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>

14 files changed, 14 insertions, 0 deletions

diff --git a/tests/qapi-schema/alternate-good.out b/tests/qapi-schema/alternate-good.out
index 3d765ffd7b..65af7278f4 100644
--- a/tests/qapi-schema/alternate-good.out
+++ b/tests/qapi-schema/alternate-good.out
@@ -1,3 +1,4 @@
+object :empty
 alternate Alt
     case value: int
     case string: Enum
diff --git a/tests/qapi-schema/args-member-array.out b/tests/qapi-schema/args-member-array.out
index b67384cc2e..b3b92dfc8e 100644
--- a/tests/qapi-schema/args-member-array.out
+++ b/tests/qapi-schema/args-member-array.out
@@ -1,3 +1,4 @@
+object :empty
 object :obj-okay-arg
     member member1: intList optional=False
     member member2: defList optional=False
diff --git a/tests/qapi-schema/comments.out b/tests/qapi-schema/comments.out
index 6161b90e91..9e2c656fa0 100644
--- a/tests/qapi-schema/comments.out
+++ b/tests/qapi-schema/comments.out
@@ -1 +1,2 @@
+object :empty
 enum Status ['good', 'bad', 'ugly']
diff --git a/tests/qapi-schema/empty.out b/tests/qapi-schema/empty.out
index e69de29bb2..272b1616f4 100644
--- a/tests/qapi-schema/empty.out
+++ b/tests/qapi-schema/empty.out
@@ -0,0 +1 @@
+object :empty
diff --git a/tests/qapi-schema/enum-empty.out b/tests/qapi-schema/enum-empty.out
index e09b00ffd6..a449d455fb 100644
--- a/tests/qapi-schema/enum-empty.out
+++ b/tests/qapi-schema/enum-empty.out
@@ -1 +1,2 @@
+object :empty
 enum MyEnum []
diff --git a/tests/qapi-schema/event-case.out b/tests/qapi-schema/event-case.out
index b5ae4c20b3..cdfd264f9c 100644
--- a/tests/qapi-schema/event-case.out
+++ b/tests/qapi-schema/event-case.out
@@ -1 +1,2 @@
+object :empty
 event oops None
diff --git a/tests/qapi-schema/flat-union-reverse-define.out b/tests/qapi-schema/flat-union-reverse-define.out
index 477fb31198..a5a9134e7e 100644
--- a/tests/qapi-schema/flat-union-reverse-define.out
+++ b/tests/qapi-schema/flat-union-reverse-define.out
@@ -1,3 +1,4 @@
+object :empty
 object TestBase
     member enum1: TestEnum optional=False
 enum TestEnum ['value1', 'value2']
diff --git a/tests/qapi-schema/ident-with-escape.out b/tests/qapi-schema/ident-with-escape.out
index 9577d1bde7..f4542b12d8 100644
--- a/tests/qapi-schema/ident-with-escape.out
+++ b/tests/qapi-schema/ident-with-escape.out
@@ -1,3 +1,4 @@
+object :empty
 object :obj-fooA-arg
     member bar1: str optional=False
 command fooA :obj-fooA-arg -> None
diff --git a/tests/qapi-schema/include-relpath.out b/tests/qapi-schema/include-relpath.out
index 6161b90e91..9e2c656fa0 100644
--- a/tests/qapi-schema/include-relpath.out
+++ b/tests/qapi-schema/include-relpath.out
@@ -1 +1,2 @@
+object :empty
 enum Status ['good', 'bad', 'ugly']
diff --git a/tests/qapi-schema/include-repetition.out b/tests/qapi-schema/include-repetition.out
index 6161b90e91..9e2c656fa0 100644
--- a/tests/qapi-schema/include-repetition.out
+++ b/tests/qapi-schema/include-repetition.out
@@ -1 +1,2 @@
+object :empty
 enum Status ['good', 'bad', 'ugly']
diff --git a/tests/qapi-schema/include-simple.out b/tests/qapi-schema/include-simple.out
index 6161b90e91..9e2c656fa0 100644
--- a/tests/qapi-schema/include-simple.out
+++ b/tests/qapi-schema/include-simple.out
@@ -1 +1,2 @@
+object :empty
 enum Status ['good', 'bad', 'ugly']
diff --git a/tests/qapi-schema/indented-expr.out b/tests/qapi-schema/indented-expr.out
index c5af55a05c..226d300798 100644
--- a/tests/qapi-schema/indented-expr.out
+++ b/tests/qapi-schema/indented-expr.out
@@ -1,3 +1,4 @@
+object :empty
 command eins None -> None
    gen=True success_response=True
 command zwei None -> None
diff --git a/tests/qapi-schema/qapi-schema-test.out b/tests/qapi-schema/qapi-schema-test.out
index a52ac31ea5..1f6e858def 100644
--- a/tests/qapi-schema/qapi-schema-test.out
+++ b/tests/qapi-schema/qapi-schema-test.out
@@ -1,3 +1,4 @@
+object :empty
 object :obj-EVENT_C-arg
     member a: int optional=True
     member b: UserDefOne optional=True
diff --git a/tests/qapi-schema/returns-int.out b/tests/qapi-schema/returns-int.out
index 1ac3e1e267..a2da259be4 100644
--- a/tests/qapi-schema/returns-int.out
+++ b/tests/qapi-schema/returns-int.out
@@ -1,2 +1,3 @@
+object :empty
 command guest-get-time None -> int
    gen=True success_response=True