qapi: New QMP command query-qmp-schema for QMP introspection

qapi/introspect.json defines the introspection schema.  It's designed
for QMP introspection, but should do for similar uses, such as QGA.

The introspection schema does not reflect all the rules and
restrictions that apply to QAPI schemata.  A valid QAPI schema has an
introspection value conforming to the introspection schema, but the
converse is not true.

Introspection lowers away a number of schema details, and makes
implicit things explicit:

* The built-in types are declared with their JSON type.

  All integer types are mapped to 'int', because how many bits we use
  internally is an implementation detail.  It could be pressed into
  external interface service as very approximate range information,
  but that's a bad idea.  If we need range information, we better do
  it properly.

* Implicit type definitions are made explicit, and given
  auto-generated names:

  - Array types, named by appending "List" to the name of their
    element type, like in generated C.

  - The enumeration types implicitly defined by simple union types,
    named by appending "Kind" to the name of their simple union type,
    like in generated C.

  - Types that don't occur in generated C.  Their names start with ':'
    so they don't clash with the user's names.

* All type references are by name.

* The struct and union types are generalized into an object type.

* Base types are flattened.

* Commands take a single argument and return a single result.

  Dictionary argument or list result is an implicit type definition.

  The empty object type is used when a command takes no arguments or
  produces no results.

  The argument is always of object type, but the introspection schema
  doesn't reflect that.

  The 'gen': false directive is omitted as implementation detail.

  The 'success-response' directive is omitted as well for now, even
  though it's not an implementation detail, because it's not used by
  QMP.

* Events carry a single data value.

  Implicit type definition and empty object type use, just like for
  commands.

  The value is of object type, but the introspection schema doesn't
  reflect that.

* Types not used by commands or events are omitted.

  Indirect use counts as use.

* Optional members have a default, which can only be null right now

  Instead of a mandatory "optional" flag, we have an optional default.
  No default means mandatory, default null means optional without
  default value.  Non-null is available for optional with default
  (possible future extension).

* Clients should *not* look up types by name, because type names are
  not ABI.  Look up the command or event you're interested in, then
  follow the references.

  TODO Should we hide the type names to eliminate the temptation?

New generator scripts/qapi-introspect.py computes an introspection
value for its input, and generates a C variable holding it.

It can generate awfully long lines.  Marked TODO.

A new test-qmp-input-visitor test case feeds its result for both
tests/qapi-schema/qapi-schema-test.json and qapi-schema.json to a
QmpInputVisitor to verify it actually conforms to the schema.

New QMP command query-qmp-schema takes its return value from that
variable.  Its reply is some 85KiBytes for me right now.

If this turns out to be too much, we have a couple of options:

* We can use shorter names in the JSON.  Not the QMP style.

* Optionally return the sub-schema for commands and events given as
  arguments.

  Right now qmp_query_schema() sends the string literal computed by
  qmp-introspect.py.  To compute sub-schema at run time, we'd have to
  duplicate parts of qapi-introspect.py in C.  Unattractive.

* Let clients cache the output of query-qmp-schema.

  It changes only on QEMU upgrades, i.e. rarely.  Provide a command
  query-qmp-schema-hash.  Clients can have a cache indexed by hash,
  and re-query the schema only when they don't have it cached.  Even
  simpler: put the hash in the QMP greeting.

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

1 files changed, 277 insertions, 0 deletions

diff --git a/qapi/introspect.json b/qapi/introspect.json
new file mode 100644
index 0000000000..9c8ad53d7a
--- /dev/null
+++ b/qapi/introspect.json
@@ -0,0 +1,277 @@
+# -*- Mode: Python -*-
+#
+# QAPI/QMP introspection
+#
+# Copyright (C) 2015 Red Hat, Inc.
+#
+# Authors:
+#  Markus Armbruster <armbru@redhat.com>
+#
+# This work is licensed under the terms of the GNU GPL, version 2 or later.
+# See the COPYING file in the top-level directory.
+
+##
+# @query-qmp-schema
+#
+# Command query-qmp-schema exposes the QMP wire ABI as an array of
+# SchemaInfo.  This lets QMP clients figure out what commands and
+# events are available in this QEMU, and their parameters and results.
+#
+# However, the SchemaInfo can't reflect all the rules and restrictions
+# that apply to QMP.  It's interface introspection (figuring out
+# what's there), not interface specification.  The specification is in
+# the QAPI schema.
+#
+# Returns: array of @SchemaInfo, where each element describes an
+# entity in the ABI: command, event, type, ...
+#
+# Note: the QAPI schema is also used to help define *internal*
+# interfaces, by defining QAPI types.  These are not part of the QMP
+# wire ABI, and therefore not returned by this command.
+#
+# Since: 2.5
+##
+{ 'command': 'query-qmp-schema',
+  'returns': [ 'SchemaInfo' ],
+  'gen': false }                # just to simplify qmp_query_json()
+
+##
+# @SchemaMetaType
+#
+# This is a @SchemaInfo's meta type, i.e. the kind of entity it
+# describes.
+#
+# @builtin: a predefined type such as 'int' or 'bool'.
+#
+# @enum: an enumeration type
+#
+# @array: an array type
+#
+# @object: an object type (struct or union)
+#
+# @alternate: an alternate type
+#
+# @command: a QMP command
+#
+# @event: a QMP event
+#
+# Since: 2.5
+##
+{ 'enum': 'SchemaMetaType',
+  'data': [ 'builtin', 'enum', 'array', 'object', 'alternate',
+            'command', 'event' ] }
+
+##
+# @SchemaInfoBase
+#
+# Members common to any @SchemaInfo.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoBase',
+  'data': { 'name': 'str', 'meta-type': 'SchemaMetaType' } }
+
+##
+# @SchemaInfo
+#
+# @name: the entity's name, inherited from @base.
+#         Entities defined in the QAPI schema have the name defined in
+#         the schema.  Implicitly defined entities have generated
+#         names.  See docs/qapi-code-gen.txt section "Client JSON
+#         Protocol introspection" for details.
+#
+# All references to other SchemaInfo are by name.
+#
+# Command and event names are part of the wire ABI, but type names are
+# not.  Therefore, looking up a type by "well-known" name is wrong.
+# Look up the command or event, then follow the references.
+#
+# @meta-type: the entity's meta type, inherited from @base.
+#
+# Additional members depend on the value of @meta-type.
+#
+# Since: 2.5
+##
+{ 'union': 'SchemaInfo',
+  'base': 'SchemaInfoBase',
+  'discriminator': 'meta-type',
+  'data': {
+      'builtin': 'SchemaInfoBuiltin',
+      'enum': 'SchemaInfoEnum',
+      'array': 'SchemaInfoArray',
+      'object': 'SchemaInfoObject',
+      'alternate': 'SchemaInfoAlternate',
+      'command': 'SchemaInfoCommand',
+      'event': 'SchemaInfoEvent' } }
+
+##
+# @SchemaInfoBuiltin
+#
+# Additional SchemaInfo members for meta-type 'builtin'.
+#
+# @json-type: the JSON type used for this type on the wire.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoBuiltin',
+  'data': { 'json-type': 'JSONType' } }
+
+##
+# @JSONType
+#
+# The four primitive and two structured types according to RFC 7159
+# section 1, plus 'int' (split off 'number'), plus the obvious top
+# type 'value'.
+#
+# Since: 2.5
+##
+{ 'enum': 'JSONType',
+  'data': [ 'string', 'number', 'int', 'boolean', 'null',
+            'object', 'array', 'value' ] }
+
+##
+# @SchemaInfoEnum
+#
+# Additional SchemaInfo members for meta-type 'enum'.
+#
+# @values: the enumeration type's values.
+#
+# Values of this type are JSON string on the wire.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoEnum',
+  'data': { 'values': ['str'] } }
+
+##
+# @SchemaInfoArray
+#
+# Additional SchemaInfo members for meta-type 'array'.
+#
+# @element-type: the array type's element type.
+#
+# Values of this type are JSON array on the wire.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoArray',
+  'data': { 'element-type': 'str' } }
+
+##
+# @SchemaInfoObject
+#
+# Additional SchemaInfo members for meta-type 'object'.
+#
+# @members: the object type's (non-variant) members.
+#
+# @tag: #optional the name of the member serving as type tag.
+#       An element of @members with this name must exist.
+#
+# @variants: #optional variant members, i.e. additional members that
+#            depend on the type tag's value.  Present exactly when
+#            @tag is present.
+#
+# Values of this type are JSON object on the wire.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoObject',
+  'data': { 'members': [ 'SchemaInfoObjectMember' ],
+            '*tag': 'str',
+            '*variants': [ 'SchemaInfoObjectVariant' ] } }
+
+##
+# @SchemaInfoObjectMember
+#
+# An object member.
+#
+# @name: the member's name, as defined in the QAPI schema.
+#
+# @type: the name of the member's type.
+#
+# @default: #optional default when used as command parameter.
+#           If absent, the parameter is mandatory.
+#           If present, the value must be null.  The parameter is
+#           optional, and behavior when it's missing is not specified
+#           here.
+#           Future extension: if present and non-null, the parameter
+#           is optional, and defaults to this value.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoObjectMember',
+  'data': { 'name': 'str', 'type': 'str', '*default': 'any' } }
+# @default's type must be null or match @type
+
+##
+# @SchemaInfoObjectVariant
+#
+# The variant members for a value of the type tag.
+#
+# @case: a value of the type tag.
+#
+# @type: the name of the object type that provides the variant members
+#        when the type tag has value @case.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoObjectVariant',
+  'data': { 'case': 'str', 'type': 'str' } }
+
+##
+# @SchemaInfoAlternate
+#
+# Additional SchemaInfo members for meta-type 'alternate'.
+#
+# @members: the alternate type's members.
+#           The members' wire encoding is distinct, see
+#           docs/qapi-code-gen.txt section Alternate types.
+#
+# On the wire, this can be any of the members.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoAlternate',
+  'data': { 'members': [ 'SchemaInfoAlternateMember' ] } }
+
+##
+# @SchemaInfoAlternateMember
+#
+# An alternate member.
+#
+# @type: the name of the member's type.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoAlternateMember',
+  'data': { 'type': 'str' } }
+
+##
+# @SchemaInfoCommand
+#
+# Additional SchemaInfo members for meta-type 'command'.
+#
+# @arg-type: the name of the object type that provides the command's
+#            parameters.
+#
+# @ret-type: the name of the command's result type.
+#
+# TODO @success-response (currently irrelevant, because it's QGA, not QMP)
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoCommand',
+  'data': { 'arg-type': 'str', 'ret-type': 'str' } }
+
+##
+# @SchemaInfoEvent
+#
+# Additional SchemaInfo members for meta-type 'event'.
+#
+# @arg-type: the name of the object type that provides the event's
+#            parameters.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoEvent',
+  'data': { 'arg-type': 'str' } }