unicorn/qemu/include/qapi/clone-visitor.h

/*
 * Clone Visitor
 *
 * Copyright (C) 2016 Red Hat, Inc.
 *
 * This work is licensed under the terms of the GNU GPL, version 2 or later.
 * See the COPYING file in the top-level directory.
 *
 */

#ifndef QAPI_CLONE_VISITOR_H
#define QAPI_CLONE_VISITOR_H

#include "qapi/visitor.h"

/*
 * The clone visitor is for direct use only by the QAPI_CLONE() macro;
 * it requires that the root visit occur on an object, list, or
 * alternate, and is not usable directly on built-in QAPI types.
 */
typedef struct QapiCloneVisitor QapiCloneVisitor;

void *qapi_clone(const void *src, void (*visit_type)(Visitor *, const char *,
                                                     void **, Error **));

/*
 * Deep-clone QAPI object @src of the given @type, and return the result.
 *
 * Not usable on QAPI scalars (integers, strings, enums), nor on a
 * QAPI object that references the 'any' type.  Safe when @src is NULL.
 */
#define QAPI_CLONE(type, src)                                           \
    ((type *)qapi_clone(src,                                            \
                        (void (*)(Visitor *, const char *, void**,      \
                                  Error **))visit_type_ ## type))

#endif
qapi: Add new clone visitor We have a couple places in the code base that want to deep-clone one QAPI object into another, and they were resorting to serializing the struct out to QObject then reparsing it. A much more efficient version can be done by adding a new clone visitor. Since cloning is still relatively uncommon, expose the use of the new visitor via a QAPI_CLONE() macro that takes care of type-punning the underlying function pointer, rather than generating lots of unused functions for types that won't be cloned. And yes, we're relying on the compiler treating all pointers equally, even though a strict C program cannot portably do so - but we're not the first one in the qemu code base to expect it to work (hello, glib!). The choice of adding a fourth visitor type deserves some explanation. On the surface, the clone visitor is mostly an input visitor (it takes arbitrary input - in this case, another QAPI object - and creates a new QAPI object during the course of the visit). But ever since commit da72ab0 consolidated enum visits based on the visitor type, using VISITOR_INPUT would cause us to run visit_type_str(), even though for cloning there is nothing to do (we just copy the enum value across, without regards to its mapping to strings). Also, since our input happens to be a QAPI object, we can also satisfy the internal checks for VISITOR_OUTPUT. So in the end, I settled with a new VISITOR_CLONE, and chose its value such that many internal checks can use 'v->type & mask', sticking to 'v->type == value' where the difference matters. Note that we can only clone objects (including alternates) and lists, not built-ins or enums. The visitor core hides integer width from the actual visitor (since commit 04e070d), and as long as that's the case, we can't clone top-level integers. Then again, those can always be cloned by direct copy, since they are not objects with deep pointers, so it's no real loss. And restricting cloning to just objects and lists is cleaner than restricting it to non-integers. As such, I documented that the clone visitor is for direct use only by code internal to QAPI, and should not be used on incomplete objects (other than a hack to work around the fact that we allow NULL in place of "" in visit_type_str() in other output visitors). Note that as written, the clone visitor will never fail on a complete object. Scalars (including enums) not at the root of the clone copy just fine with no additional effort while visiting the scalar, by virtue of a g_memdup() each time we push another struct onto the stack. Cloning a string requires deduplication of a pointer, which means it can also provide the guarantee of an input visitor of never producing NULL even when still accepting NULL in place of "" the way the QMP output visitor does. Cloning an 'any' type could be possible by incrementing the QObject refcnt, but it's not obvious whether that is better than implementing a QObject deep clone. So for now, we document it as unsupported, and intentionally omit the .type_any() callback to let a developer know their usage needs implementation. Add testsuite coverage for several different clone situations, to ensure that the code is working. I also tested that valgrind was happy with the test. Backports commit a15fcc3cf69ee3d408f60d6cc316488d2b0249b4 from qemu 2018-02-25 06:27:18 +00:00			`/*`
			`* Clone Visitor`
			`*`
			`* Copyright (C) 2016 Red Hat, Inc.`
			`*`
			`* This work is licensed under the terms of the GNU GPL, version 2 or later.`
			`* See the COPYING file in the top-level directory.`
			`*`
			`*/`

			`#ifndef QAPI_CLONE_VISITOR_H`
			`#define QAPI_CLONE_VISITOR_H`

			`#include "qapi/visitor.h"`

			`/*`
			`* The clone visitor is for direct use only by the QAPI_CLONE() macro;`
			`* it requires that the root visit occur on an object, list, or`
			`* alternate, and is not usable directly on built-in QAPI types.`
			`*/`
			`typedef struct QapiCloneVisitor QapiCloneVisitor;`

			`void qapi_clone(const void src, void (visit_type)(Visitor , const char *,`
			`void , Error ));`

			`/*`
			`* Deep-clone QAPI object @src of the given @type, and return the result.`
			`*`
			`* Not usable on QAPI scalars (integers, strings, enums), nor on a`
			`* QAPI object that references the 'any' type. Safe when @src is NULL.`
			`*/`
			`#define QAPI_CLONE(type, src) \`
			`((type *)qapi_clone(src, \`
			`(void ()(Visitor , const char , void*, \`
			`Error **))visit_type_ ## type))`

			`#endif`