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>

3 files changed, 56 insertions, 2 deletions

diff --git a/src/sbus/sbus_codegen b/src/sbus/sbus_codegen
index a1e2d02a0..86aeff094 100755
--- a/src/sbus/sbus_codegen
+++ b/src/sbus/sbus_codegen
@@ -184,6 +184,7 @@ def source_methods(iface, methods):
             out("        %s__out,", meth.fq_c_name())
         else:
             out("        NULL, /* no out_args */")
+        out("        offsetof(struct %s, %s),", iface.c_name(), meth.c_name())
         out("    },")
     out("    { NULL, }")
     out("};")
@@ -251,7 +252,7 @@ def source_interface(iface):
         out("    NULL, /* no propetries */")
     out("};")
 
-def generate_source(ifaces, filename):
+def generate_source(ifaces, filename, include_header=None):
     basename = os.path.basename(filename)
 
     out("/* The following definitions are auto-generated from %s */", basename)
@@ -260,6 +261,8 @@ def generate_source(ifaces, filename):
     out("#include \"util/util.h\"")
     out("#include \"sbus/sssd_dbus.h\"")
     out("#include \"sbus/sssd_dbus_meta.h\"")
+    if include_header:
+        out("#include \"%s\"", os.path.basename(include_header))
 
     for iface in ifaces:
 
@@ -278,6 +281,20 @@ def generate_source(ifaces, filename):
         # The sbus_interface structure
         source_interface(iface)
 
+def header_vtable(iface, methods):
+    out("")
+    out("/* vtable for %s */", iface.name)
+    out("struct %s {", iface.c_name())
+    out("    struct sbus_vtable vtable; /* derive from sbus_vtable */")
+
+    # All methods
+    for meth in iface.methods:
+        out("    sbus_msg_handler_fn %s;", meth.c_name())
+
+    # TODO: Property getters and setters will go here
+
+    out("};")
+
 def generate_header(ifaces, filename):
     basename = os.path.basename(filename)
     guard = "__%s__" % re.sub(r'([^_A-Z0-9])', "_", basename.upper())
@@ -291,6 +308,23 @@ def generate_header(ifaces, filename):
 
     out("")
     out("/* ------------------------------------------------------------------------")
+    out(" * DBus Vtable handler structures")
+    out(" *")
+    out(" * These structures are filled in by implementors of the different")
+    out(" * dbus interfaces to handle method calls.")
+    out(" *")
+    out(" * Handler functions of type sbus_msg_handler_fn accept raw messages,")
+    out(" * other handlers will be typed appropriately. If a handler that is")
+    out(" * set to NULL is invoked it will result in a")
+    out(" * org.freedesktop.DBus.Error.NotSupported error for the caller.")
+    out(" */")
+
+    for iface in ifaces:
+        if iface.methods:
+            header_vtable(iface, iface.methods)
+
+    out("")
+    out("/* ------------------------------------------------------------------------")
     out(" * DBus Interface Metadata")
     out(" *")
     out(" * These structure definitions are filled in with the information about")
@@ -465,6 +499,10 @@ def parse_options():
                       dest="output", default=None,
                       help="Set output file name (default: stdout)",
                       metavar="FILE")
+    parser.add_option("--include",
+                      dest="include", default=None,
+                      help="name of a header to #include",
+                      metavar="HEADER")
     (options, args) = parser.parse_args()
 
     if not args:
@@ -488,7 +526,7 @@ def main():
         if options.mode == "header":
             generate_header(parser.parsed_interfaces, filename)
         elif options.mode == "source":
-            generate_source(parser.parsed_interfaces, filename)
+            generate_source(parser.parsed_interfaces, filename, options.include)
         else:
             assert False, "should not be reached"
 
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"
diff --git a/src/sbus/sssd_dbus_meta.h b/src/sbus/sssd_dbus_meta.h
index 17ea7a507..abc992137 100644
--- a/src/sbus/sssd_dbus_meta.h
+++ b/src/sbus/sssd_dbus_meta.h
@@ -42,6 +42,7 @@ struct sbus_method_meta {
     const char *name;
     const struct sbus_arg_meta *in_args;
     const struct sbus_arg_meta *out_args;
+    size_t vtable_offset;
 };
 
 enum {