qapi-types/visit.py: Inheritance for structs

This introduces a new 'base' key for struct definitions that refers to
another struct type. On the JSON level, the fields of the base type are
included directly into the same namespace as the fields of the defined
type, like with unions. On the C level, a pointer to a struct of the
base type is included.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

3 files changed, 37 insertions, 2 deletions

diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index 0ce045c0b3..91f44d01b9 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -53,6 +53,23 @@ The use of '*' as a prefix to the name means the member is optional.  Optional
 members should always be added to the end of the dictionary to preserve
 backwards compatibility.
 
+
+A complex type definition can specify another complex type as its base.
+In this case, the fields of the base type are included as top-level fields
+of the new complex type's dictionary in the QMP wire format. An example
+definition is:
+
+ { 'type': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } }
+ { 'type': 'BlockdevOptionsGenericCOWFormat',
+   'base': 'BlockdevOptionsGenericFormat',
+   'data': { '*backing': 'str' } }
+
+An example BlockdevOptionsGenericCOWFormat object on the wire could use
+both fields like this:
+
+ { "file": "/some/place/my-image",
+   "backing": "/some/place/my-backing-file" }
+
 === Enumeration types ===
 
 An enumeration type is a dictionary containing a single key whose value is a
diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index 566fe5e4d9..4a1652b56f 100644
--- a/scripts/qapi-types.py
+++ b/scripts/qapi-types.py
@@ -86,6 +86,7 @@ def generate_struct(expr):
     structname = expr.get('type', "")
     fieldname = expr.get('field', "")
     members = expr['data']
+    base = expr.get('base')
 
     ret = mcgen('''
 struct %(name)s
@@ -93,6 +94,9 @@ struct %(name)s
 ''',
           name=structname)
 
+    if base:
+        ret += generate_struct_fields({'base': base})
+
     ret += generate_struct_fields(members)
 
     if len(fieldname):
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 1e44004a1e..c39e6284b8 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -17,7 +17,7 @@ import os
 import getopt
 import errno
 
-def generate_visit_struct_fields(name, field_prefix, fn_prefix, members):
+def generate_visit_struct_fields(name, field_prefix, fn_prefix, members, base = None):
     substructs = []
     ret = ''
     full_name = name if not fn_prefix else "%s_%s" % (name, fn_prefix)
@@ -42,6 +42,19 @@ static void visit_type_%(full_name)s_fields(Visitor *m, %(name)s ** obj, Error *
         name=name, full_name=full_name)
     push_indent()
 
+    if base:
+        ret += mcgen('''
+visit_start_implicit_struct(m, obj ? (void**) &(*obj)->%(c_name)s : NULL, sizeof(%(type)s), &err);
+if (!err) {
+    visit_type_%(type)s_fields(m, obj ? &(*obj)->%(c_prefix)s%(c_name)s : NULL, &err);
+    error_propagate(errp, err);
+    err = NULL;
+    visit_end_implicit_struct(m, &err);
+}
+''',
+                     c_prefix=c_var(field_prefix),
+                     type=type_name(base), c_name=c_var('base'))
+
     for argname, argentry, optional, structured in parse_args(members):
         if optional:
             ret += mcgen('''
@@ -124,8 +137,9 @@ def generate_visit_struct(expr):
 
     name = expr['type']
     members = expr['data']
+    base = expr.get('base')
 
-    ret = generate_visit_struct_fields(name, "", "", members)
+    ret = generate_visit_struct_fields(name, "", "", members, base)
 
     ret += mcgen('''