qapi: New QAPI_CLONE_MEMBERS()

QAPI_CLONE() returns a newly allocated QAPI object.  Inconvenient when
we want to clone into an existing object.  QAPI_CLONE_MEMBERS() does
exactly that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1493192202-3184-4-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/qapi/clone-visitor.h

@@ -24,6 +24,9 @@ typedef struct QapiCloneVisitor QapiCloneVisitor;
 
 void *qapi_clone(const void *src, void (*visit_type)(Visitor *, const char *,
                                                      void **, Error **));
+void qapi_clone_members(void *dst, const void *src, size_t sz,
+                        void (*visit_type_members)(Visitor *, void *,
+                                                   Error **));
 
 /*
  * Deep-clone QAPI object @src of the given @type, and return the result.
@@ -36,4 +39,15 @@ void *qapi_clone(const void *src, void (*visit_type)(Visitor *, const char *,
                         (void (*)(Visitor *, const char *, void**,      \
                                   Error **))visit_type_ ## type))
 
+/*
+ * Copy deep clones of @type members from @src to @dst.
+ *
+ * Not usable on QAPI scalars (integers, strings, enums), nor on a
+ * QAPI object that references the 'any' type.
+ */
+#define QAPI_CLONE_MEMBERS(type, dst, src)                              \
+    qapi_clone_members(dst, src, sizeof(type),                          \
+                       (void (*)(Visitor *, void *,                     \
+                                 Error **))visit_type_ ## type ## _members)
+
 #endif

qapi/qapi-clone-visitor.c

@@ -180,3 +180,16 @@ void *qapi_clone(const void *src, void (*visit_type)(Visitor *, const char *,
     visit_free(v);
     return dst;
 }
+
+void qapi_clone_members(void *dst, const void *src, size_t sz,
+                        void (*visit_type_members)(Visitor *, void *,
+                                                   Error **))
+{
+    Visitor *v;
+
+    v = qapi_clone_visitor_new();
+    memcpy(dst, src, sz);
+    to_qcv(v)->depth++;
+    visit_type_members(v, dst, &error_abort);
+    visit_free(v);
+}