sbus: Add sbus_vtable and update codegen to support it

Each interface is a vtable structure derived from
sbus_vtable, in the sense that it has an sbus_vtable
struct as its first argument. This lets us upcast the
interface vtable structure to an sbus_vtable and dispatch
to it dynamically and cleanly.

The interface metadata contains information about which
vtable offset in the interface metadata should be dispatched
to for a given function. This is a common scheme, not only
among dbus implementations, but also compiled languages.

Currently all the vtable functions are of type
sbus_msg_handler_fn. These are the handlers we are familiar
with and perform raw processing of the message. Later commits
will introduce type safe handlers that levelage compile checking
and automatic argument packing/unpacking.

Although this may seem contrived now, the remainder of the
dbus infrastructure work will build on this, including
ofd.Properties, ofd.ObjectManager, ofd.Introspect, compiler
checked type safe unpacking/packing, etc.

The codegen now generates vtable structures for each interface
along-side the metadata, and fills in vtable offsets
appropriately.

It is obviously still possible to hand-craft such vtables and
metadata if needed for a special case.

Once again examples output can be found at:

  src/tests/sbus_codegen_tests_generated.h

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
Reviewed-by: Sumit Bose <sbose@redhat.com>
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/sbus/sssd_dbus.h b/src/sbus/sssd_dbus.h
index 7604797c1..e9e7c9867 100644
--- a/src/sbus/sssd_dbus.h
+++ b/src/sbus/sssd_dbus.h
@@ -55,6 +55,21 @@ enum {
     SBUS_RECONNECT_ERROR
 };
 
+/*
+ * This represents vtable of interface handlers for methods and
+ * properties and so on. The actual vtable structs derive from this struct
+ * (ie: have this struct as their first member).
+ *
+ * The offsets for matching vtable function pointers are in sbus_method_meta
+ * These are used to dynamically dispatch the method invocations.
+ */
+struct sbus_vtable {
+    const struct sbus_interface_meta *meta;
+    int flags; /* unused for now */
+
+    /* derived structs place function pointers here. */
+};
+
 /* Special interface and method for D-BUS introspection */
 #define DBUS_INTROSPECT_INTERFACE "org.freedesktop.DBus.Introspectable"
 #define DBUS_INTROSPECT_METHOD "Introspect"