
     def fundchannel(self, node_id, amount, feerate=None, announce=True,
                     minconf=None, utxos=None, push_msat=None, close_to=None,
                     request_amt=None, compact_lease=None,
-                    mindepth: Optional[int] = None):
+                    mindepth: Optional[int] = None,
+                    reserve: Optional[str] = None):
         """
         Fund channel with {id} using {amount} satoshis with feerate
         of {feerate} (uses default feerate if unset).
@@ -756,6 +757,7 @@ class LightningRpc(UnixDomainSocketRpc):
             "request_amt": request_amt,
             "compact_lease": compact_lease,
             "mindepth": mindepth,
+            "reserve": reserve,
         }
         return self.call("fundchannel", payload)
 

doc/schemas/fundchannel.request.json

@@ -44,6 +44,10 @@
     "mindepth": {
       "description": "Number of confirmations required before we consider the channel active",
       "type": "u32"
+    },
+    "reserve": {
+      "type": "msat",
+      "description": "The amount we want the peer to maintain on its side"
     }
   }
 }

lightningd/opening_common.c

@@ -65,6 +65,13 @@ new_uncommitted_channel(struct peer *peer)
 	 /* We override this in openchannel hook if we want zeroconf */
 	uc->minimum_depth = ld->config.anchor_confirms;
 
+	/* Use default 1% reserve if not otherwise specified. If this
+	 * is not-NULL it will be used by openingd as absolute value
+	 * (clamped to dust limit). */
+	uc->reserve = NULL;
+
+	memset(&uc->cid, 0xFF, sizeof(uc->cid));
+
 	/* Declare the new channel to the HSM. */
 	new_channel_msg = towire_hsmd_new_channel(NULL, &uc->peer->id, uc->dbid);
 	if (!wire_sync_write(ld->hsm_fd, take(new_channel_msg)))

lightningd/opening_common.h

@@ -58,6 +58,12 @@ struct uncommitted_channel {
 
 	/* Our channel config. */
 	struct channel_config our_config;
+
+	/* Reserve we will impose on the other side. If this is NULL
+	 * we will use our default of 1% of the funding
+	 * amount. Otherwise it will be used by openingd as absolute
+	 * value (clamped to dust limit). */
+	struct amount_sat *reserve;
 };
 
 struct funding_channel {
@@ -66,6 +72,7 @@ struct funding_channel {
 	struct wallet_tx *wtx;
 	struct amount_msat push;
 	struct amount_sat funding_sats;
+
 	u8 channel_flags;
 	const u8 *our_upfront_shutdown_script;
 

lightningd/opening_control.c

@@ -1095,7 +1095,7 @@ static struct command_result *json_fundchannel_start(struct command *cmd,
 	bool *announce_channel;
 	u32 *feerate_per_kw, *mindepth;
 	int fds[2];
-	struct amount_sat *amount;
+	struct amount_sat *amount, *reserve;
 	struct amount_msat *push_msat;
 	u32 *upfront_shutdown_script_wallet_index;
 	struct channel_id tmp_channel_id;
@@ -1114,6 +1114,7 @@ static struct command_result *json_fundchannel_start(struct command *cmd,
 		   p_opt("close_to", param_bitcoin_address, &fc->our_upfront_shutdown_script),
 		   p_opt("push_msat", param_msat, &push_msat),
 		   p_opt_def("mindepth", param_u32, &mindepth, cmd->ld->config.anchor_confirms),
+		   p_opt("reserve", param_sat, &reserve),
 		   NULL))
 		return command_param_failed();
 
@@ -1223,6 +1224,8 @@ static struct command_result *json_fundchannel_start(struct command *cmd,
 	assert(mindepth != NULL);
 	fc->uc->minimum_depth = *mindepth;
 
+	fc->uc->reserve = reserve;
+
 	/* Needs to be stolen away from cmd */
 	if (fc->our_upfront_shutdown_script)
 		fc->our_upfront_shutdown_script

plugins/spender/fundchannel.c

@@ -54,6 +54,7 @@ json_fundchannel(struct command *cmd,
 	const jsmntok_t *request_amt;
 	const jsmntok_t *compact_lease;
 	const jsmntok_t *mindepth;
+	const jsmntok_t *reserve;
 
 	struct out_req *req;
 
@@ -69,6 +70,7 @@ json_fundchannel(struct command *cmd,
 		   p_opt("request_amt", param_tok, &request_amt),
 		   p_opt("compact_lease", param_tok, &compact_lease),
 		   p_opt("mindepth", param_tok, &mindepth),
+		   p_opt("reserve", param_tok, &reserve),
 		   NULL))
 		return command_param_failed();
 
@@ -99,6 +101,9 @@ json_fundchannel(struct command *cmd,
 	if (mindepth)
 		json_add_tok(req->js, "mindepth", mindepth, buf);
 
+	if (reserve)
+		json_add_tok(req->js, "reserve", reserve, buf);
+
 	json_object_end(req->js);
 	json_array_end(req->js);
 	if (feerate)

plugins/spender/multifundchannel.c

@@ -1125,6 +1125,11 @@ fundchannel_start_dest(struct multifundchannel_destination *dest)
 	if (dest->mindepth)
 		json_add_u32(req->js, "mindepth", *dest->mindepth);
 
+	if (dest->reserve)
+		json_add_string(
+		    req->js, "reserve",
+		    type_to_string(tmpctx, struct amount_sat, dest->reserve));
+
 	send_outreq(cmd->plugin, req);
 }
 
@@ -1914,6 +1919,7 @@ param_destinations_array(struct command *cmd, const char *name,
 				     AMOUNT_SAT(0)),
 			   p_opt("compact_lease", param_lease_hex, &rates),
 			   p_opt("mindepth", param_u32, &dest->mindepth),
+			   p_opt("reserve", param_sat, &dest->reserve),
 			   NULL))
 			return command_param_failed();
 

plugins/spender/multifundchannel.h

@@ -112,6 +112,8 @@ struct multifundchannel_destination {
 	bool all;
 	struct amount_sat amount;
 
+	struct amount_sat *reserve;
+
 	/* the output index for this destination.  */
 	unsigned int outnum;