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Tue, 16 Feb 2021 12:56:00 +0100 (CET) From: Markus Armbruster To: Kevin Wolf Subject: Re: [PATCH v2 1/6] qapi: Add interfaces for alias support to Visitor References: <20210211183118.422036-1-kwolf@redhat.com> <20210211183118.422036-2-kwolf@redhat.com> Date: Tue, 16 Feb 2021 12:56:00 +0100 In-Reply-To: <20210211183118.422036-2-kwolf@redhat.com> (Kevin Wolf's message of "Thu, 11 Feb 2021 19:31:13 +0100") Message-ID: <874kicmdkf.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=63.128.21.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: jsnow@redhat.com, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kevin Wolf writes: > This adds functions to the Visitor interface that can be used to define > aliases and alias scopes. > > Signed-off-by: Kevin Wolf > --- > include/qapi/visitor-impl.h | 12 ++++++++++ > include/qapi/visitor.h | 44 +++++++++++++++++++++++++++++++++++++ > qapi/qapi-visit-core.c | 22 +++++++++++++++++++ > 3 files changed, 78 insertions(+) > > diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h > index 7362c043be..d9a6874528 100644 > --- a/include/qapi/visitor-impl.h > +++ b/include/qapi/visitor-impl.h > @@ -113,6 +113,18 @@ struct Visitor > The core takes care of the return type in the public interface. */ > void (*optional)(Visitor *v, const char *name, bool *present); > > + /* > + * Optional; intended for input visitors. If not given, aliases are > + * ignored. > + */ > + void (*define_alias)(Visitor *v, const char *name, const char **source); > + > + /* Must be set if define_alias is set */ > + void (*start_alias_scope)(Visitor *v); > + > + /* Must be set if define_alias is set */ > + void (*end_alias_scope)(Visitor *v); > + > /* Must be set */ > VisitorType type; > > diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h > index ebc19ede7f..2ecbc20624 100644 > --- a/include/qapi/visitor.h > +++ b/include/qapi/visitor.h Still missing: an update of the big comment. That's okay, we can do that last. > @@ -459,6 +459,50 @@ void visit_end_alternate(Visitor *v, void **obj); > */ > bool visit_optional(Visitor *v, const char *name, bool *present); > > +/* > + * Defines a new alias rule. > + * > + * If @name is non-NULL, the member called @name in the external > + * representation of the currently visited object is defined as an > + * alias for the member described by @source. It is not allowed to > + * call this function when the currently visited type is not an > + * object. > + * > + * If @name is NULL, all members of the object described by @source > + * are considered to have alias members with the same key in the > + * currently visited object. > + * > + * @source is a NULL-terminated non-empty array of names that describe > + * the path to a member, starting from the currently visited object. > + * All elements in @source except the last one should describe > + * objects. If an intermediate element refers to a member with a > + * non-object type, the alias won't work (this case can legitimately > + * happen in unions where an alias only makes sense for one branch, > + * but not for another). A sufficiently paranoid reader will now realize that the system won't catch mistakes. I'm not objecting to that; I understand distinguishing aliases that may work (just not now) from aliases that can't work is non-trivial and quite probably not worth it. I'm just wondering whether we should be even more explicit, to help insufficiently paranoid readers. Here's a possible argument against: we expect this function to be used only by generated code, and the maintainers of the generator are expected to be sufficiently paranoid. The same issue might exist in qapi-code-gen.txt. > + * > + * The alias stays valid until the current alias scope ends. > + * visit_start/end_struct() implicitly start/end an alias scope. > + * Additionally, visit_start/end_alias_scope() can be used to explicitly > + * create a nested alias scope. > + */ > +void visit_define_alias(Visitor *v, const char *name, const char **source); > + > +/* > + * Begins an explicit alias scope. > + * > + * Alias definitions after here will only stay valid until the > + * corresponding visit_end_alias_scope() is called. > + */ > +void visit_start_alias_scope(Visitor *v); > + > +/* > + * Ends an explicit alias scope. > + * > + * Alias definitions between the correspoding visit_start_alias_scope() > + * call and here go out of scope and won't apply in later code any more. > + */ > +void visit_end_alias_scope(Visitor *v); > + > /* > * Visit an enum value. > * > diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c > index 7e5f40e7f0..651dd88e02 100644 > --- a/qapi/qapi-visit-core.c > +++ b/qapi/qapi-visit-core.c > @@ -135,6 +135,28 @@ bool visit_optional(Visitor *v, const char *name, bool *present) > return *present; > } > > +void visit_define_alias(Visitor *v, const char *name, const char **source) > +{ > + assert(source[0] != NULL); > + if (v->define_alias) { > + v->define_alias(v, name, source); > + } > +} > + > +void visit_start_alias_scope(Visitor *v) > +{ > + if (v->start_alias_scope) { > + v->start_alias_scope(v); > + } > +} > + > +void visit_end_alias_scope(Visitor *v) > +{ > + if (v->end_alias_scope) { > + v->end_alias_scope(v); > + } > +} > + > bool visit_is_input(Visitor *v) > { > return v->type == VISITOR_INPUT;