qapi: untangle next_list

Right now, the semantics of next_list are complicated.  The caller must:

* call start_list

* call next_list for each element *including the first*

* on the first call to next_list, the second argument should point to
NULL and the result is the head of the list.  On subsequent calls,
the second argument should point to the last node (last result of
next_list) and next_list itself tacks the element at the tail of the
list.

This works for both input and output visitor, but having the visitor
write memory when it is only reading the list is ugly.  Plus, relying
on *list to detect the first call is tricky and undocumented.

We can initialize so->entry in next_list instead of start_list, leaving
it NULL in start_list.  This way next_list sees clearly whether it is
on the first call---as a bonus, it discriminates the cases based on
internal state of the visitor rather than external state.  We can
also pull the assignment of the list head from generated code up to
next_list.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index 5831e371eae3e1adee267b81855c6f0f3a833035..ad11767a2fd81afe43553be24620c83e7a7d93e7 100644 (file)
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -194,11 +194,11 @@ Example:
 
     void visit_type_UserDefOneList(Visitor *m, UserDefOneList ** obj, const char *name, Error **errp)
     {
-        GenericList *i;
+        GenericList *i, **prev = (GenericList **)obj;
 
         visit_start_list(m, name, errp);
 
-        for (i = visit_next_list(m, (GenericList **)obj, errp); i; i = visit_next_list(m, &i, errp)) {
+        for (; (i = visit_next_list(m, prev, errp)) != NULL; prev = &i) {
             UserDefOneList *native_i = (UserDefOneList *)i;
             visit_type_UserDefOne(m, &native_i->value, NULL, errp);
         }

diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
index ef9288f1e92ed1050361b637ab0912c9c7e37cbc..413e3338eb323319922e72cde117f15ef27c5684 100644 (file)
--- a/qapi/qmp-input-visitor.c
+++ b/qapi/qmp-input-visitor.c
@@ -64,9 +64,7 @@ static const QObject *qmp_input_get_object(QmpInputVisitor *qiv,
 static void qmp_input_push(QmpInputVisitor *qiv, const QObject *obj, Error **errp)
 {
     qiv->stack[qiv->nb_stack].obj = obj;
-    if (qobject_type(obj) == QTYPE_QLIST) {
-        qiv->stack[qiv->nb_stack].entry = qlist_first(qobject_to_qlist(obj));
-    }
+    qiv->stack[qiv->nb_stack].entry = NULL;
     qiv->nb_stack++;
 
     if (qiv->nb_stack >= QIV_STACK_SIZE) {
@@ -132,18 +130,24 @@ static GenericList *qmp_input_next_list(Visitor *v, GenericList **list,
     QmpInputVisitor *qiv = to_qiv(v);
     GenericList *entry;
     StackObject *so = &qiv->stack[qiv->nb_stack - 1];
+    bool first;
+
+    if (so->entry == NULL) {
+        so->entry = qlist_first(qobject_to_qlist(so->obj));
+        first = true;
+    } else {
+        so->entry = qlist_next(so->entry);
+        first = false;
+    }
 
     if (so->entry == NULL) {
         return NULL;
     }
 
     entry = g_malloc0(sizeof(*entry));
-    if (*list) {
-        so->entry = qlist_next(so->entry);
-        if (so->entry == NULL) {
-            g_free(entry);
-            return NULL;
-        }
+    if (first) {
+        *list = entry;
+    } else {
         (*list)->next = entry;
     }
 

diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 31d50a6645dffd63f764a58da61ee5041b0e8c69..8d4e94a45f44d859221448f75512912f3ef60fe1 100644 (file)
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -86,14 +86,14 @@ def generate_visit_list(name, members):
 
 void visit_type_%(name)sList(Visitor *m, %(name)sList ** obj, const char *name, Error **errp)
 {
-    GenericList *i, **head = (GenericList **)obj;
+    GenericList *i, **prev = (GenericList **)obj;
 
     if (error_is_set(errp)) {
         return;
     }
     visit_start_list(m, name, errp);
 
-    for (*head = i = visit_next_list(m, head, errp); i; i = visit_next_list(m, &i, errp)) {
+    for (; (i = visit_next_list(m, prev, errp)) != NULL; prev = &i) {
         %(name)sList *native_i = (%(name)sList *)i;
         visit_type_%(name)s(m, &native_i->value, NULL, errp);
     }