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[Qemu-devel] [PATCH v4 08/10] qapi: Allow anonymous base for flat union

From: Eric Blake
Subject: [Qemu-devel] [PATCH v4 08/10] qapi: Allow anonymous base for flat union
Date: Sat, 5 Mar 2016 09:16:33 -0700

Rather than requiring all flat unions to explicitly create
a separate base struct, we can allow the qapi schema to specify
the common members via an inline dictionary. This is similar to
how commands can specify an inline anonymous type for its 'data'.
We already have several struct types that only exist to serve as
a single flat union's base; the next commit will clean them up.

Now that anonymous bases are legal, we need to rework the
flat-union-bad-base negative test (as previously written, it
forms what is now valid QAPI; tweak it to now provide coverage
of a new error message path), and add a positive test in
qapi-schema-test to use an anonymous base (making the integer
argument optional, for even more coverage).

Note that this patch only allows anonymous bases for flat unions;
simple unions are enough syntactic sugar that we do not want to
burden them further.  Meanwhile, while it would be easy to also
allow an anonymous base for structs, that would be quite
redundant, as the members can be put right into the struct

Signed-off-by: Eric Blake <address@hidden>

v4: rebase to use implicit type, improve commit message
[no v3]
v2: rebase onto s/fields/members/ change
v1: rebase and rework to use gen_visit_fields_call(), test new error
Previously posted as part of qapi cleanup subset F:
v6: avoid redundant error check in gen_visit_union(), rebase to
earlier gen_err_check() improvements
 scripts/qapi.py                            | 12 ++++++++++--
 scripts/qapi-types.py                      | 10 ++++++----
 docs/qapi-code-gen.txt                     | 30 +++++++++++++++---------------
 tests/qapi-schema/flat-union-bad-base.err  |  2 +-
 tests/qapi-schema/flat-union-bad-base.json |  5 ++---
 tests/qapi-schema/qapi-schema-test.json    |  6 +-----
 tests/qapi-schema/qapi-schema-test.out     | 10 +++++-----
 7 files changed, 40 insertions(+), 35 deletions(-)

diff --git a/scripts/qapi.py b/scripts/qapi.py
index 0574b8b..227f47d 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -327,6 +327,8 @@ class QAPISchemaParser(object):

 def find_base_members(base):
+    if isinstance(base, dict):
+        return base
     base_struct_define = find_struct(base)
     if not base_struct_define:
         return None
@@ -560,9 +562,10 @@ def check_union(expr, expr_info):

     # Else, it's a flat union.
-        # The object must have a string member 'base'.
+        # The object must have a string or dictionary 'base'.
         check_type(expr_info, "'base' for union '%s'" % name,
-                   base, allow_metas=['struct'])
+                   base, allow_dict=True, allow_optional=True,
+                   allow_metas=['struct'])
         if not base:
             raise QAPIExprError(expr_info,
                                 "Flat union '%s' must have a base"
@@ -1049,6 +1052,8 @@ class QAPISchemaMember(object):
             owner = owner[5:]
             if owner.endswith('-arg'):
                 return '(parameter of %s)' % owner[:-4]
+            elif owner.endswith('-base'):
+                return '(base of %s)' % owner[:-5]
                 assert owner.endswith('-wrapper')
                 # Unreachable and not implemented
@@ -1336,6 +1341,9 @@ class QAPISchema(object):
         base = expr.get('base')
         tag_name = expr.get('discriminator')
         tag_member = None
+        if isinstance(base, dict):
+            base = (self._make_implicit_object_type(
+                    name, info, 'base', self._make_members(base, info)))
         if tag_name:
             variants = [self._make_variant(key, value)
                         for (key, value) in data.iteritems()]
diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index b8499ac..c003721 100644
--- a/scripts/qapi-types.py
+++ b/scripts/qapi-types.py
@@ -72,12 +72,14 @@ struct %(c_name)s {

     if base:
-        ret += mcgen('''
+        if not base.is_implicit():
+            ret += mcgen('''
     /* Members inherited from %(c_name)s: */
-                     c_name=base.c_name())
+                         c_name=base.c_name())
         ret += gen_struct_members(base.members)
-        ret += mcgen('''
+        if not base.is_implicit():
+            ret += mcgen('''
     /* Own members: */
     ret += gen_struct_members(members)
@@ -223,7 +225,7 @@ class QAPISchemaGenTypeVisitor(QAPISchemaVisitor):
     def visit_object_type(self, name, info, base, members, variants):
         self._fwdecl += gen_fwd_object_or_array(name)
         self.decl += gen_object(name, base, members, variants)
-        if base:
+        if base and not base.is_implicit():
             self.decl += gen_upcast(name, base)
         if name[0] != ':':
             # implicit types won't be directly allocated/freed
diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index e0b2ef1..a92d4da 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -284,7 +284,7 @@ better than open-coding the member to be type 'str'.
 === Union types ===

 Usage: { 'union': STRING, 'data': DICT }
-or:    { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME,
+or:    { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME-OR-DICT,
          'discriminator': ENUM-MEMBER-OF-BASE }

 Union types are used to let the user choose between several different
@@ -320,24 +320,25 @@ an implicit C enum 'NameKind' is created, corresponding 
to the union
 the union can be named 'max', as this would collide with the implicit
 enum.  The value for each branch can be of any type.

-A flat union definition specifies a struct as its base, and
-avoids nesting on the wire.  All branches of the union must be
-complex types, and the top-level members of the union dictionary on
-the wire will be combination of members from both the base type and the
-appropriate branch type (when merging two dictionaries, there must be
-no keys in common).  The 'discriminator' member must be the name of an
-enum-typed member of the base struct.
+A flat union definition avoids nesting on the wire, and specifies a
+set of common members that occur in all variants of the union.  The
+'base' key must specifiy either a type name (the type must be a
+struct, not a union), or a dictionary representing an anonymous type.
+All branches of the union must be complex types, and the top-level
+members of the union dictionary on the wire will be combination of
+members from both the base type and the appropriate branch type (when
+merging two dictionaries, there must be no keys in common).  The
+'discriminator' member must be the name of a non-optional enum-typed
+member of the base struct.

 The following example enhances the above simple union example by
-adding a common member 'readonly', renaming the discriminator to
+adding a common member 'read-only', renaming the discriminator to
 something more applicable, and reducing the number of {} required on
 the wire:

  { 'enum': 'BlockdevDriver', 'data': [ 'file', 'qcow2' ] }
- { 'struct': 'BlockdevCommonOptions',
-   'data': { 'driver': 'BlockdevDriver', 'readonly': 'bool' } }
  { 'union': 'BlockdevOptions',
-   'base': 'BlockdevCommonOptions',
+   'base': { 'driver': 'BlockdevDriver', 'read-only': 'bool' },
    'discriminator': 'driver',
    'data': { 'file': 'FileOptions',
              'qcow2': 'Qcow2Options' } }
@@ -346,7 +347,7 @@ Resulting in these JSON objects:

  { "driver": "file", "readonly": true,
    "filename": "/some/place/my-image" }
- { "driver": "qcow2", "readonly": false,
+ { "driver": "qcow2", "read-only": false,
    "backing-file": "/some/place/my-image", "lazy-refcounts": true }

 Notice that in a flat union, the discriminator name is controlled by
@@ -366,10 +367,9 @@ union has a struct with a single member named 'data'.  
That is,
 is identical on the wire to:

  { 'enum': 'Enum', 'data': ['one', 'two'] }
- { 'struct': 'Base', 'data': { 'type': 'Enum' } }
  { 'struct': 'Branch1', 'data': { 'data': 'str' } }
  { 'struct': 'Branch2', 'data': { 'data': 'int' } }
- { 'union': 'Flat', 'base': 'Base', 'discriminator': 'type',
+ { 'union': 'Flat': 'base': { 'type': 'Enum' }, 'discriminator': 'type',
    'data': { 'one': 'Branch1', 'two': 'Branch2' } }

diff --git a/tests/qapi-schema/flat-union-bad-base.err 
index 79b8a71..bee24a2 100644
--- a/tests/qapi-schema/flat-union-bad-base.err
+++ b/tests/qapi-schema/flat-union-bad-base.err
@@ -1 +1 @@
-tests/qapi-schema/flat-union-bad-base.json:9: 'base' for union 'TestUnion' 
should be a type name
+tests/qapi-schema/flat-union-bad-base.json:8: 'string' (member of TestTypeA) 
collides with 'string' (base of TestUnion)
diff --git a/tests/qapi-schema/flat-union-bad-base.json 
index e2e622b..74dd421 100644
--- a/tests/qapi-schema/flat-union-bad-base.json
+++ b/tests/qapi-schema/flat-union-bad-base.json
@@ -1,5 +1,4 @@
-# we require the base to be an existing struct
-# TODO: should we allow an anonymous inline base type?
+# we allow anonymous base, but enforce no duplicate keys
 { 'enum': 'TestEnum',
   'data': [ 'value1', 'value2' ] }
 { 'struct': 'TestTypeA',
@@ -7,7 +6,7 @@
 { 'struct': 'TestTypeB',
   'data': { 'integer': 'int' } }
 { 'union': 'TestUnion',
-  'base': { 'enum1': 'TestEnum', 'kind': 'str' },
+  'base': { 'enum1': 'TestEnum', 'string': 'str' },
   'discriminator': 'enum1',
   'data': { 'value1': 'TestTypeA',
             'value2': 'TestTypeB' } }
diff --git a/tests/qapi-schema/qapi-schema-test.json 
index e722748..f571e1b 100644
--- a/tests/qapi-schema/qapi-schema-test.json
+++ b/tests/qapi-schema/qapi-schema-test.json
@@ -75,14 +75,10 @@
   'base': 'UserDefZero',
   'data': { 'string': 'str', 'enum1': 'EnumOne' } }

-{ 'struct': 'UserDefUnionBase2',
-  'base': 'UserDefZero',
-  'data': { 'string': 'str', 'enum1': 'QEnumTwo' } }
 # this variant of UserDefFlatUnion defaults to a union that uses members with
 # allocated types to test corner cases in the cleanup/dealloc visitor
 { 'union': 'UserDefFlatUnion2',
-  'base': 'UserDefUnionBase2',
+  'base': { '*integer': 'int', 'string': 'str', 'enum1': 'QEnumTwo' },
   'discriminator': 'enum1',
   'data': { 'value1' : 'UserDefC', # intentional forward reference
             'value2' : 'UserDefB' } }
diff --git a/tests/qapi-schema/qapi-schema-test.out 
index f531961..a67d375 100644
--- a/tests/qapi-schema/qapi-schema-test.out
+++ b/tests/qapi-schema/qapi-schema-test.out
@@ -8,6 +8,10 @@ object :obj-EVENT_D-arg
     member b: str optional=False
     member c: str optional=True
     member enum3: EnumOne optional=True
+object :obj-UserDefFlatUnion2-base
+    member integer: int optional=True
+    member string: str optional=False
+    member enum1: QEnumTwo optional=False
 object :obj-__org.qemu_x-command-arg
     member a: __org.qemu_x-EnumList optional=False
     member b: __org.qemu_x-StructList optional=False
@@ -121,7 +125,7 @@ object UserDefFlatUnion
     case value2: UserDefB
     case value3: UserDefB
 object UserDefFlatUnion2
-    base UserDefUnionBase2
+    base :obj-UserDefFlatUnion2-base
     tag enum1
     case value1: UserDefC
     case value2: UserDefB
@@ -166,10 +170,6 @@ object UserDefUnionBase
     base UserDefZero
     member string: str optional=False
     member enum1: EnumOne optional=False
-object UserDefUnionBase2
-    base UserDefZero
-    member string: str optional=False
-    member enum1: QEnumTwo optional=False
 object UserDefZero
     member integer: int optional=False
 object WrapAlternate

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