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authorEric Blake <eblake@redhat.com>2016-04-28 15:45:32 -0600
committerMarkus Armbruster <armbru@redhat.com>2016-05-12 09:47:55 +0200
commit68ab47e4b4ecc1c4649362b8cc1e49794d1a6537 (patch)
treec3fb21b91ab1fde06d3471e02f32f694aa18837a /include/qapi
parentd9f62dde1303286b24ac8ce88be27e2b9b9c5f46 (diff)
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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>
Diffstat (limited to 'include/qapi')
-rw-r--r--include/qapi/visitor.h25
1 files changed, 16 insertions, 9 deletions
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 8de6b43..4d12167 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -66,12 +66,14 @@
* member @name is not present, or is present but not the specified
* type).
*
- * FIXME: At present, visit_type_FOO() is an awkward interface: input
- * visitors may allocate an incomplete *@obj even when reporting an
- * error, but using an output visitor with an incomplete object has
- * undefined behavior. To avoid a memory leak, callers must use
- * qapi_free_FOO() even on error (this uses the dealloc visitor, and
- * safely handles an incomplete object).
+ * If an error is detected during visit_type_FOO() with an input
+ * visitor, then *@obj will be NULL for pointer types, and left
+ * unchanged for scalar types. Using an output visitor with an
+ * incomplete object has undefined behavior (other than a special case
+ * for visit_type_str() treating NULL like ""), while the dealloc
+ * visitor safely handles incomplete objects. Since input visitors
+ * never produce an incomplete object, such an object is possible only
+ * by manual construction.
*
* For the QAPI object types (structs, unions, and alternates), there
* is an additional generated function in qapi-visit.h compatible
@@ -106,7 +108,6 @@
* v = ...obtain input visitor...
* visit_type_Foo(v, NULL, &f, &err);
* if (err) {
- * qapi_free_Foo(f);
* ...handle error...
* } else {
* ...use f...
@@ -124,7 +125,6 @@
* v = ...obtain input visitor...
* visit_type_FooList(v, NULL, &l, &err);
* if (err) {
- * qapi_free_FooList(l);
* ...handle error...
* } else {
* for ( ; l; l = l->next) {
@@ -154,7 +154,9 @@
* helpers that rely on in-tree information to control the walk:
* visit_optional() for the 'has_member' field associated with
* optional 'member' in the C struct; and visit_next_list() for
- * advancing through a FooList linked list. Only the generated
+ * advancing through a FooList linked list. Similarly, the
+ * visit_is_input() helper makes it possible to write code that is
+ * visitor-agnostic everywhere except for cleanup. Only the generated
* visit_type functions need to use these helpers.
*
* It is also possible to use the visitors to do a virtual walk, where
@@ -405,6 +407,11 @@ bool visit_optional(Visitor *v, const char *name, bool *present);
void visit_type_enum(Visitor *v, const char *name, int *obj,
const char *const strings[], Error **errp);
+/*
+ * Check if visitor is an input visitor.
+ */
+bool visit_is_input(Visitor *v);
+
/*** Visiting built-in types ***/
/*