[Qemu-devel] [PATCH v7 16/31] qmp: Don't abuse stack to track qmp-output root

[Eric Blake <eblake@redhat.com>]

The previous commit documented an inconsistency in how we are
using the stack of qmp-output-visitor.  Normally, pushing a
single top-level object puts the object on the stack twice:
once as the root, and once as the current container being
appended to; but popping that struct only pops once.  However,
qmp_ouput_add() was trying to either set up the added object
as the new root (works if you parse two top-level scalars in a
row: the second replaces the first as the root) or as a member
of the current container (works as long as you have an open
container on the stack; but if you have popped the first
top-level container, it then resolves to the root and still
tries to add into that existing container).

Fix the stupidity by not tracking two separate things in the
stack.  Drop the now-useless qmp_output_first() while at it.

Saved for a later patch: we still are rather sloppy in that
qmp_output_get_object() can be called in the middle of a parse,
rather than requiring that a visit is complete.

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v7: retitle; rebase to earlier changes, drop qmp_output_first()
v6: no change
---
 qapi/qmp-output-visitor.c | 79 ++++++++++++++++-------------------------------
 1 file changed, 26 insertions(+), 53 deletions(-)

diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
index 77097b7..06ee19b 100644
--- a/qapi/qmp-output-visitor.c
+++ b/qapi/qmp-output-visitor.c
@@ -29,16 +29,8 @@ typedef QTAILQ_HEAD(QStack, QStackEntry) QStack;
 struct QmpOutputVisitor
 {
     Visitor visitor;
-    /* FIXME: we are abusing stack to hold two separate pieces of
-     * information: the current root object in slot 0, and the stack
-     * of N objects still being built in slots 1 through N (for N+1
-     * slots in use).  Worse, our behavior is inconsistent:
-     * qmp_output_add_obj() visiting two top-level scalars in a row
-     * discards the first in favor of the second, but visiting two
-     * top-level objects in a row tries to append the second object
-     * into the first (since the first object was placed in the stack
-     * in both slot 0 and 1, but only popped from slot 1).  */
-    QStack stack;
+    QStack stack; /* Stack of containers that haven't yet been finished */
+    QObject *root; /* Root of the output visit */
 };

 #define qmp_output_add(qov, name, value) \
@@ -55,6 +47,7 @@ static void qmp_output_push_obj(QmpOutputVisitor *qov, QObject *value)
 {
     QStackEntry *e = g_malloc0(sizeof(*e));

+    assert(qov->root);
     assert(value);
     e->value = value;
     if (qobject_type(e->value) == QTYPE_QLIST) {
@@ -76,26 +69,12 @@ static QObject *qmp_output_pop(QmpOutputVisitor *qov)
     return value;
 }

-/* Grab the root QObject, if any, in preparation to empty the stack */
-static QObject *qmp_output_first(QmpOutputVisitor *qov)
-{
-    QStackEntry *e = QTAILQ_LAST(&qov->stack, QStack);
-
-    if (!e) {
-        /* No root */
-        return NULL;
-    }
-    assert(e->value);
-    return e->value;
-}
-
-/* Grab the most recent QObject from the stack, which must exist */
+/* Grab the most recent QObject from the stack, if any */
 static QObject *qmp_output_last(QmpOutputVisitor *qov)
 {
     QStackEntry *e = QTAILQ_FIRST(&qov->stack);

-    assert(e && e->value);
-    return e->value;
+    return e ? e->value : NULL;
 }

 /* Add @value to the current QObject being built.
@@ -106,29 +85,25 @@ static void qmp_output_add_obj(QmpOutputVisitor *qov, const char *name,
 {
     QObject *cur;

-    if (QTAILQ_EMPTY(&qov->stack)) {
-        /* Stack was empty, track this object as root */
-        qmp_output_push_obj(qov, value);
-        return;
-    }
-
     cur = qmp_output_last(qov);

-    switch (qobject_type(cur)) {
-    case QTYPE_QDICT:
-        assert(name);
-        qdict_put_obj(qobject_to_qdict(cur), name, value);
-        break;
-    case QTYPE_QLIST:
-        qlist_append_obj(qobject_to_qlist(cur), value);
-        break;
-    default:
-        /* The previous root was a scalar, replace it with a new root */
-        /* FIXME this is abusing the stack; see comment above */
-        qobject_decref(qmp_output_pop(qov));
-        assert(QTAILQ_EMPTY(&qov->stack));
-        qmp_output_push_obj(qov, value);
-        break;
+    if (!cur) {
+        /* FIXME we should require the user to reset the visitor, rather
+         * than throwing away the previous root */
+        qobject_decref(qov->root);
+        qov->root = value;
+    } else {
+        switch (qobject_type(cur)) {
+        case QTYPE_QDICT:
+            assert(name);
+            qdict_put_obj(qobject_to_qdict(cur), name, value);
+            break;
+        case QTYPE_QLIST:
+            qlist_append_obj(qobject_to_qlist(cur), value);
+            break;
+        default:
+            g_assert_not_reached();
+        }
     }
 }

@@ -230,7 +205,9 @@ static void qmp_output_type_any(Visitor *v, QObject **obj, const char *name,
 /* Finish building, and return the root object. Will not be NULL. */
 QObject *qmp_output_get_qobject(QmpOutputVisitor *qov)
 {
-    QObject *obj = qmp_output_first(qov);
+    /* FIXME: we should require that a visit occurred, and that it is
+     * complete (no starts without a matching end) */
+    QObject *obj = qov->root;
     if (obj) {
         qobject_incref(obj);
     } else {
@@ -248,16 +225,12 @@ void qmp_output_visitor_cleanup(QmpOutputVisitor *v)
 {
     QStackEntry *e, *tmp;

-    /* The bottom QStackEntry, if any, owns the root QObject. See the
-     * qmp_output_push_obj() invocations in qmp_output_add_obj(). */
-    QObject *root = QTAILQ_EMPTY(&v->stack) ? NULL : qmp_output_first(v);
-
     QTAILQ_FOREACH_SAFE(e, &v->stack, node, tmp) {
         QTAILQ_REMOVE(&v->stack, e, node);
         g_free(e);
     }

-    qobject_decref(root);
+    qobject_decref(v->root);
     g_free(v);
 }

-- 
2.4.3