The semantics of the list visit are somewhat baroque, with the following pseudocode when FooList is used: start() for (prev = head; cur = next(prev); prev = &cur) { visit(&cur->value) } Note that these semantics (advance before visit) requires that the first call to next() return the list head, while all other calls return the next element of the list; that is, every visitor implementation is required to track extra state to decide whether to return the input as-is, or to advance. It also requires an argument of 'GenericList **' to next(), solely because the first iteration might need to modify the caller's GenericList head, so that all other calls have to do a layer of dereferencing. Thankfully, we only have two uses of list visits in the entire code base: one in spapr_drc (which completely avoids visit_next_list(), feeding in integers from a different source than uint8List), and one in qapi-visit.py. That is, all other list visitors are generated in qapi-visit.c, and share the same paradigm based on a qapi FooList type, so we can refactor how lists are laid out with minimal churn among clients. We can greatly simplify things by hoisting the special case into the start() routine, and flipping the order in the loop to visit before advance: start(head) for (tail = *head; tail; tail = next(tail)) { visit(&tail->value) } With the simpler semantics, visitors have less state to track, the argument to next() is reduced to 'GenericList *', and it also becomes obvious whether an input visitor is allocating a FooList during visit_start_list() (rather than the old way of not knowing if an allocation happened until the first visit_next_list()). As a minor drawback, we now allocate in two functions instead of one, and have to pass the size to both functions (unless we were to tweak the input visitors to cache the size to start_list for reuse during next_list, but that defeats the goal of less visitor state). The signature of visit_start_list() is chosen to match visit_start_struct(), with the new parameters after 'name'. The spapr_drc case is a virtual visit, done by passing NULL for list, similarly to how NULL is passed to visit_start_struct() when a qapi type is not used in those visits. It was easy to provide these semantics for qmp-output and dealloc visitors, and a bit harder for qmp-input (several prerequisite patches refactored things to make this patch straightforward). But it turned out that the string and opts visitors munge enough other state during visit_next_list() to make it easier to just document and require a GenericList visit for now; an assertion will remind us to adjust things if we need the semantics in the future. Several pre-requisite cleanup patches made the reshuffling of the various visitors easier; particularly the qmp input visitor. Signed-off-by: Eric Blake <eblake@redhat.com> Message-Id: <1461879932-9020-24-git-send-email-eblake@redhat.com> Signed-off-by: Markus Armbruster <armbru@redhat.com>
110 lines
3.5 KiB
C
110 lines
3.5 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/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 three 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.
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*/
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typedef enum VisitorType {
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VISITOR_INPUT,
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VISITOR_OUTPUT,
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VISITOR_DEALLOC,
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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);
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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);
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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);
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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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};
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#endif
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