qapi: Simplify semantics of visit_next_list()

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>

16 files changed, 142 insertions, 178 deletions

diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index d135847c87..fe61c5c423 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -912,18 +912,20 @@ Example:
     void visit_type_UserDefOneList(Visitor *v, const char *name, UserDefOneList **obj, Error **errp)
     {
         Error *err = NULL;
-        GenericList *i, **prev;
+        UserDefOneList *tail;
+        size_t size = sizeof(**obj);
 
-        visit_start_list(v, name, &err);
+        visit_start_list(v, name, (GenericList **)obj, size, &err);
         if (err) {
             goto out;
         }
 
-        for (prev = (GenericList **)obj;
-             !err && (i = visit_next_list(v, prev, sizeof(**obj))) != NULL;
-             prev = &i) {
-            UserDefOneList *native_i = (UserDefOneList *)i;
-            visit_type_UserDefOne(v, NULL, &native_i->value, &err);
+        for (tail = *obj; tail;
+             tail = (UserDefOneList *)visit_next_list(v, (GenericList *)tail, size)) {
+            visit_type_UserDefOne(v, NULL, &tail->value, &err);
+            if (err) {
+                break;
+            }
         }
 
         visit_end_list(v);
diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
index e3a6191ad4..94c875d752 100644
--- a/hw/ppc/spapr_drc.c
+++ b/hw/ppc/spapr_drc.c
@@ -309,7 +309,7 @@ static void prop_get_fdt(Object *obj, Visitor *v, const char *name,
             int i;
             prop = fdt_get_property_by_offset(fdt, fdt_offset, &prop_len);
             name = fdt_string(fdt, fdt32_to_cpu(prop->nameoff));
-            visit_start_list(v, name, &err);
+            visit_start_list(v, name, NULL, 0, &err);
             if (err) {
                 error_propagate(errp, err);
                 return;
diff --git a/include/qapi/opts-visitor.h b/include/qapi/opts-visitor.h
index fe37ed9a36..ae1bf7cf51 100644
--- a/include/qapi/opts-visitor.h
+++ b/include/qapi/opts-visitor.h
@@ -32,7 +32,8 @@ typedef struct OptsVisitor OptsVisitor;
  *
  * The Opts input visitor does not implement support for visiting QAPI
  * alternates, numbers (other than integers), null, or arbitrary
- * QTypes.
+ * QTypes.  It also requires a non-null list argument to
+ * visit_start_list().
  */
 OptsVisitor *opts_visitor_new(const QemuOpts *opts);
 void opts_visitor_cleanup(OptsVisitor *nv);
diff --git a/include/qapi/string-input-visitor.h b/include/qapi/string-input-visitor.h
index a8d8f6723b..7b76c2b9e3 100644
--- a/include/qapi/string-input-visitor.h
+++ b/include/qapi/string-input-visitor.h
@@ -19,7 +19,8 @@ typedef struct StringInputVisitor StringInputVisitor;
 
 /*
  * The string input visitor does not implement support for visiting
- * QAPI structs, alternates, null, or arbitrary QTypes.
+ * QAPI structs, alternates, null, or arbitrary QTypes.  It also
+ * requires a non-null list argument to visit_start_list().
  */
 StringInputVisitor *string_input_visitor_new(const char *str);
 void string_input_visitor_cleanup(StringInputVisitor *v);
diff --git a/include/qapi/string-output-visitor.h b/include/qapi/string-output-visitor.h
index 89b7e4bc9b..e10522a35b 100644
--- a/include/qapi/string-output-visitor.h
+++ b/include/qapi/string-output-visitor.h
@@ -19,7 +19,8 @@ typedef struct StringOutputVisitor StringOutputVisitor;
 
 /*
  * The string output visitor does not implement support for visiting
- * QAPI structs, alternates, null, or arbitrary QTypes.
+ * QAPI structs, alternates, null, or arbitrary QTypes.  It also
+ * requires a non-null list argument to visit_start_list().
  */
 StringOutputVisitor *string_output_visitor_new(bool human);
 void string_output_visitor_cleanup(StringOutputVisitor *v);
diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index b20a9226e0..145afd03e7 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -49,11 +49,13 @@ struct Visitor
     /* Must be set to visit structs */
     void (*end_struct)(Visitor *v);
 
-    /* Must be set */
-    void (*start_list)(Visitor *v, const char *name, Error **errp);
+    /* Must be set; implementations may require @list to be non-null,
+     * but must document it. */
+    void (*start_list)(Visitor *v, const char *name, GenericList **list,
+                       size_t size, Error **errp);
 
     /* Must be set */
-    GenericList *(*next_list)(Visitor *v, GenericList **list, size_t size);
+    GenericList *(*next_list)(Visitor *v, GenericList *tail, size_t size);
 
     /* Must be set */
     void (*end_list)(Visitor *v);
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index fe268fe78d..8de6b436fb 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -176,7 +176,7 @@
  *  if (err) {
  *      goto out;
  *  }
- *  visit_start_list(v, "list", &err);
+ *  visit_start_list(v, "list", NULL, 0, &err);
  *  if (err) {
  *      goto outobj;
  *  }
@@ -281,19 +281,27 @@ void visit_end_struct(Visitor *v);
  * @name expresses the relationship of this list to its parent
  * container; see the general description of @name above.
  *
+ * @list must be non-NULL for a real walk, in which case @size
+ * determines how much memory an input visitor will allocate into
+ * *@list (at least sizeof(GenericList)).  Some visitors also allow
+ * @list to be NULL for a virtual walk, in which case @size is
+ * ignored.
+ *
  * @errp obeys typical error usage, and reports failures such as a
- * member @name is not present, or present but not a list.
+ * member @name is not present, or present but not a list.  On error,
+ * input visitors set *@list to NULL.
  *
  * After visit_start_list() succeeds, the caller may visit its members
- * one after the other.  A real visit uses visit_next_list() for
- * traversing the linked list, while a virtual visit uses other means.
- * For each list element, call the appropriate visit_type_FOO() with
- * name set to NULL and obj set to the address of the value member of
- * the list element.  Finally, visit_end_list() needs to be called to
- * clean up, even if intermediate visits fail.  See the examples
- * above.
+ * one after the other.  A real visit (where @obj is non-NULL) uses
+ * visit_next_list() for traversing the linked list, while a virtual
+ * visit (where @obj is NULL) uses other means.  For each list
+ * element, call the appropriate visit_type_FOO() with name set to
+ * NULL and obj set to the address of the value member of the list
+ * element.  Finally, visit_end_list() needs to be called to clean up,
+ * even if intermediate visits fail.  See the examples above.
  */
-void visit_start_list(Visitor *v, const char *name, Error **errp);
+void visit_start_list(Visitor *v, const char *name, GenericList **list,
+                      size_t size, Error **errp);
 
 /*
  * Iterate over a GenericList during a non-virtual list visit.
@@ -301,20 +309,15 @@ void visit_start_list(Visitor *v, const char *name, Error **errp);
  * @size represents the size of a linked list node (at least
  * sizeof(GenericList)).
  *
- * @list must not be NULL; on the first call, @list contains the
- * address of the list head, and on subsequent calls *@list must be
- * the previously returned value.  Should be called in a loop until a
- * NULL return or error occurs; for each non-NULL return, the caller
- * then calls the appropriate visit_type_*() for the element type
- * of the list, with that function's name parameter set to NULL and
- * obj set to the address of (*@list)->value.
- *
- * FIXME: This interface is awkward; it requires all callbacks to
- * track whether it is the first or a subsequent call.  A better
- * interface would pass the head of the list through
- * visit_start_list().
+ * @tail must not be NULL; on the first call, @tail is the value of
+ * *list after visit_start_list(), and on subsequent calls @tail must
+ * be the previously returned value.  Should be called in a loop until
+ * a NULL return or error occurs; for each non-NULL return, the caller
+ * then calls the appropriate visit_type_*() for the element type of
+ * the list, with that function's name parameter set to NULL and obj
+ * set to the address of @tail->value.
  */
-GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size);
+GenericList *visit_next_list(Visitor *v, GenericList *tail, size_t size);
 
 /*
  * Complete a list visit started earlier.
diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c
index ece0598e34..4cf1cf885b 100644
--- a/qapi/opts-visitor.c
+++ b/qapi/opts-visitor.c
@@ -23,9 +23,8 @@
 enum ListMode
 {
     LM_NONE,             /* not traversing a list of repeated options */
-    LM_STARTED,          /* opts_start_list() succeeded */
 
-    LM_IN_PROGRESS,      /* opts_next_list() has been called.
+    LM_IN_PROGRESS,      /* opts_next_list() ready to be called.
                           *
                           * Generating the next list link will consume the most
                           * recently parsed QemuOpt instance of the repeated
@@ -214,35 +213,33 @@ lookup_distinct(const OptsVisitor *ov, const char *name, Error **errp)
 
 
 static void
-opts_start_list(Visitor *v, const char *name, Error **errp)
+opts_start_list(Visitor *v, const char *name, GenericList **list, size_t size,
+                Error **errp)
 {
     OptsVisitor *ov = to_ov(v);
 
     /* we can't traverse a list in a list */
     assert(ov->list_mode == LM_NONE);
+    /* we don't support visits without a list */
+    assert(list);
     ov->repeated_opts = lookup_distinct(ov, name, errp);
-    if (ov->repeated_opts != NULL) {
-        ov->list_mode = LM_STARTED;
+    if (ov->repeated_opts) {
+        ov->list_mode = LM_IN_PROGRESS;
+        *list = g_malloc0(size);
+    } else {
+        *list = NULL;
     }
 }
 
 
 static GenericList *
-opts_next_list(Visitor *v, GenericList **list, size_t size)
+opts_next_list(Visitor *v, GenericList *tail, size_t size)
 {
     OptsVisitor *ov = to_ov(v);
-    GenericList **link;
 
     switch (ov->list_mode) {
-    case LM_STARTED:
-        ov->list_mode = LM_IN_PROGRESS;
-        link = list;
-        break;
-
     case LM_SIGNED_INTERVAL:
     case LM_UNSIGNED_INTERVAL:
-        link = &(*list)->next;
-
         if (ov->list_mode == LM_SIGNED_INTERVAL) {
             if (ov->range_next.s < ov->range_limit.s) {
                 ++ov->range_next.s;
@@ -263,7 +260,6 @@ opts_next_list(Visitor *v, GenericList **list, size_t size)
             g_hash_table_remove(ov->unprocessed_opts, opt->name);
             return NULL;
         }
-        link = &(*list)->next;
         break;
     }
 
@@ -271,8 +267,8 @@ opts_next_list(Visitor *v, GenericList **list, size_t size)
         abort();
     }
 
-    *link = g_malloc0(size);
-    return *link;
+    tail->next = g_malloc0(size);
+    return tail->next;
 }
 
 
@@ -281,8 +277,7 @@ opts_end_list(Visitor *v)
 {
     OptsVisitor *ov = to_ov(v);
 
-    assert(ov->list_mode == LM_STARTED ||
-           ov->list_mode == LM_IN_PROGRESS ||
+    assert(ov->list_mode == LM_IN_PROGRESS ||
            ov->list_mode == LM_SIGNED_INTERVAL ||
            ov->list_mode == LM_UNSIGNED_INTERVAL);
     ov->repeated_opts = NULL;
diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
index 9005badb25..cd68b55a1a 100644
--- a/qapi/qapi-dealloc-visitor.c
+++ b/qapi/qapi-dealloc-visitor.c
@@ -22,7 +22,6 @@
 typedef struct StackEntry
 {
     void *value;
-    bool is_list_head;
     QTAILQ_ENTRY(StackEntry) node;
 } StackEntry;
 
@@ -43,10 +42,6 @@ static void qapi_dealloc_push(QapiDeallocVisitor *qov, void *value)
 
     e->value = value;
 
-    /* see if we're just pushing a list head tracker */
-    if (value == NULL) {
-        e->is_list_head = true;
-    }
     QTAILQ_INSERT_HEAD(&qov->stack, e, node);
 }
 
@@ -93,38 +88,22 @@ static void qapi_dealloc_end_alternate(Visitor *v)
     }
 }
 
-static void qapi_dealloc_start_list(Visitor *v, const char *name, Error **errp)
+static void qapi_dealloc_start_list(Visitor *v, const char *name,
+                                    GenericList **list, size_t size,
+                                    Error **errp)
 {
-    QapiDeallocVisitor *qov = to_qov(v);
-    qapi_dealloc_push(qov, NULL);
 }
 
-static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp,
+static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList *tail,
                                            size_t size)
 {
-    GenericList *list = *listp;
-    QapiDeallocVisitor *qov = to_qov(v);
-    StackEntry *e = QTAILQ_FIRST(&qov->stack);
-
-    if (e && e->is_list_head) {
-        e->is_list_head = false;
-        return list;
-    }
-
-    if (list) {
-        list = list->next;
-        g_free(*listp);
-        return list;
-    }
-
-    return NULL;
+    GenericList *next = tail->next;
+    g_free(tail);
+    return next;
 }
 
 static void qapi_dealloc_end_list(Visitor *v)
 {
-    QapiDeallocVisitor *qov = to_qov(v);
-    void *obj = qapi_dealloc_pop(qov);
-    assert(obj == NULL); /* should've been list head tracker with no payload */
 }
 
 static void qapi_dealloc_type_str(Visitor *v, const char *name, char **obj,
diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 58144f67b7..d6bf4bd253 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -48,15 +48,23 @@ void visit_end_struct(Visitor *v)
     v->end_struct(v);
 }
 
-void visit_start_list(Visitor *v, const char *name, Error **errp)
+void visit_start_list(Visitor *v, const char *name, GenericList **list,
+                      size_t size, Error **errp)
 {
-    v->start_list(v, name, errp);
+    Error *err = NULL;
+
+    assert(!list || size >= sizeof(GenericList));
+    v->start_list(v, name, list, size, &err);
+    if (list && v->type == VISITOR_INPUT) {
+        assert(!(err && *list));
+    }
+    error_propagate(errp, err);
 }
 
-GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size)
+GenericList *visit_next_list(Visitor *v, GenericList *tail, size_t size)
 {
-    assert(list && size >= sizeof(GenericList));
-    return v->next_list(v, list, size);
+    assert(tail && size >= sizeof(GenericList));
+    return v->next_list(v, tail, size);
 }
 
 void visit_end_list(Visitor *v)
diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
index 90b9df11c4..aea90a1378 100644
--- a/qapi/qmp-input-visitor.c
+++ b/qapi/qmp-input-visitor.c
@@ -29,7 +29,6 @@ typedef struct StackObject
 
     GHashTable *h;           /* If obj is dict: unvisited keys */
     const QListEntry *entry; /* If obj is list: unvisited tail */
-    bool first;              /* If obj is list: next_list() not yet called? */
 } StackObject;
 
 struct QmpInputVisitor
@@ -81,7 +80,6 @@ static QObject *qmp_input_get_object(QmpInputVisitor *qiv,
     } else {
         assert(qobject_type(qobj) == QTYPE_QLIST);
         assert(!name);
-        assert(!tos->first);
         ret = qlist_entry_obj(tos->entry);
         if (consume) {
             tos->entry = qlist_next(tos->entry);
@@ -97,7 +95,8 @@ static void qdict_add_key(const char *key, QObject *obj, void *opaque)
     g_hash_table_insert(h, (gpointer) key, NULL);
 }
 
-static void qmp_input_push(QmpInputVisitor *qiv, QObject *obj, Error **errp)
+static const QListEntry *qmp_input_push(QmpInputVisitor *qiv, QObject *obj,
+                                        Error **errp)
 {
     GHashTable *h;
     StackObject *tos = &qiv->stack[qiv->nb_stack];
@@ -105,7 +104,7 @@ static void qmp_input_push(QmpInputVisitor *qiv, QObject *obj, Error **errp)
     assert(obj);
     if (qiv->nb_stack >= QIV_STACK_SIZE) {
         error_setg(errp, "An internal buffer overran");
-        return;
+        return NULL;
     }
 
     tos->obj = obj;
@@ -118,10 +117,10 @@ static void qmp_input_push(QmpInputVisitor *qiv, QObject *obj, Error **errp)
         tos->h = h;
     } else if (qobject_type(obj) == QTYPE_QLIST) {
         tos->entry = qlist_first(qobject_to_qlist(obj));
-        tos->first = true;
     }
 
     qiv->nb_stack++;
+    return tos->entry;
 }
 
 
@@ -192,40 +191,43 @@ static void qmp_input_start_struct(Visitor *v, const char *name, void **obj,
 }
 
 
-static void qmp_input_start_list(Visitor *v, const char *name, Error **errp)
+static void qmp_input_start_list(Visitor *v, const char *name,
+                                 GenericList **list, size_t size, Error **errp)
 {
     QmpInputVisitor *qiv = to_qiv(v);
     QObject *qobj = qmp_input_get_object(qiv, name, true);
+    const QListEntry *entry;
 
     if (!qobj || qobject_type(qobj) != QTYPE_QLIST) {
+        if (list) {
+            *list = NULL;
+        }
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
                    "list");
         return;
     }
 
-    qmp_input_push(qiv, qobj, errp);
+    entry = qmp_input_push(qiv, qobj, errp);
+    if (list) {
+        if (entry) {
+            *list = g_malloc0(size);
+        } else {
+            *list = NULL;
+        }
+    }
 }
 
-static GenericList *qmp_input_next_list(Visitor *v, GenericList **list,
+static GenericList *qmp_input_next_list(Visitor *v, GenericList *tail,
                                         size_t size)
 {
     QmpInputVisitor *qiv = to_qiv(v);
-    GenericList *entry;
     StackObject *so = &qiv->stack[qiv->nb_stack - 1];
 
     if (!so->entry) {
         return NULL;
     }
-
-    entry = g_malloc0(size);
-    if (so->first) {
-        *list = entry;
-        so->first = false;
-    } else {
-        (*list)->next = entry;
-    }
-
-    return entry;
+    tail->next = g_malloc0(size);
+    return tail->next;
 }
 
 
diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
index 232ad58221..4d3cf78333 100644
--- a/qapi/qmp-output-visitor.c
+++ b/qapi/qmp-output-visitor.c
@@ -22,7 +22,6 @@
 typedef struct QStackEntry
 {
     QObject *value;
-    bool is_list_head;
     QTAILQ_ENTRY(QStackEntry) node;
 } QStackEntry;
 
@@ -52,9 +51,6 @@ static void qmp_output_push_obj(QmpOutputVisitor *qov, QObject *value)
     assert(qov->root);
     assert(value);
     e->value = value;
-    if (qobject_type(e->value) == QTYPE_QLIST) {
-        e->is_list_head = true;
-    }
     QTAILQ_INSERT_HEAD(&qov->stack, e, node);
 }
 
@@ -118,7 +114,9 @@ static void qmp_output_end_struct(Visitor *v)
     assert(qobject_type(value) == QTYPE_QDICT);
 }
 
-static void qmp_output_start_list(Visitor *v, const char *name, Error **errp)
+static void qmp_output_start_list(Visitor *v, const char *name,
+                                  GenericList **listp, size_t size,
+                                  Error **errp)
 {
     QmpOutputVisitor *qov = to_qov(v);
     QList *list = qlist_new();
@@ -127,20 +125,10 @@ static void qmp_output_start_list(Visitor *v, const char *name, Error **errp)
     qmp_output_push(qov, list);
 }
 
-static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp,
+static GenericList *qmp_output_next_list(Visitor *v, GenericList *tail,
                                          size_t size)
 {
-    GenericList *list = *listp;
-    QmpOutputVisitor *qov = to_qov(v);
-    QStackEntry *e = QTAILQ_FIRST(&qov->stack);
-
-    assert(e);
-    if (e->is_list_head) {
-        e->is_list_head = false;
-        return list;
-    }
-
-    return list ? list->next : NULL;
+    return tail->next;
 }
 
 static void qmp_output_end_list(Visitor *v)
diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c
index ad150dccf6..30b58791c9 100644
--- a/qapi/string-input-visitor.c
+++ b/qapi/string-input-visitor.c
@@ -25,8 +25,6 @@ struct StringInputVisitor
 {
     Visitor visitor;
 
-    bool head;
-
     GList *ranges;
     GList *cur_range;
     int64_t cur;
@@ -128,11 +126,16 @@ error:
 }
 
 static void
-start_list(Visitor *v, const char *name, Error **errp)
+start_list(Visitor *v, const char *name, GenericList **list, size_t size,
+           Error **errp)
 {
     StringInputVisitor *siv = to_siv(v);
 
+    /* We don't support visits without a list */
+    assert(list);
+
     if (parse_str(siv, name, errp) < 0) {
+        *list = NULL;
         return;
     }
 
@@ -142,13 +145,15 @@ start_list(Visitor *v, const char *name, Error **errp)
         if (r) {
             siv->cur = r->begin;
         }
+        *list = g_malloc0(size);
+    } else {
+        *list = NULL;
     }
 }
 
-static GenericList *next_list(Visitor *v, GenericList **list, size_t size)
+static GenericList *next_list(Visitor *v, GenericList *tail, size_t size)
 {
     StringInputVisitor *siv = to_siv(v);
-    GenericList **link;
     Range *r;
 
     if (!siv->ranges || !siv->cur_range) {
@@ -172,21 +177,12 @@ static GenericList *next_list(Visitor *v, GenericList **list, size_t size)
         siv->cur = r->begin;
     }
 
-    if (siv->head) {
-        link = list;
-        siv->head = false;
-    } else {
-        link = &(*list)->next;
-    }
-
-    *link = g_malloc0(size);
-    return *link;
+    tail->next = g_malloc0(size);
+    return tail->next;
 }
 
 static void end_list(Visitor *v)
 {
-    StringInputVisitor *siv = to_siv(v);
-    siv->head = true;
 }
 
 static void parse_type_int64(Visitor *v, const char *name, int64_t *obj,
@@ -369,6 +365,5 @@ StringInputVisitor *string_input_visitor_new(const char *str)
     v->visitor.optional = parse_optional;
 
     v->string = str;
-    v->head = true;
     return v;
 }
diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
index 0d44d7e6a5..d01319628b 100644
--- a/qapi/string-output-visitor.c
+++ b/qapi/string-output-visitor.c
@@ -20,7 +20,7 @@
 
 enum ListMode {
     LM_NONE,             /* not traversing a list of repeated options */
-    LM_STARTED,          /* start_list() succeeded */
+    LM_STARTED,          /* next_list() ready to be called */
 
     LM_IN_PROGRESS,      /* next_list() has been called.
                           *
@@ -48,7 +48,7 @@ enum ListMode {
 
     LM_UNSIGNED_INTERVAL,/* Same as above, only for an unsigned interval. */
 
-    LM_END
+    LM_END,              /* next_list() called, about to see last element. */
 };
 
 typedef enum ListMode ListMode;
@@ -58,7 +58,6 @@ struct StringOutputVisitor
     Visitor visitor;
     bool human;
     GString *string;
-    bool head;
     ListMode list_mode;
     union {
         int64_t s;
@@ -266,39 +265,29 @@ static void print_type_number(Visitor *v, const char *name, double *obj,
 }
 
 static void
-start_list(Visitor *v, const char *name, Error **errp)
+start_list(Visitor *v, const char *name, GenericList **list, size_t size,
+           Error **errp)
 {
     StringOutputVisitor *sov = to_sov(v);
 
     /* we can't traverse a list in a list */
     assert(sov->list_mode == LM_NONE);
-    sov->list_mode = LM_STARTED;
-    sov->head = true;
+    /* We don't support visits without a list */
+    assert(list);
+    /* List handling is only needed if there are at least two elements */
+    if (*list && (*list)->next) {
+        sov->list_mode = LM_STARTED;
+    }
 }
 
-static GenericList *next_list(Visitor *v, GenericList **list, size_t size)
+static GenericList *next_list(Visitor *v, GenericList *tail, size_t size)
 {
     StringOutputVisitor *sov = to_sov(v);
-    GenericList *ret = NULL;
-    if (*list) {
-        if (sov->head) {
-            ret = *list;
-        } else {
-            ret = (*list)->next;
-        }
+    GenericList *ret = tail->next;
 
-        if (sov->head) {
-            if (ret && ret->next == NULL) {
-                sov->list_mode = LM_NONE;
-            }
-            sov->head = false;
-        } else {
-            if (ret && ret->next == NULL) {
-                sov->list_mode = LM_END;
-            }
-        }
+    if (ret && !ret->next) {
+        sov->list_mode = LM_END;
     }
-
     return ret;
 }
 
@@ -311,8 +300,6 @@ static void end_list(Visitor *v)
            sov->list_mode == LM_NONE ||
            sov->list_mode == LM_IN_PROGRESS);
     sov->list_mode = LM_NONE;
-    sov->head = true;
-
 }
 
 char *string_output_get_string(StringOutputVisitor *sov)
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index bdf8971440..8b7efcc74a 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -117,18 +117,20 @@ def gen_visit_list(name, element_type):
 void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error **errp)
 {
     Error *err = NULL;
-    GenericList *i, **prev;
+    %(c_name)s *tail;
+    size_t size = sizeof(**obj);
 
-    visit_start_list(v, name, &err);
+    visit_start_list(v, name, (GenericList **)obj, size, &err);
     if (err) {
         goto out;
     }
 
-    for (prev = (GenericList **)obj;
-         !err && (i = visit_next_list(v, prev, sizeof(**obj))) != NULL;
-         prev = &i) {
-        %(c_name)s *native_i = (%(c_name)s *)i;
-        visit_type_%(c_elt_type)s(v, NULL, &native_i->value, &err);
+    for (tail = *obj; tail;
+         tail = (%(c_name)s *)visit_next_list(v, (GenericList *)tail, size)) {
+        visit_type_%(c_elt_type)s(v, NULL, &tail->value, &err);
+        if (err) {
+            break;
+        }
     }
 
     visit_end_list(v);
diff --git a/tests/test-string-input-visitor.c b/tests/test-string-input-visitor.c
index f99824d240..5a56920222 100644
--- a/tests/test-string-input-visitor.c
+++ b/tests/test-string-input-visitor.c
@@ -98,8 +98,6 @@ static void test_visitor_in_intList(TestInputVisitorData *data,
 
     v = visitor_input_test_init(data, "not an int list");
 
-    /* FIXME: res should be NULL on failure, regardless of starting value */
-    res = NULL;
     visit_type_int16List(v, NULL, &res, &err);
     error_free_or_abort(&err);
     g_assert(!res);