qapi: Extend -compat to set policy for unstable interfaces

New option parameters unstable-input and unstable-output set policy
for unstable interfaces just like deprecated-input and
deprecated-output set policy for deprecated interfaces (see commit
6dd75472d5 "qemu-options: New -compat to set policy for deprecated
interfaces").  This is intended for testing users of the management
interfaces.  It is experimental.

For now, this covers only syntactic aspects of QMP, i.e. stuff tagged
with feature 'unstable'.  We may want to extend it to cover semantic
aspects, or the command line.

Note that there is no good way for management application to detect
presence of these new option parameters: they are 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>
Acked-by: John Snow <jsnow@redhat.com>
Message-Id: <20211028102520.747396-10-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Doc comments fixed up]

include/qapi/util.h

@@ -13,6 +13,7 @@
 
 typedef enum {
     QAPI_DEPRECATED,
+    QAPI_UNSTABLE,
 } QapiSpecialFeature;
 
 typedef struct QEnumLookup {

qapi/compat.json

@@ -47,9 +47,15 @@
 #
 # @deprecated-input: how to handle deprecated input (default 'accept')
 # @deprecated-output: how to handle deprecated output (default 'accept')
+# @unstable-input: how to handle unstable input (default 'accept')
+#                  (since 6.2)
+# @unstable-output: how to handle unstable output (default 'accept')
+#                   (since 6.2)
 #
 # Since: 6.0
 ##
 { 'struct': 'CompatPolicy',
   'data': { '*deprecated-input': 'CompatPolicyInput',
-            '*deprecated-output': 'CompatPolicyOutput' } }
+            '*deprecated-output': 'CompatPolicyOutput',
+            '*unstable-input': 'CompatPolicyInput',
+            '*unstable-output': 'CompatPolicyOutput' } }

qapi/qapi-util.c

@@ -49,6 +49,12 @@ bool compat_policy_input_ok(unsigned special_features,
                                     error_class, kind, name, errp)) {
         return false;
     }
+    if ((special_features & (1u << QAPI_UNSTABLE))
+        && !compat_policy_input_ok1("Unstable",
+                                    policy->unstable_input,
+                                    error_class, kind, name, errp)) {
+        return false;
+    }
     return true;
 }
 

qapi/qobject-output-visitor.c

@@ -212,8 +212,12 @@ static bool qobject_output_type_null(Visitor *v, const char *name,
 static bool qobject_output_policy_skip(Visitor *v, const char *name,
                                        unsigned special_features)
 {
-    return !(special_features & 1u << QAPI_DEPRECATED)
-        || v->compat_policy.deprecated_output == COMPAT_POLICY_OUTPUT_HIDE;
+    CompatPolicy *pol = &v->compat_policy;
+
+    return ((special_features & 1u << QAPI_DEPRECATED)
+            && pol->deprecated_output == COMPAT_POLICY_OUTPUT_HIDE)
+        || ((special_features & 1u << QAPI_UNSTABLE)
+            && pol->unstable_output == COMPAT_POLICY_OUTPUT_HIDE);
 }
 
 /* Finish building, and return the root object.

qemu-options.hx

@@ -3641,7 +3641,9 @@ DEFHEADING(Debug/Expert options:)
 
 DEF("compat", HAS_ARG, QEMU_OPTION_compat,
     "-compat [deprecated-input=accept|reject|crash][,deprecated-output=accept|hide]\n"
-    "                Policy for handling deprecated management interfaces\n",
+    "                Policy for handling deprecated management interfaces\n"
+    "-compat [unstable-input=accept|reject|crash][,unstable-output=accept|hide]\n"
+    "                Policy for handling unstable management interfaces\n",
     QEMU_ARCH_ALL)
 SRST
 ``-compat [deprecated-input=@var{input-policy}][,deprecated-output=@var{output-policy}]``
@@ -3659,6 +3661,22 @@ SRST
         Suppress deprecated command results and events
 
     Limitation: covers only syntactic aspects of QMP.
+
+``-compat [unstable-input=@var{input-policy}][,unstable-output=@var{output-policy}]``
+    Set policy for handling unstable management interfaces (experimental):
+
+    ``unstable-input=accept`` (default)
+        Accept unstable commands and arguments
+    ``unstable-input=reject``
+        Reject unstable commands and arguments
+    ``unstable-input=crash``
+        Crash on unstable commands and arguments
+    ``unstable-output=accept`` (default)
+        Emit unstable command results and events
+    ``unstable-output=hide``
+        Suppress unstable command results and events
+
+    Limitation: covers only syntactic aspects of QMP.
 ERST
 
 DEF("fw_cfg", HAS_ARG, QEMU_OPTION_fwcfg,

scripts/qapi/events.py

@@ -109,13 +109,15 @@ def gen_event_send(name: str,
         if not boxed:
             ret += gen_param_var(arg_type)
 
-    if 'deprecated' in [f.name for f in features]:
-        ret += mcgen('''
+    for f in features:
+        if f.is_special():
+            ret += mcgen('''
 
-    if (compat_policy.deprecated_output == COMPAT_POLICY_OUTPUT_HIDE) {
+    if (compat_policy.%(feat)s_output == COMPAT_POLICY_OUTPUT_HIDE) {
         return;
     }
-''')
+''',
+                         feat=f.name)
 
     ret += mcgen('''
 

scripts/qapi/schema.py

@@ -254,9 +254,11 @@ class QAPISchemaType(QAPISchemaEntity):
 
     def check(self, schema):
         QAPISchemaEntity.check(self, schema)
-        if 'deprecated' in [f.name for f in self.features]:
-            raise QAPISemError(
-                self.info, "feature 'deprecated' is not supported for types")
+        for feat in self.features:
+            if feat.is_special():
+                raise QAPISemError(
+                    self.info,
+                    f"feature '{feat.name}' is not supported for types")
 
     def describe(self):
         assert self.meta
@@ -726,7 +728,7 @@ class QAPISchemaFeature(QAPISchemaMember):
     role = 'feature'
 
     def is_special(self):
-        return self.name in ('deprecated')
+        return self.name in ('deprecated', 'unstable')
 
 
 class QAPISchemaObjectTypeMember(QAPISchemaMember):