qapi: Split visit_end_struct() into pieces

As mentioned in previous patches, we want to call visit_end_struct()
functions unconditionally, so that visitors can release resources
tied up since the matching visit_start_struct() without also having
to worry about error priority if more than one error occurs.

Even though error_propagate() can be safely used to ignore a second
error during cleanup caused by a first error, it is simpler if the
cleanup cannot set an error.  So, split out the error checking
portion (basically, input visitors checking for unvisited keys) into
a new function visit_check_struct(), which can be safely skipped if
any earlier errors are encountered, and leave the cleanup portion
(which never fails, but must be called unconditionally if
visit_start_struct() succeeded) in visit_end_struct().

Generated code in qapi-visit.c has diffs resembling:

|@@ -59,10 +59,12 @@ void visit_type_ACPIOSTInfo(Visitor *v,
|         goto out_obj;
|     }
|     visit_type_ACPIOSTInfo_members(v, obj, &err);
|-    error_propagate(errp, err);
|-    err = NULL;
|+    if (err) {
|+        goto out_obj;
|+    }
|+    visit_check_struct(v, &err);
| out_obj:
|-    visit_end_struct(v, &err);
|+    visit_end_struct(v);
| out:

and in qapi-event.c:

@@ -47,7 +47,10 @@ void qapi_event_send_acpi_device_ost(ACP
|         goto out;
|     }
|     visit_type_q_obj_ACPI_DEVICE_OST_arg_members(v, &param, &err);
|-    visit_end_struct(v, err ? NULL : &err);
|+    if (!err) {
|+        visit_check_struct(v, &err);
|+    }
|+    visit_end_struct(v);
|     if (err) {
|         goto out;

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-20-git-send-email-eblake@redhat.com>
[Conflict with a doc fixup resolved]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

2 files changed, 4 insertions, 2 deletions

diff --git a/tests/test-qmp-input-visitor.c b/tests/test-qmp-input-visitor.c
index 10e00ecbc7..6617276bce 100644
--- a/tests/test-qmp-input-visitor.c
+++ b/tests/test-qmp-input-visitor.c
@@ -303,7 +303,8 @@ static void test_visitor_in_null(TestInputVisitorData *data,
     error_free_or_abort(&err);
     visit_type_null(v, "b", &err);
     error_free_or_abort(&err);
-    visit_end_struct(v, &error_abort);
+    visit_check_struct(v, &error_abort);
+    visit_end_struct(v);
 }
 
 static void test_visitor_in_union_flat(TestInputVisitorData *data,
diff --git a/tests/test-qmp-output-visitor.c b/tests/test-qmp-output-visitor.c
index 5543511c21..1f80e696ea 100644
--- a/tests/test-qmp-output-visitor.c
+++ b/tests/test-qmp-output-visitor.c
@@ -498,7 +498,8 @@ static void test_visitor_out_null(TestOutputVisitorData *data,
 
     visit_start_struct(data->ov, NULL, NULL, 0, &error_abort);
     visit_type_null(data->ov, "a", &error_abort);
-    visit_end_struct(data->ov, &error_abort);
+    visit_check_struct(data->ov, &error_abort);
+    visit_end_struct(data->ov);
     arg = qmp_output_get_qobject(data->qov);
     g_assert(qobject_type(arg) == QTYPE_QDICT);
     qdict = qobject_to_qdict(arg);