[PATCH v2 1/6] qapi: Add interfaces for alias support to Visitor

[Kevin Wolf <kwolf@redhat.com>]

This adds functions to the Visitor interface that can be used to define
aliases and alias scopes.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 include/qapi/visitor-impl.h | 12 ++++++++++
 include/qapi/visitor.h      | 44 +++++++++++++++++++++++++++++++++++++
 qapi/qapi-visit-core.c      | 22 +++++++++++++++++++
 3 files changed, 78 insertions(+)

diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index 7362c043be..d9a6874528 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -113,6 +113,18 @@ struct Visitor
        The core takes care of the return type in the public interface. */
     void (*optional)(Visitor *v, const char *name, bool *present);
 
+    /*
+     * Optional; intended for input visitors. If not given, aliases are
+     * ignored.
+     */
+    void (*define_alias)(Visitor *v, const char *name, const char **source);
+
+    /* Must be set if define_alias is set */
+    void (*start_alias_scope)(Visitor *v);
+
+    /* Must be set if define_alias is set */
+    void (*end_alias_scope)(Visitor *v);
+
     /* Must be set */
     VisitorType type;
 
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index ebc19ede7f..2ecbc20624 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -459,6 +459,50 @@ void visit_end_alternate(Visitor *v, void **obj);
  */
 bool visit_optional(Visitor *v, const char *name, bool *present);
 
+/*
+ * Defines a new alias rule.
+ *
+ * If @name is non-NULL, the member called @name in the external
+ * representation of the currently visited object is defined as an
+ * alias for the member described by @source.  It is not allowed to
+ * call this function when the currently visited type is not an
+ * object.
+ *
+ * If @name is NULL, all members of the object described by @source
+ * are considered to have alias members with the same key in the
+ * currently visited object.
+ *
+ * @source is a NULL-terminated non-empty array of names that describe
+ * the path to a member, starting from the currently visited object.
+ * All elements in @source except the last one should describe
+ * objects.  If an intermediate element refers to a member with a
+ * non-object type, the alias won't work (this case can legitimately
+ * happen in unions where an alias only makes sense for one branch,
+ * but not for another).
+ *
+ * The alias stays valid until the current alias scope ends.
+ * visit_start/end_struct() implicitly start/end an alias scope.
+ * Additionally, visit_start/end_alias_scope() can be used to explicitly
+ * create a nested alias scope.
+ */
+void visit_define_alias(Visitor *v, const char *name, const char **source);
+
+/*
+ * Begins an explicit alias scope.
+ *
+ * Alias definitions after here will only stay valid until the
+ * corresponding visit_end_alias_scope() is called.
+ */
+void visit_start_alias_scope(Visitor *v);
+
+/*
+ * Ends an explicit alias scope.
+ *
+ * Alias definitions between the correspoding visit_start_alias_scope()
+ * call and here go out of scope and won't apply in later code any more.
+ */
+void visit_end_alias_scope(Visitor *v);
+
 /*
  * Visit an enum value.
  *
diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 7e5f40e7f0..651dd88e02 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -135,6 +135,28 @@ bool visit_optional(Visitor *v, const char *name, bool *present)
     return *present;
 }
 
+void visit_define_alias(Visitor *v, const char *name, const char **source)
+{
+    assert(source[0] != NULL);
+    if (v->define_alias) {
+        v->define_alias(v, name, source);
+    }
+}
+
+void visit_start_alias_scope(Visitor *v)
+{
+    if (v->start_alias_scope) {
+        v->start_alias_scope(v);
+    }
+}
+
+void visit_end_alias_scope(Visitor *v)
+{
+    if (v->end_alias_scope) {
+        v->end_alias_scope(v);
+    }
+}
+
 bool visit_is_input(Visitor *v)
 {
     return v->type == VISITOR_INPUT;
-- 
2.29.2