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Re: [PATCH v3 12/34] qapi: Add feature flags to remaining definitions

From: Eric Blake
Subject: Re: [PATCH v3 12/34] qapi: Add feature flags to remaining definitions
Date: Mon, 16 Mar 2020 12:55:38 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 3/15/20 9:46 AM, Markus Armbruster wrote:
In v4.1.0, we added feature flags just to struct types (commit
6a8c0b5102^..f3ed93d545), to satisfy an immediate need (commit
c9d4070991 "file-posix: Add dynamic-auto-read-only QAPI feature").  In
v4.2.0, we added them to commands (commit 23394b4c39 "qapi: Add
feature flags to commands") to satisfy another immediate need (commit
d76744e65e "qapi: Allow introspecting fix for savevm's cooperation
with blockdev").

Add them to the remaining definitions: enumeration types, union types,
alternate types, and events.

Signed-off-by: Markus Armbruster <address@hidden>

+++ b/qapi/introspect.json
@@ -89,12 +89,18 @@
  # @meta-type: the entity's meta type, inherited from @base.
+# @features: names of features associated with the entity, in no
+#            particular order.
+#            (since 4.1 for object types, 4.2 for commands, 5.0 for
+#            the rest)

Odd versioning hint, but accurate, and I don't see any way to improve it.

  # Additional members depend on the value of @meta-type.
  # Since: 2.5
  { 'union': 'SchemaInfo',
-  'base': { 'name': 'str', 'meta-type': 'SchemaMetaType' },
+  'base': { 'name': 'str', 'meta-type': 'SchemaMetaType',
+            '*features': [ 'str' ] },
    'discriminator': 'meta-type',
    'data': {
        'builtin': 'SchemaInfoBuiltin',
@@ -174,9 +180,6 @@
  #            and may even differ from the order of the values of the
  #            enum type of the @tag.
-# @features: names of features associated with the type, in no particular
-#            order. (since: 4.1)
  # Values of this type are JSON object on the wire.
  # Since: 2.5
@@ -184,8 +187,7 @@
  { 'struct': 'SchemaInfoObject',
    'data': { 'members': [ 'SchemaInfoObjectMember' ],
              '*tag': 'str',
-            '*variants': [ 'SchemaInfoObjectVariant' ],
-            '*features': [ 'str' ] } }
+            '*variants': [ 'SchemaInfoObjectVariant' ] } }

The code motion from use in some of the union branches to now being present in the base class of all of the branches is backwards-compatible.

The generator changes also look correct, and have enough testsuite coverage to make it easier to be confident about the patch.

Reviewed-by: Eric Blake <address@hidden>

+++ b/tests/qapi-schema/doc-good.json
@@ -53,10 +53,14 @@
  # @Enum:
  # @one: The _one_ {and only}
+# Features:
+# @enum-feat: Also _one_ {and only}

All our existing public features are a single word (matching naming conventions elsewhere in QAPI). Are we sure we want to allow feature names that include whitespace? Of course, the fact that our testsuite covers it (even if we don't use it publically) means that we are sure that our generator can handle it, regardless of whether we decide that a separate patch should restrict feature names. But I don't see it holding up this patch.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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