qapi: Simplify visits of optional fields

None of the visitor callbacks would set an error when testing
if an optional field was present; make this part of the interface
contract by eliminating the errp argument.

The resulting generated code has a nice diff:

|- visit_optional(v, &has_fdset_id, "fdset-id", &err);
|- if (err) {
|- goto out;
|- }
|+ visit_optional(v, &has_fdset_id, "fdset-id");
| if (has_fdset_id) {
| visit_type_int(v, &fdset_id, "fdset-id", &err);
| if (err) {
| goto out;
| }
| }

Backports commit 5cdc8831a795fb8452d7e34f644202fd724e122a from qemu

msvc/unicorn/qapi-visit.c

@@ -62,10 +62,7 @@ static void visit_type_X86CPUFeatureWordInfo_fields(Visitor *v, X86CPUFeatureWor
     if (err) {
         goto out;
     }
-    visit_optional(v, &(*obj)->has_cpuid_input_ecx, "cpuid-input-ecx", &err);
-    if (err) {
-        goto out;
-    }
+    visit_optional(v, &(*obj)->has_cpuid_input_ecx, "cpuid-input-ecx");
     if ((*obj)->has_cpuid_input_ecx) {
         visit_type_int(v, &(*obj)->cpuid_input_ecx, "cpuid-input-ecx", &err);
         if (err) {

qemu/include/qapi/visitor-impl.h

@@ -52,9 +52,8 @@ struct Visitor
     void (*type_any)(Visitor *v, QObject **obj, const char *name,
                      Error **errp);
 
-    /* May be NULL */
-    void (*optional)(Visitor *v, bool *present, const char *name,
-                     Error **errp);
+    /* May be NULL; most useful for input visitors. */
+    void (*optional)(Visitor *v, bool *present, const char *name);
 
     void (*type_uint8)(Visitor *v, uint8_t *obj, const char *name, Error **errp);
     void (*type_uint16)(Visitor *v, uint16_t *obj, const char *name, Error **errp);

qemu/include/qapi/visitor.h

@@ -36,8 +36,15 @@ void visit_end_implicit_struct(Visitor *v);
 void visit_start_list(Visitor *v, const char *name, Error **errp);
 GenericList *visit_next_list(Visitor *v, GenericList **list);
 void visit_end_list(Visitor *v);
-void visit_optional(Visitor *v, bool *present, const char *name,
-                    Error **errp);
+
+/**
+ * Check if an optional member @name of an object needs visiting.
+ * For input visitors, set *@present according to whether the
+ * corresponding visit_type_*() needs calling; for other visitors,
+ * leave *@present unchanged.
+ */
+void visit_optional(Visitor *v, bool *present, const char *name);
+
 /**
  * Determine the qtype of the item @name in the current object visit.
  * For input visitors, set *@type to the correct qtype of a qapi

qemu/qapi/qapi-visit-core.c

@@ -74,11 +74,10 @@ void visit_end_union(Visitor *v, bool data_present, Error **errp)
     }
 }
 
-void visit_optional(Visitor *v, bool *present, const char *name,
-                    Error **errp)
+void visit_optional(Visitor *v, bool *present, const char *name)
 {
     if (v->optional) {
-        v->optional(v, present, name, errp);
+        v->optional(v, present, name);
     }
 }
 

qemu/qapi/qmp-input-visitor.c

@@ -319,8 +319,7 @@ static void qmp_input_type_any(Visitor *v, QObject **obj, const char *name,
     *obj = qobj;
 }
 
-static void qmp_input_optional(Visitor *v, bool *present, const char *name,
-                               Error **errp)
+static void qmp_input_optional(Visitor *v, bool *present, const char *name)
 {
     QmpInputVisitor *qiv = to_qiv(v);
     QObject *qobj = qmp_input_get_object(qiv, name, true);

qemu/qapi/string-input-visitor.c

@@ -297,8 +297,7 @@ static void parse_type_number(Visitor *v, double *obj, const char *name,
     *obj = val;
 }
 
-static void parse_optional(Visitor *v, bool *present, const char *name,
-                           Error **errp)
+static void parse_optional(Visitor *v, bool *present, const char *name)
 {
     StringInputVisitor *siv = to_siv(v);
 

qemu/scripts/qapi.py

@@ -1656,15 +1656,11 @@ def gen_visit_fields(members, prefix='', need_cast=False, skiperr=False):
     for memb in members:
         if memb.optional:
             ret += mcgen('''
-    visit_optional(v, &%(prefix)shas_%(c_name)s, "%(name)s", %(errp)s);
+    visit_optional(v, &%(prefix)shas_%(c_name)s, "%(name)s");
+    if (%(prefix)shas_%(c_name)s) {
 ''',
                          prefix=prefix, c_name=c_name(memb.name),
                          name=memb.name, errp=errparg)
-            ret += gen_err_check(skiperr=skiperr)
-            ret += mcgen('''
-    if (%(prefix)shas_%(c_name)s) {
-''',
-                         prefix=prefix, c_name=c_name(memb.name))
             push_indent()
 
         # Ugly: sometimes we need to cast away const