[Qemu-devel] [PATCH v13 18/18] qapi: Change visit_type_FOO() to no longer return partial objects

[Eric Blake]

Returning a partial object on error is an invitation for a careless
caller to leak memory.  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 enhancing qapi-visit-core to return true
for input visitors (the callbacks themselves do not need
modification); since we've documented that visit_end_* must be
called after any successful visit_start_*, that is a sufficient
point for knowing that something was allocated during start.

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 <address@hidden>

---
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         | 42 ++++++++++++++++++++++++++++++------------
 include/qapi/visitor-impl.h    |  8 +++++---
 scripts/qapi-visit.py          | 25 +++++++++++++------------
 qapi/qapi-visit-core.c         | 41 ++++++++++++++++++++++++++++++++++-------
 tests/test-qmp-commands.c      | 13 ++++++-------
 tests/test-qmp-input-strict.c  | 19 ++++++++-----------
 tests/test-qmp-input-visitor.c | 10 ++--------
 docs/qapi-code-gen.txt         | 10 ++++++++--
 8 files changed, 106 insertions(+), 62 deletions(-)

diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 8d4837d..5b7c86e 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -66,14 +66,16 @@
  * }' if an error is encountered on "value" (or to have the visitor
  * core auto-generate the nicer name).
  *
- * FIXME: At present, input visitors may allocate an incomplete address@hidden
- * even when visit_type_FOO() reports an error.  Using an output
- * visitor with an incomplete object has undefined behavior; callers
- * must call qapi_free_FOO() (which uses the dealloc visitor, and
- * safely handles an incomplete object) to avoid a memory leak.
+ * If an error is detected during visit_type_FOO() with an input
+ * visitor, then address@hidden 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.
  *
- * Likewise, the QAPI object types (structs, unions, and alternates)
- * have a generated function in qapi-visit.h compatible with:
+ * Additionally, the QAPI object types (structs, unions, and
+ * alternates) have a generated function in qapi-visit.h compatible
+ * with:
  *
  * void visit_type_FOO_members(Visitor *v, FOO *obj, Error **errp);
  *
@@ -105,7 +107,6 @@
  *  v = ...obtain input visitor...
  *  visit_type_Foo(v, NULL, &f, &err);
  *  if (err) {
- *      qapi_free_Foo(f);
  *      ...handle error...
  *  } else {
  *      ...use f...
@@ -113,7 +114,6 @@
  *  v = ...obtain input visitor...
  *  visit_type_FooList(v, NULL, &l, &err);
  *  if (err) {
- *      qapi_free_FooList(l);
  *      ...handle error...
  *  } else {
  *      while (l) {
@@ -257,8 +257,14 @@ void visit_check_struct(Visitor *v, Error **errp);
  * even if intermediate processing was skipped due to errors, to allow
  * the backend to release any resources.  Destroying the visitor may
  * behave as if this was implicitly called.
+ *
+ * Returns true if this is an input visitor (that is, an allocation
+ * occurred during visit_start_struct() if obj was non-NULL).  The
+ * caller can use this, along with tracking whether a local error
+ * occurred in the meantime, to decide when to undo allocation before
+ * returning control from a visit_type_FOO() function.
  */
-void visit_end_struct(Visitor *v);
+bool visit_end_struct(Visitor *v);


 /* === Visiting lists */
@@ -314,8 +320,14 @@ GenericList *visit_next_list(Visitor *v, GenericList 
*tail, size_t size);
  * even if intermediate processing was skipped due to errors, to allow
  * the backend to release any resources.  Destroying the visitor may
  * behave as if this was implicitly called.
+ *
+ * Returns true if this is an input visitor (that is, an allocation
+ * occurred during visit_start_list() if list was non-NULL).  The
+ * caller can use this, along with tracking whether a local error
+ * occurred in the meantime, to decide when to undo allocation before
+ * returning control from a visit_type_FOO() function.
  */
-void visit_end_list(Visitor *v);
+bool visit_end_list(Visitor *v);


 /* === Visiting alternates */
@@ -348,10 +360,16 @@ void visit_start_alternate(Visitor *v, const char *name,
  * the backend to release any resources.  Destroying the visitor may
  * behave as if this was implicitly called.
  *
+ * Returns true if this is an input visitor (that is, an allocation
+ * occurred during visit_start_alternate() if obj was non-NULL).  The
+ * caller can use this, along with tracking whether a local error
+ * occurred in the meantime, to decide when to undo allocation before
+ * returning control from a visit_type_FOO() function.
+ *
  * TODO: Should all the visit_end_* interfaces take obj parameter, so
  * that dealloc visitor need not track what was passed in visit_start?
  */
-void visit_end_alternate(Visitor *v);
+bool visit_end_alternate(Visitor *v);


 /* === Other helpers */
diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index 0471465..f113869 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -42,7 +42,8 @@ struct Visitor
     /* Optional; intended for input visitors.  */
     void (*check_struct)(Visitor *v, Error **errp);

-    /* Must be set to visit structs.  */
+    /* Must be set to visit structs.  The core takes care of the
+     * return value.  */
     void (*end_struct)(Visitor *v);

     /* Must be set; document if @list may not be NULL.  */
@@ -52,7 +53,7 @@ struct Visitor
     /* Must be set.  */
     GenericList *(*next_list)(Visitor *v, GenericList *tail, size_t size);

-    /* Must be set.  */
+    /* Must be set.  The core takes care of the return value.  */
     void (*end_list)(Visitor *v);

     /* Must be set by input and dealloc visitors to visit alternates;
@@ -61,7 +62,8 @@ struct Visitor
                             GenericAlternate **obj, size_t size,
                             bool promote_int, Error **errp);

-    /* Optional, needed for dealloc visitor.  */
+    /* Optional, needed for dealloc visitor.  The core takes care of
+     * the return value.  */
     void (*end_alternate)(Visitor *v);

     /* Must be set.  */
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index fdb10c0..e95285e 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -91,10 +91,6 @@ 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)
@@ -114,7 +110,10 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, 
%(c_name)s **obj, Error
             break;
         }
     }
-    visit_end_list(v);
+    if (visit_end_list(v) && err) {
+        qapi_free_%(c_name)s(*obj);
+        *obj = NULL;
+    }
 out:
     error_propagate(errp, err);
 }
@@ -191,21 +190,20 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, 
%(c_name)s **obj, Error
         error_setg(&err, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
                    "%(name)s");
     }
-    visit_end_alternate(v);
+    if (visit_end_alternate(v) && err) {
+        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)
@@ -225,7 +223,10 @@ 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 (visit_end_struct(v) && err) {
+        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 bbcedb1..7c36b24 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -22,11 +22,17 @@
 void visit_start_struct(Visitor *v, const char *name, void **obj,
                         size_t size, Error **errp)
 {
+    Error *err = NULL;
+
     if (obj) {
         assert(size);
         assert(v->type != VISITOR_OUTPUT || *obj);
     }
-    v->start_struct(v, name, obj, size, errp);
+    v->start_struct(v, name, obj, size, &err);
+    if (obj && v->type == VISITOR_INPUT) {
+        assert(err || *obj);
+    }
+    error_propagate(errp, err);
 }

 void visit_check_struct(Visitor *v, Error **errp)
@@ -36,11 +42,13 @@ void visit_check_struct(Visitor *v, Error **errp)
     }
 }

-void visit_end_struct(Visitor *v)
+bool visit_end_struct(Visitor *v)
 {
     v->end_struct(v);
+    return v->type == VISITOR_INPUT;
 }

+
 void visit_start_list(Visitor *v, const char *name, GenericList **list,
                       size_t size, Error **errp)
 {
@@ -54,26 +62,34 @@ GenericList *visit_next_list(Visitor *v, GenericList *tail, 
size_t size)
     return v->next_list(v, tail, size);
 }

-void visit_end_list(Visitor *v)
+bool visit_end_list(Visitor *v)
 {
     v->end_list(v);
+    return v->type == VISITOR_INPUT;
 }

 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(err || *obj);
+        }
+        error_propagate(errp, err);
     }
 }

-void visit_end_alternate(Visitor *v)
+bool visit_end_alternate(Visitor *v)
 {
     if (v->end_alternate) {
         v->end_alternate(v);
     }
+    return v->type == VISITOR_INPUT;
 }

 bool visit_optional(Visitor *v, const char *name, bool *present)
@@ -205,12 +221,17 @@ 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)
 {
+    Error *err = NULL;
     assert(obj);
     /* TODO: Fix callers to not pass NULL when they mean "", so that we
      * can enable:
     assert(v->type != VISITOR_OUTPUT || *obj);
      */
-    v->type_str(v, name, obj, errp);
+    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,
@@ -222,9 +243,15 @@ void visit_type_number(Visitor *v, const char *name, 
double *obj,

 void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp)
 {
+    Error *err = NULL;
+
     assert(obj);
     assert(v->type != VISITOR_OUTPUT || *obj);
-    v->type_any(v, name, obj, errp);
+    v->type_any(v, name, obj, &err);
+    if (v->type == VISITOR_INPUT) {
+        assert(err || *obj);
+    }
+    error_propagate(errp, err);
 }

 void visit_type_null(Visitor *v, const char *name, Error **errp)
diff --git a/tests/test-qmp-commands.c b/tests/test-qmp-commands.c
index 14a9ebb..d6c494d 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 6a33aa4..9587b14 100644
--- a/tests/test-qmp-input-strict.c
+++ b/tests/test-qmp-input-strict.c
@@ -181,10 +181,7 @@ 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,
@@ -198,7 +195,7 @@ 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,
@@ -212,7 +209,7 @@ 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,
@@ -227,7 +224,7 @@ 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,
@@ -241,7 +238,7 @@ 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,
@@ -256,13 +253,13 @@ 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,
                                          const void *unused)
 {
-    UserDefAlternate *tmp = NULL;
+    UserDefAlternate *tmp;
     Visitor *v;
     Error *err = NULL;

@@ -270,7 +267,7 @@ 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 a62c2b1..19bed0a 100644
--- a/tests/test-qmp-input-visitor.c
+++ b/tests/test-qmp-input-visitor.c
@@ -759,18 +759,12 @@ 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 6107fa4..b702885 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -904,7 +904,10 @@ Example:
         }
         visit_check_struct(v, &err);
     out_obj:
-        visit_end_struct(v);
+        if (visit_end_struct(v) && err) {
+            qapi_free_UserDefOne(*obj);
+            *obj = NULL;
+        }
     out:
         error_propagate(errp, err);
     }
@@ -927,7 +930,10 @@ Example:
             }
         }

-        visit_end_list(v);
+        if (visit_end_list(v) && err) {
+            qapi_free_UserDefOneList(*obj);
+            *obj = NULL;
+        }
     out:
         error_propagate(errp, err);
     }
-- 
2.5.0