qapi: Fix generated code when flat union has member 'kind'

A flat union's tag member gets renamed to 'kind' in the generated
code.  Breaks when another member named 'kind' exists.

Example, adapted from qapi-schema-test.json:

    { 'struct': 'UserDefUnionBase',
      'data': { 'kind': 'str', 'enum1': 'EnumOne' } }

We generate:

    struct UserDefFlatUnion
    {
        EnumOne kind;
        union {
            void *data;
            UserDefA *value1;
            UserDefB *value2;
            UserDefB *value3;
        };
        char *kind;
    };

Kill the silly rename.
Reported-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

scripts/qapi-types.py

@@ -200,11 +200,12 @@ def generate_union(expr, meta):
     ret = mcgen('''
 struct %(name)s
 {
-    %(discriminator_type_name)s kind;
+    %(discriminator_type_name)s %(discriminator)s;
     union {
         void *data;
 ''',
                 name=name,
+                discriminator=c_name(discriminator or 'kind'),
                 discriminator_type_name=c_name(discriminator_type_name))
 
     for key in typeinfo:

scripts/qapi-visit.py

@@ -288,20 +288,23 @@ void visit_type_%(name)s(Visitor *m, %(name)s **obj, const char *name, Error **e
                      name=c_name(name))
 
     if not discriminator:
+        tag = 'kind'
         disc_key = "type"
     else:
+        tag = discriminator
         disc_key = discriminator
     ret += mcgen('''
-        visit_type_%(disc_type)s(m, &(*obj)->kind, "%(disc_key)s", &err);
+        visit_type_%(disc_type)s(m, &(*obj)->%(c_tag)s, "%(disc_key)s", &err);
         if (err) {
             goto out_obj;
         }
         if (!visit_start_union(m, !!(*obj)->data, &err) || err) {
             goto out_obj;
         }
-        switch ((*obj)->kind) {
+        switch ((*obj)->%(c_tag)s) {
 ''',
                  disc_type = disc_type,
+                 c_tag=c_name(tag),
                  disc_key = disc_key)
 
     for key in members:

tests/test-qmp-input-visitor.c

@@ -313,7 +313,7 @@ static void test_visitor_in_union_flat(TestInputVisitorData *data,
 
     visit_type_UserDefFlatUnion(v, &tmp, NULL, &err);
     g_assert(err == NULL);
-    g_assert_cmpint(tmp->kind, ==, ENUM_ONE_VALUE1);
+    g_assert_cmpint(tmp->enum1, ==, ENUM_ONE_VALUE1);
     g_assert_cmpstr(tmp->string, ==, "str");
     /* TODO g_assert_cmpint(tmp->integer, ==, 41); */
     g_assert_cmpint(tmp->value1->boolean, ==, true);

tests/test-qmp-output-visitor.c

@@ -437,7 +437,7 @@ static void test_visitor_out_union_flat(TestOutputVisitorData *data,
     Error *err = NULL;
 
     UserDefFlatUnion *tmp = g_malloc0(sizeof(UserDefFlatUnion));
-    tmp->kind = ENUM_ONE_VALUE1;
+    tmp->enum1 = ENUM_ONE_VALUE1;
     tmp->string = g_strdup("str");
     tmp->value1 = g_malloc0(sizeof(UserDefA));
     /* TODO when generator bug is fixed: tmp->integer = 41; */