qapi: Disallow qmp_marshal_FOO(NULL, ...)

For QMP commands without arguments, gen_marshal() laboriously
generates a qmp_marshal_FOO() that copes with null @args.  Turns
there's just one caller that passes null instead of an empty QDict.
Adjust that caller, and simplify gen_marshal().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200424084338.26803-15-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

3 files changed, 7 insertions, 26 deletions

diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
index c6dd1891c3..a7794ef658 100644
--- a/docs/devel/qapi-code-gen.txt
+++ b/docs/devel/qapi-code-gen.txt
@@ -1579,8 +1579,8 @@ Example:
     void qmp_marshal_my_command(QDict *args, QObject **ret, Error **errp)
     {
         Error *err = NULL;
-        UserDefOne *retval;
         Visitor *v;
+        UserDefOne *retval;
         q_obj_my_command_arg arg = {0};
 
         v = qobject_input_visitor_new(QOBJECT(args));
diff --git a/monitor/qmp.c b/monitor/qmp.c
index f89e7daf27..d433ceae5b 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -322,9 +322,12 @@ static QDict *qmp_greeting(MonitorQMP *mon)
 {
     QList *cap_list = qlist_new();
     QObject *ver = NULL;
+    QDict *args;
     QMPCapability cap;
 
-    qmp_marshal_query_version(NULL, &ver, NULL);
+    args = qdict_new();
+    qmp_marshal_query_version(args, &ver, NULL);
+    qobject_unref(args);
 
     for (cap = 0; cap < QMP_CAPABILITY__MAX; cap++) {
         if (mon->capab_offered[cap]) {
diff --git a/scripts/qapi/commands.py b/scripts/qapi/commands.py
index bc30876c88..f545903567 100644
--- a/scripts/qapi/commands.py
+++ b/scripts/qapi/commands.py
@@ -104,6 +104,7 @@ def gen_marshal(name, arg_type, boxed, ret_type):
 %(proto)s
 {
     Error *err = NULL;
+    Visitor *v;
 ''',
                 proto=build_marshal_proto(name))
 
@@ -117,21 +118,14 @@ def gen_marshal(name, arg_type, boxed, ret_type):
         visit_members = ('visit_type_%s_members(v, &arg, &err);'
                          % arg_type.c_name())
         ret += mcgen('''
-    Visitor *v;
     %(c_name)s arg = {0};
-
 ''',
                      c_name=arg_type.c_name())
     else:
         visit_members = ''
-        ret += mcgen('''
-    Visitor *v = NULL;
-
-    if (args) {
-''')
-        push_indent()
 
     ret += mcgen('''
+
     v = qobject_input_visitor_new(QOBJECT(args));
     visit_start_struct(v, NULL, NULL, 0, &err);
     if (err) {
@@ -148,12 +142,6 @@ def gen_marshal(name, arg_type, boxed, ret_type):
 ''',
                  visit_members=visit_members)
 
-    if not have_args:
-        pop_indent()
-        ret += mcgen('''
-    }
-''')
-
     ret += gen_call(name, arg_type, boxed, ret_type)
 
     ret += mcgen('''
@@ -168,10 +156,6 @@ out:
                          % arg_type.c_name())
     else:
         visit_members = ''
-        ret += mcgen('''
-    if (args) {
-''')
-        push_indent()
 
     ret += mcgen('''
     v = qapi_dealloc_visitor_new();
@@ -182,12 +166,6 @@ out:
 ''',
                  visit_members=visit_members)
 
-    if not have_args:
-        pop_indent()
-        ret += mcgen('''
-    }
-''')
-
     ret += mcgen('''
 }
 ''')