qapi: Guarantee NULL obj on input visitor callback error

Our existing input visitors were not very consistent on errors in a
function taking 'TYPE **obj'.  These are start_struct(),
start_alternate(), type_str(), and type_any().  next_list() is
similar, but can't fail (see commit 08f9541).  While all of them set
'*obj' to allocated storage on success, it was not obvious whether
'*obj' was guaranteed safe on failure, or whether it was left
uninitialized.  But a future patch wants to guarantee that
visit_type_FOO() does not leak a partially-constructed obj back to
the caller; it is easier to implement this if we can reliably state
that input visitors assign '*obj' regardless of success or failure,
and that on failure *obj is NULL.  Add assertions to enforce
consistency in the final setting of err vs. *obj.

The opts-visitor start_struct() doesn't set an error, but it
also was doing a weird check for 0 size; all callers pass in
non-zero size if obj is non-NULL.

The testsuite has at least one spot where we no longer need
to pre-initialize a variable prior to a visit; valgrind confirms
that the test is still fine with the cleanup.

A later patch will document the design constraint implemented
here.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-3-git-send-email-eblake@redhat.com>
[visit_start_alternate()'s assertion tightened, commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/opts-visitor.c

@@ -133,7 +133,7 @@ opts_start_struct(Visitor *v, const char *name, void **obj,
     const QemuOpt *opt;
 
     if (obj) {
-        *obj = g_malloc0(size > 0 ? size : 1);
+        *obj = g_malloc0(size);
     }
     if (ov->depth++ > 0) {
         return;
@@ -314,6 +314,7 @@ opts_type_str(Visitor *v, const char *name, char **obj, Error **errp)
 
     opt = lookup_scalar(ov, name, errp);
     if (!opt) {
+        *obj = NULL;
         return;
     }
     *obj = g_strdup(opt->str ? opt->str : "");

qapi/qapi-visit-core.c

@@ -23,7 +23,13 @@
 void visit_start_struct(Visitor *v, const char *name, void **obj,
                         size_t size, Error **errp)
 {
-    v->start_struct(v, name, obj, size, errp);
+    Error *err = NULL;
+
+    v->start_struct(v, name, obj, size, &err);
+    if (obj && v->type == VISITOR_INPUT) {
+        assert(!err != !*obj);
+    }
+    error_propagate(errp, err);
 }
 
 void visit_end_struct(Visitor *v, Error **errp)
@@ -51,10 +57,16 @@ void visit_start_alternate(Visitor *v, const char *name,
                            GenericAlternate **obj, size_t size,
                            bool promote_int, Error **errp)
 {
+    Error *err = NULL;
+
     assert(obj && size >= sizeof(GenericAlternate));
     if (v->start_alternate) {
-        v->start_alternate(v, name, obj, size, promote_int, errp);
+        v->start_alternate(v, name, obj, size, promote_int, &err);
     }
+    if (v->type == VISITOR_INPUT) {
+        assert(v->start_alternate && !err != !*obj);
+    }
+    error_propagate(errp, err);
 }
 
 void visit_end_alternate(Visitor *v)
@@ -188,7 +200,14 @@ void visit_type_bool(Visitor *v, const char *name, bool *obj, Error **errp)
 
 void visit_type_str(Visitor *v, const char *name, char **obj, Error **errp)
 {
-    v->type_str(v, name, obj, errp);
+    Error *err = NULL;
+
+    assert(obj);
+    v->type_str(v, name, obj, &err);
+    if (v->type == VISITOR_INPUT) {
+        assert(!err != !*obj);
+    }
+    error_propagate(errp, err);
 }
 
 void visit_type_number(Visitor *v, const char *name, double *obj,
@@ -199,7 +218,14 @@ void visit_type_number(Visitor *v, const char *name, double *obj,
 
 void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp)
 {
-    v->type_any(v, name, obj, errp);
+    Error *err = NULL;
+
+    assert(obj);
+    v->type_any(v, name, obj, &err);
+    if (v->type == VISITOR_INPUT) {
+        assert(!err != !*obj);
+    }
+    error_propagate(errp, err);
 }
 
 static void output_type_enum(Visitor *v, const char *name, int *obj,

qapi/qmp-input-visitor.c

@@ -120,6 +120,9 @@ static void qmp_input_start_struct(Visitor *v, const char *name, void **obj,
     QObject *qobj = qmp_input_get_object(qiv, name, true);
     Error *err = NULL;
 
+    if (obj) {
+        *obj = NULL;
+    }
     if (!qobj || qobject_type(qobj) != QTYPE_QDICT) {
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
                    "QDict");
@@ -267,6 +270,7 @@ static void qmp_input_type_str(Visitor *v, const char *name, char **obj,
     QString *qstr = qobject_to_qstring(qmp_input_get_object(qiv, name, true));
 
     if (!qstr) {
+        *obj = NULL;
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
                    "string");
         return;

qapi/string-input-visitor.c

@@ -293,6 +293,7 @@ static void parse_type_str(Visitor *v, const char *name, char **obj,
     if (siv->string) {
         *obj = g_strdup(siv->string);
     } else {
+        *obj = NULL;
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
                    "string");
     }

tests/test-qmp-input-strict.c

@@ -263,7 +263,7 @@ static void test_validate_fail_union_flat_no_discrim(TestInputVisitorData *data,
 static void test_validate_fail_alternate(TestInputVisitorData *data,
                                          const void *unused)
 {
-    UserDefAlternate *tmp = NULL;
+    UserDefAlternate *tmp;
     Visitor *v;
     Error *err = NULL;