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

qemu/include/qapi/qmp-output-visitor.h

@@ -19,9 +19,12 @@
 
 typedef struct QmpOutputVisitor QmpOutputVisitor;
 
-QmpOutputVisitor *qmp_output_visitor_new(void);
-
-QObject *qmp_output_get_qobject(QmpOutputVisitor *v);
-Visitor *qmp_output_get_visitor(QmpOutputVisitor *v);
+/*
+ * Create a new QMP output visitor.
+ *
+ * If everything else succeeds, pass @result to visit_complete() to
+ * collect the result of the visit.
+ */
+Visitor *qmp_output_visitor_new(QObject **result);
 
 #endif

qemu/include/qapi/visitor-impl.h

@@ -106,6 +106,9 @@ struct Visitor
     /* Must be set */
     VisitorType type;
 
+    /* Must be set for output visitors, optional otherwise. */
+    void (*complete)(Visitor *v, void *opaque);
+
     /* Must be set */
     void (*free)(Visitor *v);
 };

qemu/include/qapi/visitor.h

@@ -41,21 +41,15 @@
  *
  * All of the visitors are created via:
  *
- * Type *subtype_visitor_new(parameters...);
- *
- * where Type is either directly 'Visitor *', or is a subtype that can
- * be trivially upcast to Visitor * via another function:
- *
- * Visitor *subtype_get_visitor(SubtypeVisitor *);
+ * Visitor *subtype_visitor_new(parameters...);
  *
  * A visitor should be used for exactly one top-level visit_type_FOO()
- * or virtual walk, then passed to visit_free() to clean up resources.
+ * or virtual walk; if that is successful, the caller can optionally
+ * call visit_complete() (for now, useful only for output visits, but
+ * safe to call on all visits).  Then, regardless of success or
+ * failure, the user should call visit_free() to clean up resources.
  * It is okay to free the visitor without completing the visit, if
- * some other error is detected in the meantime.  Output visitors
- * provide an additional function, for collecting the final results of
- * a successful visit: string_output_get_string() and
- * qmp_output_get_qobject(); this collection function should not be
- * called if any errors were reported during the visit.
+ * some other error is detected in the meantime.
  *
  * All QAPI types have a corresponding function with a signature
  * roughly compatible with this:
@@ -125,14 +119,14 @@
  *  Error *err = NULL;
  *  Visitor *v;
  *
- *  v = ...obtain input visitor...
+ *  v = FOO_visitor_new(...);
  *  visit_type_Foo(v, NULL, &f, &err);
  *  if (err) {
  *      ...handle error...
  *  } else {
  *      ...use f...
  *  }
- *  ...clean up v...
+ *  visit_free(v);
  *  qapi_free_Foo(f);
  * </example>
  *
@@ -142,7 +136,7 @@
  *  Error *err = NULL;
  *  Visitor *v;
  *
- *  v = ...obtain input visitor...
+ *  v = FOO_visitor_new(...);
  *  visit_type_FooList(v, NULL, &l, &err);
  *  if (err) {
  *      ...handle error...
@@ -151,7 +145,7 @@
  *          ...use l->value...
  *      }
  *  }
- *  ...clean up v...
+ *  visit_free(v);
  *  qapi_free_FooList(l);
  * </example>
  *
@@ -161,13 +155,17 @@
  *  Foo *f = ...obtain populated object...
  *  Error *err = NULL;
  *  Visitor *v;
+ *  Type *result;
  *
- *  v = ...obtain output visitor...
+ *  v = FOO_visitor_new(..., &result);
  *  visit_type_Foo(v, NULL, &f, &err);
  *  if (err) {
  *      ...handle error...
+ *  } else {
+ *      visit_complete(v, &result);
+ *      ...use result...
  *  }
- *  ...clean up v...
+ *  visit_free(v);
  * </example>
  *
  * When visiting a real QAPI struct, this file provides several
@@ -193,7 +191,7 @@
  *  Error *err = NULL;
  *  int value;
  *
- *  v = ...obtain visitor...
+ *  v = FOO_visitor_new(...);
  *  visit_start_struct(v, NULL, NULL, 0, &err);
  *  if (err) {
  *      goto out;
@@ -221,7 +219,7 @@
  *  visit_end_struct(v, NULL);
  * out:
  *  error_propagate(errp, err);
- *  ...clean up v...
+ *  visit_free(v);
  * </example>
  */
 
@@ -245,6 +243,18 @@ typedef struct GenericAlternate {
 
 /*** Visitor cleanup ***/
 
+/*
+ * Complete the visit, collecting any output.
+ *
+ * May only be called only once after a successful top-level
+ * visit_type_FOO() or visit_end_ITEM(), and marks the end of the
+ * visit.  The @opaque pointer should match the output parameter
+ * passed to the subtype_visitor_new() used to create an output
+ * visitor, or NULL for any other visitor.  Needed for output
+ * visitors, but may also be called with other visitors.
+ */
+void visit_complete(Visitor *v, void *opaque);
+
 /*
  * Free @v and any resources it has tied up.
  *

qemu/qapi/qapi-visit-core.c

@@ -21,6 +21,14 @@
 #include "qapi/visitor.h"
 #include "qapi/visitor-impl.h"
 
+void visit_complete(Visitor *v, void *opaque)
+{
+    assert(v->type != VISITOR_OUTPUT || v->complete);
+    if (v->complete) {
+        v->complete(v, opaque);
+    }
+}
+
 void visit_free(Visitor *v)
 {
     if (v) {

qemu/qapi/qmp-output-visitor.c

@@ -34,6 +34,7 @@ struct QmpOutputVisitor
     Visitor visitor;
     QStack stack; /* Stack of containers that haven't yet been finished */
     QObject *root; /* Root of the output visit */
+    QObject **result; /* User's storage location for result */
 };
 
 #define qmp_output_add(qov, name, value) \
@@ -201,18 +202,16 @@ static void qmp_output_type_null(Visitor *v, const char *name, Error **errp)
 /* Finish building, and return the root object.
  * The root object is never null. The caller becomes the object's
  * owner, and should use qobject_decref() when done with it.  */
-QObject *qmp_output_get_qobject(QmpOutputVisitor *qov)
+static void qmp_output_complete(Visitor *v, void *opaque)
 {
+    QmpOutputVisitor *qov = to_qov(v);
+
     /* A visit must have occurred, with each start paired with end.  */
     assert(qov->root && QTAILQ_EMPTY(&qov->stack));
+    assert(opaque == qov->result);
 
-    qobject_incref(qov->root);
-    return qov->root;
-}
-
-Visitor *qmp_output_get_visitor(QmpOutputVisitor *v)
-{
-    return &v->visitor;
+    *qov->result = qov->root;
+    qov->result = NULL;
 }
 
 static void qmp_output_free(Visitor *v)
@@ -229,7 +228,7 @@ static void qmp_output_free(Visitor *v)
     g_free(qov);
 }
 
-QmpOutputVisitor *qmp_output_visitor_new(void)
+Visitor *qmp_output_visitor_new(QObject **result)
 {
     QmpOutputVisitor *v;
 
@@ -248,9 +247,12 @@ QmpOutputVisitor *qmp_output_visitor_new(void)
     v->visitor.type_number = qmp_output_type_number;
     v->visitor.type_any = qmp_output_type_any;
     v->visitor.type_null = qmp_output_type_null;
+    v->visitor.complete = qmp_output_complete;
     v->visitor.free = qmp_output_free;
 
     QTAILQ_INIT(&v->stack);
+    *result = NULL;
+    v->result = result;
 
-    return v;
+    return &v->visitor;
 }

qemu/qom/qom-qobject.c

@@ -33,14 +33,14 @@ QObject *object_property_get_qobject(struct uc_struct *uc, Object *obj, const ch
 {
     QObject *ret = NULL;
     Error *local_err = NULL;
-    QmpOutputVisitor *qov;
+    Visitor *v;
 
-    qov = qmp_output_visitor_new();
-    object_property_get(uc, obj, qmp_output_get_visitor(qov), name, &local_err);
+    v = qmp_output_visitor_new(&ret);
+    object_property_get(uc, obj, v, name, &local_err);
     if (!local_err) {
-        ret = qmp_output_get_qobject(qov);
+        visit_complete(v, &ret);
     }
     error_propagate(errp, local_err);
-    visit_free(qmp_output_get_visitor(qov));
+    visit_free(v);
     return ret;
 }

qemu/scripts/qapi-event.py

@@ -71,7 +71,7 @@ def gen_event_send(name, arg_type):
     if arg_type and arg_type.members:
         assert not arg_type.variants
         ret += mcgen('''
-    QmpOutputVisitor *qov;
+    QObject *obj;
     Visitor *v;
 ''')
         ret += gen_param_var(arg_type)
@@ -90,8 +90,7 @@ def gen_event_send(name, arg_type):
 
     if arg_type and arg_type.members:
         ret += mcgen('''
-    qov = qmp_output_visitor_new();
-    v = qmp_output_get_visitor(qov);
+    v = qmp_output_visitor_new(&obj);
 
     visit_start_struct(v, "%(name)s", NULL, 0, &err);
     if (err) {
@@ -106,7 +105,8 @@ def gen_event_send(name, arg_type):
         goto out;
     }
 
-    qdict_put_obj(qmp, "data", qmp_output_get_qobject(qov));
+    visit_complete(v, &obj);
+    qdict_put_obj(qmp, "data", obj);
 ''',
                      name=name, c_name=arg_type.c_name())