qemu-options: New -compat to set policy for deprecated interfaces

New option -compat lets you configure what to do when deprecated
interfaces get used.  This is intended for testing users of the
management interfaces.  It is experimental.

-compat deprecated-input=<input-policy> configures what to do when
deprecated input is received.  Input policy can be "accept" (accept
silently), or "reject" (reject the request with an error).

-compat deprecated-output=<out-policy> configures what to do when
deprecated output is sent.  Output policy can be "accept" (pass on
unchanged), or "hide" (filter out the deprecated parts).

Default is "accept".  Policies other than "accept" are implemented
later in this series.

For now, -compat covers only syntactic aspects of QMP, i.e. stuff
tagged with feature 'deprecated'.  We may want to extend it to cover
semantic aspects, CLI, and experimental features.

Note that there is no good way for management application to detect
presence of -compat: it's not visible output of query-qmp-schema or
query-command-line-options.  Tolerable, because it's meant for
testing.  If running with -compat fails, skip the test.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210318155519.1224118-3-armbru@redhat.com>

qapi/compat.json

@@ -0,0 +1,51 @@
+# -*- Mode: Python -*-
+
+##
+# = Compatibility policy
+##
+
+##
+# @CompatPolicyInput:
+#
+# Policy for handling "funny" input.
+#
+# @accept: Accept silently
+# @reject: Reject with an error
+#
+# Since: 6.0
+##
+{ 'enum': 'CompatPolicyInput',
+  'data': [ 'accept', 'reject' ] }
+
+##
+# @CompatPolicyOutput:
+#
+# Policy for handling "funny" output.
+#
+# @accept: Pass on unchanged
+# @hide: Filter out
+#
+# Since: 6.0
+##
+{ 'enum': 'CompatPolicyOutput',
+  'data': [ 'accept', 'hide' ] }
+
+##
+# @CompatPolicy:
+#
+# Policy for handling deprecated management interfaces.
+#
+# This is intended for testing users of the management interfaces.
+#
+# Limitation: covers only syntactic aspects of QMP, i.e. stuff tagged
+# with feature 'deprecated'.  We may want to extend it to cover
+# semantic aspects, CLI, and experimental features.
+#
+# @deprecated-input: how to handle deprecated input (default 'accept')
+# @deprecated-output: how to handle deprecated output (default 'accept')
+#
+# Since: 6.0
+##
+{ 'struct': 'CompatPolicy',
+  'data': { '*deprecated-input': 'CompatPolicyInput',
+            '*deprecated-output': 'CompatPolicyOutput' } }

qapi/meson.build

@@ -25,6 +25,7 @@ qapi_all_modules = [
   'block-export',
   'char',
   'common',
+  'compat',
   'control',
   'crypto',
   'dump',

qapi/qapi-schema.json

@@ -79,6 +79,7 @@
 { 'include': 'migration.json' }
 { 'include': 'transaction.json' }
 { 'include': 'trace.json' }
+{ 'include': 'compat.json' }
 { 'include': 'control.json' }
 { 'include': 'introspect.json' }
 { 'include': 'qom.json' }

qapi/qmp-dispatch.c

@@ -14,6 +14,7 @@
 #include "qemu/osdep.h"
 
 #include "block/aio.h"
+#include "qapi/compat-policy.h"
 #include "qapi/error.h"
 #include "qapi/qmp/dispatch.h"
 #include "qapi/qmp/qdict.h"
@@ -23,6 +24,8 @@
 #include "qemu/coroutine.h"
 #include "qemu/main-loop.h"
 
+CompatPolicy compat_policy;
+
 static QDict *qmp_dispatch_check_obj(QDict *dict, bool allow_oob,
                                      Error **errp)
 {