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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
120 lines
4 KiB
C
120 lines
4 KiB
C
/*
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* Core Definitions for QAPI Visitor implementations
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*
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* Copyright (C) 2012-2016 Red Hat, Inc.
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*
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* Author: Paolo Bonizni <pbonzini@redhat.com>
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*
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* This work is licensed under the terms of the GNU LGPL, version 2.1 or later.
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* See the COPYING.LIB file in the top-level directory.
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*
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*/
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#ifndef QAPI_VISITOR_IMPL_H
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#define QAPI_VISITOR_IMPL_H
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#include "qapi/error.h"
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#include "qapi/visitor.h"
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/*
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* This file describes the callback interface for implementing a QAPI
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* visitor. For the client interface, see visitor.h. When
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* implementing the callbacks, it is easiest to declare a struct with
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* 'Visitor visitor;' as the first member. A callback's contract
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* matches the corresponding public functions' contract unless stated
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* otherwise. In the comments below, some callbacks are marked "must
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* be set for $TYPE visits to work"; if a visitor implementation omits
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* that callback, it should also document that it is only useful for a
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* subset of QAPI.
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*/
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/*
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* There are four classes of visitors; setting the class determines
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* how QAPI enums are visited, as well as what additional restrictions
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* can be asserted. The values are intentionally chosen so as to
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* permit some assertions based on whether a given bit is set (that
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* is, some assertions apply to input and clone visitors, some
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* assertions apply to output and clone visitors).
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*/
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typedef enum VisitorType {
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VISITOR_INPUT = 1,
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VISITOR_OUTPUT = 2,
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VISITOR_CLONE = 3,
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VISITOR_DEALLOC = 4,
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} VisitorType;
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struct Visitor
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{
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/* Must be set to visit structs */
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void (*start_struct)(Visitor *v, const char *name, void **obj,
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size_t size, Error **errp);
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/* Optional; intended for input visitors */
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void (*check_struct)(Visitor *v, Error **errp);
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/* Must be set to visit structs */
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void (*end_struct)(Visitor *v, void **obj);
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/* Must be set; implementations may require @list to be non-null,
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* but must document it. */
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void (*start_list)(Visitor *v, const char *name, GenericList **list,
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size_t size, Error **errp);
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/* Must be set */
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GenericList *(*next_list)(Visitor *v, GenericList *tail, size_t size);
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/* Must be set */
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void (*end_list)(Visitor *v, void **list);
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/* Must be set by input and dealloc visitors to visit alternates;
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* optional for output visitors. */
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void (*start_alternate)(Visitor *v, const char *name,
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GenericAlternate **obj, size_t size,
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bool promote_int, Error **errp);
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/* Optional, needed for dealloc visitor */
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void (*end_alternate)(Visitor *v, void **obj);
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/* Must be set */
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void (*type_int64)(Visitor *v, const char *name, int64_t *obj,
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Error **errp);
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/* Must be set */
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void (*type_uint64)(Visitor *v, const char *name, uint64_t *obj,
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Error **errp);
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/* Optional; fallback is type_uint64() */
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void (*type_size)(Visitor *v, const char *name, uint64_t *obj,
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Error **errp);
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/* Must be set */
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void (*type_bool)(Visitor *v, const char *name, bool *obj, Error **errp);
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/* Must be set */
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void (*type_str)(Visitor *v, const char *name, char **obj, Error **errp);
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/* Must be set to visit numbers */
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void (*type_number)(Visitor *v, const char *name, double *obj,
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Error **errp);
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/* Must be set to visit arbitrary QTypes */
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void (*type_any)(Visitor *v, const char *name, QObject **obj,
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Error **errp);
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/* Must be set to visit explicit null values. */
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void (*type_null)(Visitor *v, const char *name, Error **errp);
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/* Must be set for input visitors, optional otherwise. The core
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* takes care of the return type in the public interface. */
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void (*optional)(Visitor *v, const char *name, bool *present);
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/* Must be set */
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VisitorType type;
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/* Must be set for output visitors, optional otherwise. */
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void (*complete)(Visitor *v, void *opaque);
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/* Must be set */
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void (*free)(Visitor *v);
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};
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#endif
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