Simple unions predate flat unions.  Having both complicates the QAPI
schema language and the QAPI generator.  We haven't been using simple
unions in new code for a long time, because they are less flexible and
somewhat awkward on the wire.

To prepare for their removal, convert simple union ImageInfoSpecific
to an equivalent flat one.  Adds some boilerplate to the schema, which
is a bit ugly, but a lot easier to maintain than the simple union
feature.

Implicit enum ImageInfoSpecificKind becomes explicit.  It duplicates
part of enum BlockdevDriver.  We could reuse BlockdevDriver instead.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
---
 qapi/block-core.json | 56 ++++++++++++++++++++++++++++++++++++++------
 1 file changed, 49 insertions(+), 7 deletions(-)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index c8ce1d9d5d..b2858b144f 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -139,6 +139,49 @@
       '*encryption-format': 'RbdImageEncryptionFormat'
   } }
 
+##
+# @ImageInfoSpecificKind:
+#
+# Since: 6.1
+##
+{ 'enum': 'ImageInfoSpecificKind',
+  'data': [ 'qcow2', 'vmdk', 'luks', 'rbd' ] }
+
+##
+# @ImageInfoSpecificQCow2Wrapper:
+#
+# Since: 6.1
+##
+{ 'struct': 'ImageInfoSpecificQCow2Wrapper',
+  'data': { 'data': 'ImageInfoSpecificQCow2' } }
+
+##
+# @ImageInfoSpecificVmdkWrapper:
+#
+# Since: 6.1
+##
+{ 'struct': 'ImageInfoSpecificVmdkWrapper',
+  'data': { 'data': 'ImageInfoSpecificVmdk' } }
+
+##
+# @ImageInfoSpecificLUKSWrapper:
+#
+# Since: 6.1
+##
+{ 'struct': 'ImageInfoSpecificLUKSWrapper',
+  'data': { 'data': 'QCryptoBlockInfoLUKS' } }
+# If we need to add block driver specific parameters for
+# LUKS in future, then we'll subclass QCryptoBlockInfoLUKS
+# to define a ImageInfoSpecificLUKS
+
+##
+# @ImageInfoSpecificRbdWrapper:
+#
+# Since: 6.1
+##
+{ 'struct': 'ImageInfoSpecificRbdWrapper',
+  'data': { 'data': 'ImageInfoSpecificRbd' } }
+
 ##
 # @ImageInfoSpecific:
 #
@@ -147,14 +190,13 @@
 # Since: 1.7
 ##
 { 'union': 'ImageInfoSpecific',
+  'base': { 'type': 'ImageInfoSpecificKind' },
+  'discriminator': 'type',
   'data': {
-      'qcow2': 'ImageInfoSpecificQCow2',
-      'vmdk': 'ImageInfoSpecificVmdk',
-      # If we need to add block driver specific parameters for
-      # LUKS in future, then we'll subclass QCryptoBlockInfoLUKS
-      # to define a ImageInfoSpecificLUKS
-      'luks': 'QCryptoBlockInfoLUKS',
-      'rbd': 'ImageInfoSpecificRbd'
+      'qcow2': 'ImageInfoSpecificQCow2Wrapper',
+      'vmdk': 'ImageInfoSpecificVmdkWrapper',
+      'luks': 'ImageInfoSpecificLUKSWrapper',
+      'rbd': 'ImageInfoSpecificRbdWrapper'
   } }
 
 ##
-- 
2.31.1

On 13.09.21 14:39, Markus Armbruster wrote:
> Simple unions predate flat unions.  Having both complicates the QAPI
> schema language and the QAPI generator.  We haven't been using simple
> unions in new code for a long time, because they are less flexible and
> somewhat awkward on the wire.
>
> To prepare for their removal, convert simple union ImageInfoSpecific
> to an equivalent flat one.  Adds some boilerplate to the schema, which
> is a bit ugly, but a lot easier to maintain than the simple union
> feature.
>
> Implicit enum ImageInfoSpecificKind becomes explicit.  It duplicates
> part of enum BlockdevDriver.  We could reuse BlockdevDriver instead.
>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Hanna Reitz <hreitz@redhat.com>
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>   qapi/block-core.json | 56 ++++++++++++++++++++++++++++++++++++++------
>   1 file changed, 49 insertions(+), 7 deletions(-)

Acked-by: Hanna Reitz <hreitz@redhat.com>