bus: optionally call a callbacks for cleanup

This adds a function sd_bus_slot_set_destroy_callback() to set a function
which can free userdata or perform other cleanups.

sd_bus_slot_get_destory_callback() queries the callback, and is included
for completeness.

Without something like this, for floating asynchronous callbacks, which might
be called or not, depending on the sequence of events, it's hard to perform
resource cleanup. The alternative would be to always perform the cleanup from
the caller too, but that requires more coordination and keeping of some shared
state. It's nicer to keep the cleanup contained between the callback and the
function that requests the callback.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/systemd/sd-bus.h b/src/systemd/sd-bus.h
index 740e40439..b2394763c 100644
--- a/src/systemd/sd-bus.h
+++ b/src/systemd/sd-bus.h
@@ -111,6 +111,7 @@ typedef int (*sd_bus_property_set_t) (sd_bus *bus, const char *path, const char
 typedef int (*sd_bus_object_find_t) (sd_bus *bus, const char *path, const char *interface, void *userdata, void **ret_found, sd_bus_error *ret_error);
 typedef int (*sd_bus_node_enumerator_t) (sd_bus *bus, const char *prefix, void *userdata, char ***ret_nodes, sd_bus_error *ret_error);
 typedef int (*sd_bus_track_handler_t) (sd_bus_track *track, void *userdata);
+typedef void (*sd_bus_destroy_t)(void *userdata);
 
 #include "sd-bus-protocol.h"
 #include "sd-bus-vtable.h"
@@ -230,6 +231,8 @@ int sd_bus_slot_set_description(sd_bus_slot *slot, const char *description);
 int sd_bus_slot_get_description(sd_bus_slot *slot, const char **description);
 int sd_bus_slot_get_floating(sd_bus_slot *slot);
 int sd_bus_slot_set_floating(sd_bus_slot *slot, int b);
+int sd_bus_slot_set_destroy_callback(sd_bus_slot *s, sd_bus_destroy_t callback);
+int sd_bus_slot_get_destroy_callback(sd_bus_slot *s, sd_bus_destroy_t *callback);
 
 sd_bus_message* sd_bus_slot_get_current_message(sd_bus_slot *slot);
 sd_bus_message_handler_t sd_bus_slot_get_current_handler(sd_bus_slot *bus);