qstring: Make conversion from QObject * accept null

qobject_to_qstring() crashes on null, which is a trap for the unwary.
Return null instead, and simplify a few callers.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1444918537-18107-7-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>

qapi/qmp-input-visitor.c

@@ -255,15 +255,15 @@ static void qmp_input_type_str(Visitor *v, char **obj, const char *name,
                                Error **errp)
 {
     QmpInputVisitor *qiv = to_qiv(v);
-    QObject *qobj = qmp_input_get_object(qiv, name, true);
+    QString *qstr = qobject_to_qstring(qmp_input_get_object(qiv, name, true));
 
-    if (!qobj || qobject_type(qobj) != QTYPE_QSTRING) {
+    if (!qstr) {
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
                    "string");
         return;
     }
 
-    *obj = g_strdup(qstring_get_str(qobject_to_qstring(qobj)));
+    *obj = g_strdup(qstring_get_str(qstr));
 }
 
 static void qmp_input_type_number(Visitor *v, double *obj, const char *name,

qobject/qdict.c

@@ -282,8 +282,7 @@ QDict *qdict_get_qdict(const QDict *qdict, const char *key)
  */
 const char *qdict_get_str(const QDict *qdict, const char *key)
 {
-    QObject *obj = qdict_get_obj(qdict, key, QTYPE_QSTRING);
-    return qstring_get_str(qobject_to_qstring(obj));
+    return qstring_get_str(qobject_to_qstring(qdict_get(qdict, key)));
 }
 
 /**
@@ -325,13 +324,9 @@ bool qdict_get_try_bool(const QDict *qdict, const char *key, bool def_value)
  */
 const char *qdict_get_try_str(const QDict *qdict, const char *key)
 {
-    QObject *obj;
-
-    obj = qdict_get(qdict, key);
-    if (!obj || qobject_type(obj) != QTYPE_QSTRING)
-        return NULL;
+    QString *qstr = qobject_to_qstring(qdict_get(qdict, key));
 
-    return qstring_get_str(qobject_to_qstring(obj));
+    return qstr ? qstring_get_str(qstr) : NULL;
 }
 
 /**

qobject/qstring.c

@@ -117,9 +117,9 @@ void qstring_append_chr(QString *qstring, int c)
  */
 QString *qobject_to_qstring(const QObject *obj)
 {
-    if (qobject_type(obj) != QTYPE_QSTRING)
+    if (!obj || qobject_type(obj) != QTYPE_QSTRING) {
         return NULL;
-
+    }
     return container_of(obj, QString, base);
 }