lightningd: add withhold option to fundchannel_complete.

This is just a polite way of telling us that if we close, don't bother broadcasting
since we didn't broadcast the funding tx.

Changelog-Added: JSON-RPC: `fundchannel_complete` new parameter `withhold` (default false).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

contrib/msggen/msggen/schema.json

@@ -13092,6 +13092,13 @@
             "description": [
               "Transaction to use for funding (does not need to be signed but must be otherwise complete)."
             ]
+          },
+          "withhold": {
+            "type": "boolean",
+            "added": "v25.12",
+            "description": [
+              "Mark this channel 'withheld' so we know we haven't broadcast the funding transaction.  If the channel is closed before we call `sendpsbt` on this psbt, it will simply be closed immediately."
+            ]
           }
         }
       },

doc/schemas/fundchannel_complete.json

@@ -26,6 +26,13 @@
         "description": [
           "Transaction to use for funding (does not need to be signed but must be otherwise complete)."
         ]
+      },
+      "withhold": {
+        "type": "boolean",
+        "added": "v25.12",
+        "description": [
+          "Mark this channel 'withheld' so we know we haven't broadcast the funding transaction.  If the channel is closed before we call `sendpsbt` on this psbt, it will simply be closed immediately."
+        ]
       }
     }
   },

lightningd/opening_control.c

@@ -1025,10 +1025,12 @@ static struct command_result *json_fundchannel_complete(struct command *cmd,
 	struct wally_psbt *funding_psbt;
 	u32 *funding_txout_num = NULL;
 	struct funding_channel *fc;
+	bool *withhold;
 
 	if (!param_check(cmd, buffer, params,
 			 p_req("id", param_node_id, &id),
 			 p_req("psbt", param_psbt, &funding_psbt),
+			 p_opt_def("withhold", param_bool, &withhold, false),
 			 NULL))
 		return command_param_failed();
 
@@ -1101,9 +1103,7 @@ static struct command_result *json_fundchannel_complete(struct command *cmd,
 		return command_check_done(cmd);
 
 	fc->funding_psbt = tal_steal(fc, funding_psbt);
-
-	/* FIXME: Set by option */
-	fc->withheld = false;
+	fc->withheld = *withhold;
 
 	/* Set the cmd to this new cmd */
 	peer->uncommitted_channel->fc->cmd = cmd;