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qapi: Add new visit_complete() function
Making each output visitor provide its own output collection function was the only remaining reason for exposing visitor sub-types to the rest of the code base. Add a polymorphic visit_complete() function which is a no-op for input visitors, and which populates an opaque pointer for output visitors. For maximum type-safety, also add a parameter to the output visitor constructors with a type-correct version of the output pointer, and assert that the two uses match. This approach was considered superior to either passing the output parameter only during construction (action at a distance during visit_free() feels awkward) or only during visit_complete() (defeating type safety makes it easier to use incorrectly). Most callers were function-local, and therefore a mechanical conversion; the testsuite was a bit trickier, but the previous cleanup patch minimized the churn here. The visit_complete() function may be called at most once; doing so lets us use transfer semantics rather than duplication or ref-count semantics to get the just-built output back to the caller, even though it means our behavior is not idempotent. Generated code is simplified as follows for events: |@@ -26,7 +26,7 @@ void qapi_event_send_acpi_device_ost(ACP | QDict *qmp; | Error *err = NULL; | QMPEventFuncEmit emit; |- QmpOutputVisitor *qov; |+ QObject *obj; | Visitor *v; | q_obj_ACPI_DEVICE_OST_arg param = { | info |@@ -39,8 +39,7 @@ void qapi_event_send_acpi_device_ost(ACP | | qmp = qmp_event_build_dict("ACPI_DEVICE_OST"); | |- qov = qmp_output_visitor_new(); |- v = qmp_output_get_visitor(qov); |+ v = qmp_output_visitor_new(&obj); | | visit_start_struct(v, "ACPI_DEVICE_OST", NULL, 0, &err); | if (err) { |@@ -55,7 +54,8 @@ void qapi_event_send_acpi_device_ost(ACP | goto out; | } | |- qdict_put_obj(qmp, "data", qmp_output_get_qobject(qov)); |+ visit_complete(v, &obj); |+ qdict_put_obj(qmp, "data", obj); | emit(QAPI_EVENT_ACPI_DEVICE_OST, qmp, &err); and for commands: | { | Error *err = NULL; |- QmpOutputVisitor *qov = qmp_output_visitor_new(); | Visitor *v; | |- v = qmp_output_get_visitor(qov); |+ v = qmp_output_visitor_new(ret_out); | visit_type_AddfdInfo(v, "unused", &ret_in, &err); |- if (err) { |- goto out; |+ if (!err) { |+ visit_complete(v, ret_out); | } |- *ret_out = qmp_output_get_qobject(qov); |- |-out: | error_propagate(errp, err); Backports commit 3b098d56979d2f7fd707c5be85555d114353a28d from qemu
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@ -19,9 +19,12 @@
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typedef struct QmpOutputVisitor QmpOutputVisitor;
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QmpOutputVisitor *qmp_output_visitor_new(void);
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QObject *qmp_output_get_qobject(QmpOutputVisitor *v);
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Visitor *qmp_output_get_visitor(QmpOutputVisitor *v);
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/*
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* Create a new QMP output visitor.
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*
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* If everything else succeeds, pass @result to visit_complete() to
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* collect the result of the visit.
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*/
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Visitor *qmp_output_visitor_new(QObject **result);
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#endif
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@ -106,6 +106,9 @@ struct Visitor
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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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@ -41,21 +41,15 @@
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*
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* All of the visitors are created via:
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*
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* Type *subtype_visitor_new(parameters...);
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*
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* where Type is either directly 'Visitor *', or is a subtype that can
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* be trivially upcast to Visitor * via another function:
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*
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* Visitor *subtype_get_visitor(SubtypeVisitor *);
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* Visitor *subtype_visitor_new(parameters...);
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*
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* A visitor should be used for exactly one top-level visit_type_FOO()
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* or virtual walk, then passed to visit_free() to clean up resources.
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* or virtual walk; if that is successful, the caller can optionally
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* call visit_complete() (for now, useful only for output visits, but
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* safe to call on all visits). Then, regardless of success or
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* failure, the user should call visit_free() to clean up resources.
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* It is okay to free the visitor without completing the visit, if
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* some other error is detected in the meantime. Output visitors
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* provide an additional function, for collecting the final results of
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* a successful visit: string_output_get_string() and
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* qmp_output_get_qobject(); this collection function should not be
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* called if any errors were reported during the visit.
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* some other error is detected in the meantime.
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*
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* All QAPI types have a corresponding function with a signature
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* roughly compatible with this:
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@ -125,14 +119,14 @@
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* Error *err = NULL;
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* Visitor *v;
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*
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* v = ...obtain input visitor...
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* v = FOO_visitor_new(...);
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* visit_type_Foo(v, NULL, &f, &err);
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* if (err) {
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* ...handle error...
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* } else {
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* ...use f...
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* }
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* ...clean up v...
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* visit_free(v);
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* qapi_free_Foo(f);
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* </example>
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*
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@ -142,7 +136,7 @@
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* Error *err = NULL;
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* Visitor *v;
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*
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* v = ...obtain input visitor...
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* v = FOO_visitor_new(...);
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* visit_type_FooList(v, NULL, &l, &err);
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* if (err) {
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* ...handle error...
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@ -151,7 +145,7 @@
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* ...use l->value...
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* }
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* }
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* ...clean up v...
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* visit_free(v);
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* qapi_free_FooList(l);
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* </example>
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*
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@ -161,13 +155,17 @@
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* Foo *f = ...obtain populated object...
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* Error *err = NULL;
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* Visitor *v;
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* Type *result;
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*
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* v = ...obtain output visitor...
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* v = FOO_visitor_new(..., &result);
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* visit_type_Foo(v, NULL, &f, &err);
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* if (err) {
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* ...handle error...
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* } else {
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* visit_complete(v, &result);
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* ...use result...
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* }
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* ...clean up v...
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* visit_free(v);
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* </example>
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*
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* When visiting a real QAPI struct, this file provides several
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@ -193,7 +191,7 @@
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* Error *err = NULL;
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* int value;
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*
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* v = ...obtain visitor...
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* v = FOO_visitor_new(...);
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* visit_start_struct(v, NULL, NULL, 0, &err);
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* if (err) {
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* goto out;
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@ -221,7 +219,7 @@
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* visit_end_struct(v, NULL);
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* out:
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* error_propagate(errp, err);
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* ...clean up v...
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* visit_free(v);
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* </example>
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*/
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@ -245,6 +243,18 @@ typedef struct GenericAlternate {
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/*** Visitor cleanup ***/
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/*
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* Complete the visit, collecting any output.
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*
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* May only be called only once after a successful top-level
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* visit_type_FOO() or visit_end_ITEM(), and marks the end of the
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* visit. The @opaque pointer should match the output parameter
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* passed to the subtype_visitor_new() used to create an output
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* visitor, or NULL for any other visitor. Needed for output
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* visitors, but may also be called with other visitors.
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*/
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void visit_complete(Visitor *v, void *opaque);
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/*
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* Free @v and any resources it has tied up.
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*
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@ -21,6 +21,14 @@
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#include "qapi/visitor.h"
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#include "qapi/visitor-impl.h"
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void visit_complete(Visitor *v, void *opaque)
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{
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assert(v->type != VISITOR_OUTPUT || v->complete);
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if (v->complete) {
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v->complete(v, opaque);
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}
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}
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void visit_free(Visitor *v)
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{
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if (v) {
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@ -34,6 +34,7 @@ struct QmpOutputVisitor
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Visitor visitor;
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QStack stack; /* Stack of containers that haven't yet been finished */
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QObject *root; /* Root of the output visit */
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QObject **result; /* User's storage location for result */
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};
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#define qmp_output_add(qov, name, value) \
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/* Finish building, and return the root object.
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* The root object is never null. The caller becomes the object's
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* owner, and should use qobject_decref() when done with it. */
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QObject *qmp_output_get_qobject(QmpOutputVisitor *qov)
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static void qmp_output_complete(Visitor *v, void *opaque)
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{
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QmpOutputVisitor *qov = to_qov(v);
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/* A visit must have occurred, with each start paired with end. */
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assert(qov->root && QTAILQ_EMPTY(&qov->stack));
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assert(opaque == qov->result);
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qobject_incref(qov->root);
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return qov->root;
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}
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Visitor *qmp_output_get_visitor(QmpOutputVisitor *v)
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{
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return &v->visitor;
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*qov->result = qov->root;
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qov->result = NULL;
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}
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static void qmp_output_free(Visitor *v)
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g_free(qov);
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}
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QmpOutputVisitor *qmp_output_visitor_new(void)
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Visitor *qmp_output_visitor_new(QObject **result)
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{
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QmpOutputVisitor *v;
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v->visitor.type_number = qmp_output_type_number;
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v->visitor.type_any = qmp_output_type_any;
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v->visitor.type_null = qmp_output_type_null;
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v->visitor.complete = qmp_output_complete;
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v->visitor.free = qmp_output_free;
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QTAILQ_INIT(&v->stack);
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*result = NULL;
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v->result = result;
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return v;
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return &v->visitor;
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}
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@ -33,14 +33,14 @@ QObject *object_property_get_qobject(struct uc_struct *uc, Object *obj, const ch
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{
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QObject *ret = NULL;
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Error *local_err = NULL;
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QmpOutputVisitor *qov;
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Visitor *v;
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qov = qmp_output_visitor_new();
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object_property_get(uc, obj, qmp_output_get_visitor(qov), name, &local_err);
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v = qmp_output_visitor_new(&ret);
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object_property_get(uc, obj, v, name, &local_err);
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if (!local_err) {
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ret = qmp_output_get_qobject(qov);
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visit_complete(v, &ret);
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}
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error_propagate(errp, local_err);
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visit_free(qmp_output_get_visitor(qov));
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visit_free(v);
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return ret;
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}
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@ -71,7 +71,7 @@ def gen_event_send(name, arg_type):
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if arg_type and arg_type.members:
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assert not arg_type.variants
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ret += mcgen('''
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QmpOutputVisitor *qov;
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QObject *obj;
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Visitor *v;
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''')
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ret += gen_param_var(arg_type)
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if arg_type and arg_type.members:
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ret += mcgen('''
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qov = qmp_output_visitor_new();
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v = qmp_output_get_visitor(qov);
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v = qmp_output_visitor_new(&obj);
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visit_start_struct(v, "%(name)s", NULL, 0, &err);
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if (err) {
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goto out;
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}
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qdict_put_obj(qmp, "data", qmp_output_get_qobject(qov));
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visit_complete(v, &obj);
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qdict_put_obj(qmp, "data", obj);
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''',
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name=name, c_name=arg_type.c_name())
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