qapi: Permit alternates with just one branch

A union or alternate without branches makes no sense and doesn't work:
it can't be instantiated.  A union or alternate with just one branch
works, but is degenerate.  We accept the former, but reject the
latter.  Weird.  docs/devel/qapi-code-gen.txt doesn't mention the
difference.  It claims an alternate definition is "is similar to a
simple union type".

Permit degenerate alternates to make them consistent with unions.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-10-armbru@redhat.com>

scripts/qapi/common.py

@@ -920,11 +920,9 @@ def check_alternate(expr, info):
     members = expr['data']
     types_seen = {}
 
-    # Check every branch; require at least two branches
-    if len(members) < 2:
+    if len(members) == 0:
         raise QAPISemError(info,
-                           "Alternate '%s' should have at least two branches "
-                           "in 'data'" % name)
+                           "Alternate '%s' cannot have empty 'data'" % name)
     for (key, value) in members.items():
         check_name(info, "Member of alternate '%s'" % name, key)
         check_known_keys(info,

tests/qapi-schema/alternate-empty.err

@@ -1 +1 @@
-tests/qapi-schema/alternate-empty.json:2: Alternate 'Alt' should have at least two branches in 'data'
+tests/qapi-schema/alternate-empty.json:2: Alternate 'Alt' cannot have empty 'data'

tests/qapi-schema/alternate-empty.json

@@ -1,2 +1,2 @@
-# alternates must list at least two types to be useful
-{ 'alternate': 'Alt', 'data': { 'i': 'int' } }
+# alternates cannot be empty
+{ 'alternate': 'Alt', 'data': { } }

tests/qapi-schema/qapi-schema-test.json

@@ -186,19 +186,21 @@
 
 # test that we correctly compile downstream extensions, as well as munge
 # ticklish names
+# also test union and alternate with just one branch
 { 'enum': '__org.qemu_x-Enum', 'data': [ '__org.qemu_x-value' ] }
 { 'struct': '__org.qemu_x-Base',
   'data': { '__org.qemu_x-member1': '__org.qemu_x-Enum' } }
 { 'struct': '__org.qemu_x-Struct', 'base': '__org.qemu_x-Base',
   'data': { '__org.qemu_x-member2': 'str', '*wchar-t': 'int' } }
 { 'union': '__org.qemu_x-Union1', 'data': { '__org.qemu_x-branch': 'str' } }
+{ 'alternate': '__org.qemu_x-Alt1', 'data': { '__org.qemu_x-branch': 'str' } }
 { 'struct': '__org.qemu_x-Struct2',
   'data': { 'array': ['__org.qemu_x-Union1'] } }
 { 'union': '__org.qemu_x-Union2', 'base': '__org.qemu_x-Base',
   'discriminator': '__org.qemu_x-member1',
   'data': { '__org.qemu_x-value': '__org.qemu_x-Struct2' } }
 { 'alternate': '__org.qemu_x-Alt',
-  'data': { '__org.qemu_x-branch': 'str', 'b': '__org.qemu_x-Base' } }
+  'data': { '__org.qemu_x-branch': '__org.qemu_x-Base' } }
 { 'event': '__ORG.QEMU_X-EVENT', 'data': '__org.qemu_x-Struct' }
 { 'command': '__org.qemu_x-command',
   'data': { 'a': ['__org.qemu_x-Enum'], 'b': ['__org.qemu_x-Struct'],

tests/qapi-schema/qapi-schema-test.out

@@ -274,6 +274,9 @@ object __org.qemu_x-Union1
     member type: __org.qemu_x-Union1Kind optional=False
     tag type
     case __org.qemu_x-branch: q_obj_str-wrapper
+alternate __org.qemu_x-Alt1
+    tag type
+    case __org.qemu_x-branch: str
 array __org.qemu_x-Union1List __org.qemu_x-Union1
 object __org.qemu_x-Struct2
     member array: __org.qemu_x-Union1List optional=False
@@ -283,8 +286,7 @@ object __org.qemu_x-Union2
     case __org.qemu_x-value: __org.qemu_x-Struct2
 alternate __org.qemu_x-Alt
     tag type
-    case __org.qemu_x-branch: str
-    case b: __org.qemu_x-Base
+    case __org.qemu_x-branch: __org.qemu_x-Base
 event __ORG.QEMU_X-EVENT __org.qemu_x-Struct
    boxed=False
 array __org.qemu_x-EnumList __org.qemu_x-Enum