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

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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>
This commit is contained in:
Eric Blake 2016-04-28 15:45:32 -06:00 • committed by Markus Armbruster
parent d9f62dde13
commit 68ab47e4b4
7 changed files with 57 additions and 43 deletions
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10
tests/test-qmp-input-visitor.c
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@ -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,