qapi-visit: Simplify how we visit common union members

For a simple union SU, gen_visit_union() generates a visit of its
single tag member, like this:

visit_type_SUKind(v, "type", &(*obj)->type, &err);

For a flat union FU with base B, it generates a visit of its base
fields:

visit_type_B_fields(v, (B **)obj, &err);

Instead, we can simply visit the common members using the same fields
visit function we use for structs, generated with
gen_visit_struct_fields(). This function visits the base if any, then
the local members.

For a simple union SU, visit_type_SU_fields() contains exactly the old
tag member visit, because there is no base, and the tag member is the
only member. For instance, the code generated for qapi-schema.json's
KeyValue changes like this:

+static void visit_type_KeyValue_fields(Visitor *v, KeyValue **obj, Error **errp)
+{
+ Error *err = NULL;
+
+ visit_type_KeyValueKind(v, "type", &(*obj)->type, &err);
+ if (err) {
+ goto out;
+ }
+
+out:
+ error_propagate(errp, err);
+}
+
void visit_type_KeyValue(Visitor *v, const char *name, KeyValue **obj, Error **errp)
{
Error *err = NULL;
@@ -4863,7 +4911,7 @@ void visit_type_KeyValue(Visitor *v, con
if (!*obj) {
goto out_obj;
}
- visit_type_KeyValueKind(v, "type", &(*obj)->type, &err);
+ visit_type_KeyValue_fields(v, obj, &err);
if (err) {
goto out_obj;
}

For a flat union FU, visit_type_FU_fields() contains exactly the old
base fields visit, because there is a base, but no members. For
instance, the code generated for qapi-schema.json's CpuInfo changes
like this:

static void visit_type_CpuInfoBase_fields(Visitor *v, CpuInfoBase **obj, Error **errp);

+static void visit_type_CpuInfo_fields(Visitor *v, CpuInfo **obj, Error **errp)
+{
+ Error *err = NULL;
+
+ visit_type_CpuInfoBase_fields(v, (CpuInfoBase **)obj, &err);
+ if (err) {
+ goto out;
+ }
+
+out:
+ error_propagate(errp, err);
+}
+
static void visit_type_CpuInfoX86_fields(Visitor *v, CpuInfoX86 **obj, Error **errp)
...
@@ -3485,7 +3509,7 @@ void visit_type_CpuInfo(Visitor *v, cons
if (!*obj) {
goto out_obj;
}
- visit_type_CpuInfoBase_fields(v, (CpuInfoBase **)obj, &err);
+ visit_type_CpuInfo_fields(v, obj, &err);
if (err) {
goto out_obj;
}

As you see, the generated code grows a bit, but in practice, it's lost
in the noise: qapi-schema.json's qapi-visit.c gains roughly 1%.

This simplification became possible with commit 441cbac "qapi-visit:
Convert to QAPISchemaVisitor, fixing bugs". It's a step towards
unifying gen_struct() and gen_union().

Backports commit d7445b57f4342d50c25b5bc1746048fa3720511b from qemu

qemu/scripts/qapi-visit.py

@@ -239,11 +239,8 @@ out:
     return ret
 
 
-def gen_visit_union(name, base, variants):
+def gen_visit_union(name, base, members, variants):
-    ret = ''
+    ret = gen_visit_struct_fields(name, base, members)
-
-    if base:
-        ret += gen_visit_fields_decl(base)
 
     for var in variants.variants:
         # Ugly special case for simple union TODO get rid of it
@@ -263,21 +260,10 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
     if (!*obj) {
         goto out_obj;
     }
+    visit_type_%(c_name)s_fields(v, obj, &err);
 ''',
                  c_name=c_name(name))
 
-    if base:
-        ret += mcgen('''
-    visit_type_%(c_name)s_fields(v, (%(c_name)s **)obj, &err);
-''',
-                     c_name=base.c_name())
-    else:
-        ret += mcgen('''
-    visit_type_%(c_type)s(v, "%(name)s", &(*obj)->%(c_name)s, &err);
-''',
-                     c_type=variants.tag_member.type.c_name(),
-                     c_name=c_name(variants.tag_member.name),
-                     name=variants.tag_member.name)
     ret += gen_err_check(label='out_obj')
     ret += mcgen('''
     if (!visit_start_union(v, !!(*obj)->u.data, &err) || err) {
@@ -380,11 +366,7 @@ class QAPISchemaGenVisitVisitor(QAPISchemaVisitor):
     def visit_object_type(self, name, info, base, members, variants):
         self.decl += gen_visit_decl(name)
         if variants:
-            if members:
+            self.defn += gen_visit_union(name, base, members, variants)
-                # Members other than variants.tag_member not implemented
-                assert len(members) == 1
-                assert members[0] == variants.tag_member
-            self.defn += gen_visit_union(name, base, variants)
         else:
             self.defn += gen_visit_struct(name, base, members)