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 <ebl...@redhat.com>
---
v16: drop spurious hunk
v15: rebase, redo semantics on how generated code learns
about input visitors, hoist unrelated assertions earlier
v14: no change
v13: rebase to latest, documentation tweaks
v12: rebase to latest, don't modify callback signatures, use newer
approach for detecting input visitors, avoid 'allocated' boolean
[no v10, v11]
v9: fix bug in use of errp
v8: rebase to earlier changes
v7: rebase to earlier changes, enhance commit message, also fix
visit_type_str() and visit_type_any()
v6: rebase on top of earlier doc and formatting improvements, mention
that *obj can be uninitialized on entry to an input visitor, rework
semantics to keep valgrind happy on uninitialized input, break some
long lines
---
include/qapi/visitor.h | 25 ++++++++++++++++---------
scripts/qapi-visit.py | 22 +++++++++++++---------
qapi/qapi-visit-core.c | 5 +++++
tests/test-qmp-commands.c | 13 ++++++-------
tests/test-qmp-input-strict.c | 17 +++++++----------
tests/test-qmp-input-visitor.c | 10 ++--------
docs/qapi-code-gen.txt | 8 ++++++++
7 files changed, 57 insertions(+), 43 deletions(-)
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 3095455..a430c19 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
@@ -404,6 +406,11 @@ 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 ***/
/*
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 8b7efcc..70ea8ca 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);
}
diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 2aa9137..f5d4b52 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -104,6 +104,11 @@ bool visit_optional(Visitor *v, const char *name, bool
*present)
return *present;
}
+bool visit_is_input(Visitor *v)
+{
+ return v->type == VISITOR_INPUT;
+}
+
void visit_type_int(Visitor *v, const char *name, int64_t *obj, Error **errp)
{
assert(obj);
diff --git a/tests/test-qmp-commands.c b/tests/test-qmp-commands.c
index 597fb44..5c3edd7 100644
--- a/tests/test-qmp-commands.c
+++ b/tests/test-qmp-commands.c
@@ -228,14 +228,13 @@ static void test_dealloc_partial(void)
QDECREF(ud2_dict);
}
- /* verify partial success */
- assert(ud2 != NULL);
- assert(ud2->string0 != NULL);
- assert(strcmp(ud2->string0, text) == 0);
- assert(ud2->dict1 == NULL);
-
- /* confirm & release construction error */
+ /* verify that visit_type_XXX() cleans up properly on error */
error_free_or_abort(&err);
+ assert(!ud2);
+
+ /* Manually create a partial object, leaving ud2->dict1 at NULL */
+ ud2 = g_new0(UserDefTwo, 1);
+ ud2->string0 = g_strdup(text);
/* tear down partial object */
qapi_free_UserDefTwo(ud2);
diff --git a/tests/test-qmp-input-strict.c b/tests/test-qmp-input-strict.c
index 2b053a2..4602529 100644
--- a/tests/test-qmp-input-strict.c
+++ b/tests/test-qmp-input-strict.c
@@ -182,10 +182,7 @@ static void test_validate_fail_struct(TestInputVisitorData
*data,
visit_type_TestStruct(v, NULL, &p, &err);
error_free_or_abort(&err);
- if (p) {
- g_free(p->string);
- }
- g_free(p);
+ g_assert(!p);
}
static void test_validate_fail_struct_nested(TestInputVisitorData *data,
@@ -199,7 +196,7 @@ static void
test_validate_fail_struct_nested(TestInputVisitorData *data,
visit_type_UserDefTwo(v, NULL, &udp, &err);
error_free_or_abort(&err);
- qapi_free_UserDefTwo(udp);
+ g_assert(!udp);
}
static void test_validate_fail_list(TestInputVisitorData *data,
@@ -213,7 +210,7 @@ static void test_validate_fail_list(TestInputVisitorData
*data,
visit_type_UserDefOneList(v, NULL, &head, &err);
error_free_or_abort(&err);
- qapi_free_UserDefOneList(head);
+ g_assert(!head);
}
static void test_validate_fail_union_native_list(TestInputVisitorData *data,
@@ -228,7 +225,7 @@ static void
test_validate_fail_union_native_list(TestInputVisitorData *data,
visit_type_UserDefNativeListUnion(v, NULL, &tmp, &err);
error_free_or_abort(&err);
- qapi_free_UserDefNativeListUnion(tmp);
+ g_assert(!tmp);
}
static void test_validate_fail_union_flat(TestInputVisitorData *data,
@@ -242,7 +239,7 @@ static void
test_validate_fail_union_flat(TestInputVisitorData *data,
visit_type_UserDefFlatUnion(v, NULL, &tmp, &err);
error_free_or_abort(&err);
- qapi_free_UserDefFlatUnion(tmp);
+ g_assert(!tmp);
}
static void test_validate_fail_union_flat_no_discrim(TestInputVisitorData
*data,
@@ -257,7 +254,7 @@ static void
test_validate_fail_union_flat_no_discrim(TestInputVisitorData *data,
visit_type_UserDefFlatUnion2(v, NULL, &tmp, &err);
error_free_or_abort(&err);
- qapi_free_UserDefFlatUnion2(tmp);
+ g_assert(!tmp);
}
static void test_validate_fail_alternate(TestInputVisitorData *data,
@@ -271,7 +268,7 @@ static void
test_validate_fail_alternate(TestInputVisitorData *data,
visit_type_UserDefAlternate(v, NULL, &tmp, &err);
error_free_or_abort(&err);
- qapi_free_UserDefAlternate(tmp);
+ g_assert(!tmp);
}
static void do_test_validate_qmp_introspect(TestInputVisitorData *data,
diff --git a/tests/test-qmp-input-visitor.c b/tests/test-qmp-input-visitor.c
index 6617276..cee07ce 100644
--- a/tests/test-qmp-input-visitor.c
+++ b/tests/test-qmp-input-visitor.c
@@ -773,18 +773,12 @@ static void test_visitor_in_errors(TestInputVisitorData
*data,
visit_type_TestStruct(v, NULL, &p, &err);
error_free_or_abort(&err);
- /* FIXME - a failed parse should not leave a partially-allocated p
- * for us to clean up; this could cause callers to leak memory. */
- g_assert(p->string == NULL);
-
- g_free(p->string);
- g_free(p);
+ g_assert(!p);
v = visitor_input_test_init(data, "[ '1', '2', false, '3' ]");
visit_type_strList(v, NULL, &q, &err);
error_free_or_abort(&err);
- assert(q);
- qapi_free_strList(q);
+ assert(!q);
}
static void test_visitor_in_wrong_type(TestInputVisitorData *data,
diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index fe61c5c..d7d6987 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -905,6 +905,10 @@ Example:
visit_check_struct(v, &err);
out_obj:
visit_end_struct(v);
+ if (err && visit_is_input(v)) {
+ qapi_free_UserDefOne(*obj);
+ *obj = NULL;
+ }
out:
error_propagate(errp, err);
}
@@ -929,6 +933,10 @@ Example:
}
visit_end_list(v);
+ if (err && visit_is_input(v)) {
+ qapi_free_UserDefOneList(*obj);
+ *obj = NULL;
+ }
out:
error_propagate(errp, err);
}
--
2.5.5
