mctpd: Update interface vtables when we change to BusOwner role

Currently, setting an interface's Role property only sets the
link->role.

However, we also want to expose the BusOwner1 interface when changing
from Unknown to BusOwner, so register the bus_link_owner_vtable on the
interface object when this happens.

dbus does not like the vtables changing during a call, so we defer this
to the main loop context.

Fixes: #130
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>

Author: jk-ozlabs

CHANGELOG.md

@@ -11,6 +11,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 1. `mctpd` now queries endpoints for their vendor-defined message support,
    and publishes as the newly-specced `VendorDefinedMessageTypes` dbus property.
 
+### Fixes
+
+1. mctpd's interface objects now expose the BusOwner1 interface when set
+   as a BusOwner via the Role property
+
 ## [2.5] - 2026-02-17
 
 ### Added

src/mctpd.c

@@ -144,6 +144,7 @@ struct link {
 	char *path;
 	sd_bus_slot *slot_iface;
 	sd_bus_slot *slot_busowner;
+	sd_event_source *role_defer;
 
 	struct ctx *ctx;
 };
@@ -4207,6 +4208,27 @@ static int bus_link_get_prop(sd_bus *bus, const char *path,
 	return rc;
 }
 
+/* deferred handler for link changes, which may alter vtable state */
+static int link_set_role(sd_event_source *ev, void *userdata)
+{
+	struct link *link = userdata;
+	int rc;
+
+	sd_event_source_unref(link->role_defer);
+	link->role_defer = NULL;
+
+	if (link->role != ENDPOINT_ROLE_BUS_OWNER)
+		return 0;
+
+	rc = sd_bus_add_object_vtable(link->ctx->bus, &link->slot_busowner,
+				      link->path, CC_MCTP_DBUS_IFACE_BUSOWNER,
+				      bus_link_owner_vtable, link);
+	if (rc)
+		warnx("adding link owner vtable failed: %d", rc);
+
+	return 0;
+}
+
 static int bus_link_set_prop(sd_bus *bus, const char *path,
 			     const char *interface, const char *property,
 			     sd_bus_message *value, void *userdata,
@@ -4247,8 +4269,15 @@ static int bus_link_set_prop(sd_bus *bus, const char *path,
 		rc = -EINVAL;
 		goto out;
 	}
+
+	printf("Role for %s set to %s, via dbus\n", link->path, role.conf_val);
 	link->role = role.role;
 
+	/* We need to defer the link role change, as we cannot update the vtables
+	 * during the call.
+	 */
+	sd_event_add_defer(ctx->event, &link->role_defer, link_set_role, link);
+
 out:
 	set_berr(ctx, rc, berr);
 	return rc;
@@ -4683,6 +4712,7 @@ static int prune_old_nets(struct ctx *ctx)
 
 static void free_link(struct link *link)
 {
+	sd_event_source_disable_unref(link->role_defer);
 	sd_bus_slot_unref(link->slot_iface);
 	sd_bus_slot_unref(link->slot_busowner);
 	free(link->path);