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>
43 lines
1.3 KiB
C
43 lines
1.3 KiB
C
/*
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* Options Visitor
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*
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* Copyright Red Hat, Inc. 2012
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*
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* Author: Laszlo Ersek <lersek@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 OPTS_VISITOR_H
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#define OPTS_VISITOR_H
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#include "qapi/visitor.h"
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#include "qemu/option.h"
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/* Inclusive upper bound on the size of any flattened range. This is a safety
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* (= anti-annoyance) measure; wrong ranges should not cause long startup
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* delays nor exhaust virtual memory.
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*/
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#define OPTS_VISITOR_RANGE_MAX 65536
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typedef struct OptsVisitor OptsVisitor;
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/* Contrarily to qemu-option.c::parse_option_number(), OptsVisitor's "int"
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* parser relies on strtoll() instead of strtoull(). Consequences:
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* - string representations of negative numbers yield negative values,
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* - values below INT64_MIN or LLONG_MIN are rejected,
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* - values above INT64_MAX or LLONG_MAX are rejected.
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*
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* The Opts input visitor does not implement support for visiting QAPI
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* alternates, numbers (other than integers), null, or arbitrary
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* QTypes. It also requires a non-null list argument to
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* visit_start_list().
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*/
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OptsVisitor *opts_visitor_new(const QemuOpts *opts);
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void opts_visitor_cleanup(OptsVisitor *nv);
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Visitor *opts_get_visitor(OptsVisitor *nv);
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#endif
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