QMP: Introduce qmp_check_input_obj()

This is similar to qmp_check_client_args(), but it checks if
the input object follows the specification (QMP/qmp-spec.txt
section 2.3).

As we're limited to three keys, the work here is quite simple:
we iterate over the input object, checking each time if the
current argument complies to the specification.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -4169,6 +4169,62 @@ out:
     return err;
 }
 
+/*
+ * Input object checking rules
+ *
+ * 1. Input object must be a dict
+ * 2. The "execute" key must exist
+ * 3. The "execute" key must be a string
+ * 4. If the "arguments" key exists, it must be a dict
+ * 5. If the "id" key exists, it can be anything (ie. json-value)
+ * 6. Any argument not listed above is considered invalid
+ */
+static QDict *qmp_check_input_obj(QObject *input_obj)
+{
+    const QDictEntry *ent;
+    int has_exec_key = 0;
+    QDict *input_dict;
+
+    if (qobject_type(input_obj) != QTYPE_QDICT) {
+        qerror_report(QERR_QMP_BAD_INPUT_OBJECT, "object");
+        return NULL;
+    }
+
+    input_dict = qobject_to_qdict(input_obj);
+
+    for (ent = qdict_first(input_dict); ent; ent = qdict_next(input_dict, ent)){
+        const char *arg_name = qdict_entry_key(ent);
+        const QObject *arg_obj = qdict_entry_value(ent);
+
+        if (!strcmp(arg_name, "execute")) {
+            if (qobject_type(arg_obj) != QTYPE_QSTRING) {
+                qerror_report(QERR_QMP_BAD_INPUT_OBJECT_MEMBER, "execute",
+                              "string");
+                return NULL;
+            }
+            has_exec_key = 1;
+        } else if (!strcmp(arg_name, "arguments")) {
+            if (qobject_type(arg_obj) != QTYPE_QDICT) {
+                qerror_report(QERR_QMP_BAD_INPUT_OBJECT_MEMBER, "arguments",
+                              "object");
+                return NULL;
+            }
+        } else if (!strcmp(arg_name, "id")) {
+            /* FIXME: check duplicated IDs for async commands */
+        } else {
+            qerror_report(QERR_QMP_EXTRA_MEMBER, arg_name);
+            return NULL;
+        }
+    }
+
+    if (!has_exec_key) {
+        qerror_report(QERR_QMP_BAD_INPUT_OBJECT, "execute");
+        return NULL;
+    }
+
+    return input_dict;
+}
+
 static void handle_qmp_command(JSONMessageParser *parser, QList *tokens)
 {
     int err;
@@ -4191,7 +4247,11 @@ static void handle_qmp_command(JSONMessageParser *parser, QList *tokens)
         goto err_out;
     }
 
-    input = qobject_to_qdict(obj);
+    input = qmp_check_input_obj(obj);
+    if (!input) {
+        qobject_decref(obj);
+        goto err_out;
+    }
 
     mon->mc->id = qdict_get(input, "id");
     qobject_incref(mon->mc->id);