qapi: Change visit_type_FOO() to no longer return partial objects

Returning a partial object on error is an invitation for a careless
caller to leak memory.  We already fixed things in an earlier
patch to guarantee NULL if visit_start fails ("qapi: Guarantee
NULL obj on input visitor callback error"), but that does not
help the case where visit_start succeeds but some other failure
happens before visit_end, such that we leak a partially constructed
object outside visit_type_FOO(). As no one outside the testsuite
was actually relying on these semantics, it is cleaner to just
document and guarantee that ALL pointer-based visit_type_FOO()
functions always leave a safe value in *obj during an input visitor
(either the new object on success, or NULL if an error is
encountered), so callers can now unconditionally use
qapi_free_FOO() to clean up regardless of whether an error occurred.

The decision is done by adding visit_is_input(), then updating the
generated code to check if additional cleanup is needed based on
the type of visitor in use.

Note that we still leave *obj unchanged after a scalar-based
visit_type_FOO(); I did not feel like auditing all uses of
visit_type_Enum() to see if the callers would tolerate a specific
sentinel value (not to mention having to decide whether it would
be better to use 0 or ENUM__MAX as that sentinel).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-25-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 13 insertions, 9 deletions

diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 8b7efcc74a..70ea8caef5 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -108,10 +108,6 @@ out:
 
 
 def gen_visit_list(name, element_type):
-    # FIXME: if *obj is NULL on entry, and the first visit_next_list()
-    # assigns to *obj, while a later one fails, we should clean up *obj
-    # rather than leaving it non-NULL. As currently written, the caller must
-    # call qapi_free_FOOList() to avoid a memory leak of the partial FOOList.
     return mcgen('''
 
 void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error **errp)
@@ -134,6 +130,10 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
     }
 
     visit_end_list(v);
+    if (err && visit_is_input(v)) {
+        qapi_free_%(c_name)s(*obj);
+        *obj = NULL;
+    }
 out:
     error_propagate(errp, err);
 }
@@ -211,20 +211,20 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
                    "%(name)s");
     }
     visit_end_alternate(v);
+    if (err && visit_is_input(v)) {
+        qapi_free_%(c_name)s(*obj);
+        *obj = NULL;
+    }
 out:
     error_propagate(errp, err);
 }
 ''',
-                 name=name)
+                 name=name, c_name=c_name(name))
 
     return ret
 
 
 def gen_visit_object(name, base, members, variants):
-    # FIXME: if *obj is NULL on entry, and visit_start_struct() assigns to
-    # *obj, but then visit_type_FOO_members() fails, we should clean up *obj
-    # rather than leaving it non-NULL. As currently written, the caller must
-    # call qapi_free_FOO() to avoid a memory leak of the partial FOO.
     return mcgen('''
 
 void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error **errp)
@@ -245,6 +245,10 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
     visit_check_struct(v, &err);
 out_obj:
     visit_end_struct(v);
+    if (err && visit_is_input(v)) {
+        qapi_free_%(c_name)s(*obj);
+        *obj = NULL;
+    }
 out:
     error_propagate(errp, err);
 }