[PATCH v3 2/3] qapi: Do not generate empty enum

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Per the C++ standard, empty enum are ill-formed. Do not generate
them in order to avoid:

  In file included from qga/qga-qapi-emit-events.c:14:
  qga/qga-qapi-emit-events.h:20:1: error: empty enum is invalid
     20 | } qga_QAPIEvent;
        | ^

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 docs/devel/qapi-code-gen.rst | 6 +++---
 scripts/qapi/events.py       | 2 ++
 scripts/qapi/schema.py       | 5 ++++-
 scripts/qapi/types.py        | 2 ++
 4 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/docs/devel/qapi-code-gen.rst b/docs/devel/qapi-code-gen.rst
index 23e7f2fb1c..d684c7c24d 100644
--- a/docs/devel/qapi-code-gen.rst
+++ b/docs/devel/qapi-code-gen.rst
@@ -206,6 +206,9 @@ Syntax::
 
 Member 'enum' names the enum type.
 
+Empty enumeration (no member) does not generate anything (not even
+constant PREFIX__MAX).
+
 Each member of the 'data' array defines a value of the enumeration
 type.  The form STRING is shorthand for :code:`{ 'name': STRING }`.  The
 'name' values must be be distinct.
@@ -214,9 +217,6 @@ Example::
 
  { 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] }
 
-Nothing prevents an empty enumeration, although it is probably not
-useful.
-
 On the wire, an enumeration type's value is represented by its
 (string) name.  In C, it's represented by an enumeration constant.
 These are of the form PREFIX_NAME, where PREFIX is derived from the
diff --git a/scripts/qapi/events.py b/scripts/qapi/events.py
index 3cf01e96b6..48f4ca9613 100644
--- a/scripts/qapi/events.py
+++ b/scripts/qapi/events.py
@@ -207,6 +207,8 @@ def _begin_user_module(self, name: str) -> None:
 
     def visit_end(self) -> None:
         self._add_module('./emit', ' * QAPI Events emission')
+        if not self._event_enum_members:
+            return
         self._genc.preamble_add(mcgen('''
 #include "qemu/osdep.h"
 #include "%(prefix)sqapi-emit-events.h"
diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py
index 207e4d71f3..28045dbe93 100644
--- a/scripts/qapi/schema.py
+++ b/scripts/qapi/schema.py
@@ -309,6 +309,7 @@ class QAPISchemaEnumType(QAPISchemaType):
 
     def __init__(self, name, info, doc, ifcond, features, members, prefix):
         super().__init__(name, info, doc, ifcond, features)
+        assert members
         for m in members:
             assert isinstance(m, QAPISchemaEnumMember)
             m.set_defined_in(name)
@@ -1047,8 +1048,10 @@ def _make_implicit_object_type(self, name, info, ifcond, role, members):
         return name
 
     def _def_enum_type(self, expr: QAPIExpression):
-        name = expr['enum']
         data = expr['data']
+        if not data:
+            return
+        name = expr['enum']
         prefix = expr.get('prefix')
         ifcond = QAPISchemaIfCond(expr.get('if'))
         info = expr.info
diff --git a/scripts/qapi/types.py b/scripts/qapi/types.py
index c39d054d2c..7a7be7315f 100644
--- a/scripts/qapi/types.py
+++ b/scripts/qapi/types.py
@@ -42,6 +42,7 @@
 def gen_enum_lookup(name: str,
                     members: List[QAPISchemaEnumMember],
                     prefix: Optional[str] = None) -> str:
+    assert members
     max_index = c_enum_const(name, '_MAX', prefix)
     feats = ''
     ret = mcgen('''
@@ -86,6 +87,7 @@ def gen_enum_lookup(name: str,
 def gen_enum(name: str,
              members: List[QAPISchemaEnumMember],
              prefix: Optional[str] = None) -> str:
+    assert members
     # append automatically generated _MAX value
     enum_members = members + [QAPISchemaEnumMember('_MAX', None)]
 
-- 
2.38.1