lightningd: deprecate "message": null in channel_state_changed notifications.

Somehow I missed this when deprecating `short_channel_id` being null.

Changelog-Deprecated: Plugins: `channel_state_changed` notification `message` field being `null`: it will be omitted instead.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/json_stream.c

@@ -313,11 +313,6 @@ void json_add_bool(struct json_stream *result, const char *fieldname, bool value
 	json_add_primitive(result, fieldname, value ? "true" : "false");
 }
 
-void json_add_null(struct json_stream *stream, const char *fieldname)
-{
-	json_add_primitive(stream, fieldname, "null");
-}
-
 void json_add_hex(struct json_stream *js, const char *fieldname,
 		  const void *data, size_t len)
 {

common/json_stream.h

@@ -240,8 +240,7 @@ void json_add_s32(struct json_stream *result, const char *fieldname,
 void json_add_bool(struct json_stream *result, const char *fieldname,
 		   bool value);
 
-/* '"fieldname" : null' or 'null' if fieldname is NULL */
-void json_add_null(struct json_stream *stream, const char *fieldname);
+/* Looking for json_add_null?  Don't do that: we omit fields, don't 'null' them! */
 
 /* '"fieldname" : "0189abcdef..."' or "0189abcdef..." if fieldname is NULL */
 void json_add_hex(struct json_stream *result, const char *fieldname,

doc/developers-guide/deprecated-features.md

@@ -25,7 +25,8 @@ privacy:
 | pay_notifications.raw_fields                       | Field              | v25.09           | v26.09         | `channel_hint_update`, `pay_failure` and `pay_success` notifications now wrap members in an object of the same name       |
 | encrypted_hsm                                      | Config             | v25.12           | v26.12         | `hsm-passphrase` is a name which also makes sense for modern hsm_secrets which use BIP 39                                 |
 | newaddr.addresstype.defaultbech32                  | Parameter          | v25.12           | v26.12         | Use `p2tr` in the response (present since v23.08 if `addresstype` is `p2tr`,  and always present since v24.12).           |
-
+| channel_state_changed.null_message                 | Notification Field | v25.12           | v26.12         | In channel_state_changed notification, `message` will be missing instead of `null`                                        |
+ 
 Inevitably there are features which need to change: either to be generalized, or removed when they can no longer be supported.
 
 Types of deprecation:

doc/schemas/notification/channel_state_changed.json

@@ -15,8 +15,7 @@
       "channel_id",
       "timestamp",
       "new_state",
-      "cause",
-      "message"
+      "cause"
     ],
     "properties": {
       "peer_id": {

lightningd/notification.c

@@ -273,6 +273,12 @@ void notify_channel_opened(struct lightningd *ld,
 	notify_send(ld, n);
 }
 
+/* Don't use this: omit fields instead! */
+static void json_add_null(struct json_stream *stream, const char *fieldname)
+{
+	json_add_primitive(stream, fieldname, "null");
+}
+
 static void channel_state_changed_notification_serialize(struct json_stream *stream,
 							 struct lightningd *ld,
 							 const struct node_id *peer_id,
@@ -302,8 +308,12 @@ static void channel_state_changed_notification_serialize(struct json_stream *str
 	json_add_string(stream, "cause", channel_change_state_reason_str(cause));
 	if (message != NULL)
 		json_add_string(stream, "message", message);
-	else
+	else if (lightningd_deprecated_out_ok(ld, ld->deprecated_ok,
+					      "channel_state_changed",
+					      "null_message",
+					      "v25.12", "v26.12")) {
 		json_add_null(stream, "message");
+	}
 }
 
 REGISTER_NOTIFICATION(channel_state_changed)

plugins/bcli.c

@@ -418,6 +418,12 @@ static struct command_result *command_err_bcli_badjson(struct bitcoin_cli *bcli,
 	return command_done_err(bcli->cmd, BCLI_ERROR, err, NULL);
 }
 
+/* Don't use this in general: it's better to omit fields. */
+static void json_add_null(struct json_stream *stream, const char *fieldname)
+{
+	json_add_primitive(stream, fieldname, "null");
+}
+
 static struct command_result *process_getutxout(struct bitcoin_cli *bcli)
 {
 	const jsmntok_t *tokens;