connected_hook: allow hook to specify an error message.

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

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Config: Adds parameter `min-capacity-sat` to reject tiny channels.
 - JSON API: `listforwards` now includes the time an HTLC was received and when it was resolved. Both are expressed as UNIX timestamps to facilitate parsing (Issue [#2491](https://github.com/ElementsProject/lightning/issues/2491), PR [#2528](https://github.com/ElementsProject/lightning/pull/2528))
 - JSON API: new plugin `invoice_payment` hook for intercepting invoices before they're paid.
+- plugin: the `connected` hook can now send an `error_message` to the rejected peer.
 
 ### Changed
 

doc/PLUGINS.md

@@ -268,6 +268,12 @@ gossiped list of known addresses. In particular this means that the port for
 incoming connections is an ephemeral port, that may not be available for
 reconnections.
 
+The returned result must contain a `result` member which is either
+the string `disconnect` or `continue`.  If `disconnect` and
+there's a member `error_message`, that member is sent to the peer
+before disconnection.
+
+
 #### `db_write`
 
 This hook is called whenever a change is about to be committed to the database.

lightningd/peer_control.c

@@ -706,6 +706,15 @@ peer_connected_hook_cb(struct peer_connected_hook_payload *payload,
 		}
 
 		if (json_tok_streq(buffer, resulttok, "disconnect")) {
+			const jsmntok_t *m = json_get_member(buffer, toks,
+							     "error_message");
+			if (m) {
+				error = towire_errorfmt(tmpctx, NULL,
+							"%.*s",
+							m->end - m->start,
+							buffer + m->start);
+				goto send_error;
+			}
 			close(peer_fd);
 			tal_free(payload);
 			return;

tests/plugins/reject.py

@@ -16,7 +16,7 @@ plugin = Plugin()
 def on_connected(peer, plugin):
     if peer['id'] in plugin.reject_ids:
         print("{} is in reject list, disconnecting".format(peer['id']))
-        return {'result': 'disconnect'}
+        return {'result': 'disconnect', 'error_message': 'You are in reject list'}
 
     print("{} is allowed".format(peer['id']))
     return {'result': 'continue'}

tests/test_plugin.py

@@ -164,6 +164,11 @@ def test_plugin_connected_hook(node_factory):
     l3.connect(l1)
     l1.daemon.wait_for_log(r"{} is in reject list".format(l3.info['id']))
 
+    # FIXME: this error occurs *after* connection, so we connect then drop.
+    l3.daemon.wait_for_log(r"lightning_openingd-{} chan #1: peer_in WIRE_ERROR"
+                           .format(l1.info['id']))
+    l3.daemon.wait_for_log(r"You are in reject list")
+
     peer = l1.rpc.listpeers(l3.info['id'])['peers']
     assert(peer == [] or not peer[0]['connected'])