[v4,1/2] qapi: allow empty branches in flat unions

Message ID 1528905228-25694-2-git-send-email-anton.nefedov@virtuozzo.com
State New
Headers show
Series
  • qapi: allow empty branches in flat unions
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Commit Message

Anton Nefedov June 13, 2018, 3:53 p.m.
It often happens that just a few discriminator values imply extra data in
a flat union. Existing checks did not make possible to leave other values
uncovered. Such cases had to be worked around by either stating a dummy
(empty) type or introducing another (subset) discriminator enumeration.

Both options create redundant entities in qapi files for little profit.

With this patch it is not necessary anymore to add designated union
fields for every possible value of a discriminator enumeration.

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 docs/devel/qapi-code-gen.txt                        |  8 +++++---
 tests/qapi-schema/flat-union-incomplete-branch.json |  9 ---------
 tests/qapi-schema/qapi-schema-test.json             |  4 ++--
 scripts/qapi/common.py                              | 11 ++++-------
 scripts/qapi/types.py                               |  4 +++-
 scripts/qapi/visit.py                               | 17 +++++++++++++----
 tests/Makefile.include                              |  1 -
 tests/qapi-schema/flat-union-incomplete-branch.err  |  1 -
 tests/qapi-schema/flat-union-incomplete-branch.exit |  1 -
 tests/qapi-schema/flat-union-incomplete-branch.out  |  0
 tests/qapi-schema/qapi-schema-test.out              |  3 ++-
 11 files changed, 29 insertions(+), 30 deletions(-)
 delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.json
 delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.err
 delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.exit
 delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.out

Comments

Markus Armbruster June 14, 2018, 7:19 a.m. | #1
Anton Nefedov <anton.nefedov@virtuozzo.com> writes:

> It often happens that just a few discriminator values imply extra data in
> a flat union. Existing checks did not make possible to leave other values
> uncovered. Such cases had to be worked around by either stating a dummy
> (empty) type or introducing another (subset) discriminator enumeration.
>
> Both options create redundant entities in qapi files for little profit.
>
> With this patch it is not necessary anymore to add designated union
> fields for every possible value of a discriminator enumeration.
>
> Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> ---
>  docs/devel/qapi-code-gen.txt                        |  8 +++++---
>  tests/qapi-schema/flat-union-incomplete-branch.json |  9 ---------
>  tests/qapi-schema/qapi-schema-test.json             |  4 ++--
>  scripts/qapi/common.py                              | 11 ++++-------
>  scripts/qapi/types.py                               |  4 +++-
>  scripts/qapi/visit.py                               | 17 +++++++++++++----
>  tests/Makefile.include                              |  1 -
>  tests/qapi-schema/flat-union-incomplete-branch.err  |  1 -
>  tests/qapi-schema/flat-union-incomplete-branch.exit |  1 -
>  tests/qapi-schema/flat-union-incomplete-branch.out  |  0
>  tests/qapi-schema/qapi-schema-test.out              |  3 ++-
>  11 files changed, 29 insertions(+), 30 deletions(-)
>  delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.json
>  delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.err
>  delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.exit
>  delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.out
>
> diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
> index 1366228..88a70e4 100644
> --- a/docs/devel/qapi-code-gen.txt
> +++ b/docs/devel/qapi-code-gen.txt
> @@ -496,9 +496,11 @@ Resulting in these JSON objects:
>  
>  Notice that in a flat union, the discriminator name is controlled by
>  the user, but because it must map to a base member with enum type, the
> -code generator can ensure that branches exist for all values of the
> -enum (although the order of the keys need not match the declaration of
> -the enum).  In the resulting generated C data types, a flat union is
> +code generator ensures that branches match the existing values of the
> +enum. The order of the keys need not match the declaration of the enum.
> +The keys need not cover all possible enum values. Omitted enum values
> +are still valid branches that add no additional members to the data type.
> +In the resulting generated C data types, a flat union is
>  represented as a struct with the base members included directly, and
>  then a union of structures for each branch of the struct.
>  
> diff --git a/tests/qapi-schema/flat-union-incomplete-branch.json b/tests/qapi-schema/flat-union-incomplete-branch.json
> deleted file mode 100644
> index 25a411b..0000000
> --- a/tests/qapi-schema/flat-union-incomplete-branch.json
> +++ /dev/null
> @@ -1,9 +0,0 @@
> -# we require all branches of the union to be covered
> -{ 'enum': 'TestEnum',
> -  'data': [ 'value1', 'value2' ] }
> -{ 'struct': 'TestTypeA',
> -  'data': { 'string': 'str' } }
> -{ 'union': 'TestUnion',
> -  'base': { 'type': 'TestEnum' },
> -  'discriminator': 'type',
> -  'data': { 'value1': 'TestTypeA' } }
> diff --git a/tests/qapi-schema/qapi-schema-test.json b/tests/qapi-schema/qapi-schema-test.json
> index 46c7282..881b53b 100644
> --- a/tests/qapi-schema/qapi-schema-test.json
> +++ b/tests/qapi-schema/qapi-schema-test.json
> @@ -39,7 +39,7 @@
>              '*enum1': 'EnumOne' } }   # intentional forward reference
>  
>  { 'enum': 'EnumOne',
> -  'data': [ 'value1', 'value2', 'value3' ] }
> +  'data': [ 'value1', 'value2', 'value3', 'value4' ] }
>  
>  { 'struct': 'UserDefZero',
>    'data': { 'integer': 'int' } }
> @@ -76,7 +76,7 @@
>    'discriminator': 'enum1',
>    'data': { 'value1' : 'UserDefA',
>              'value2' : 'UserDefB',
> -            'value3' : 'UserDefB' } }
> +            'value3' : 'UserDefB' } } # skip 'value4'

Perhaps

               'value3' : 'UserDefB'
                # 'value4' defaults to empty
             } }

would be clearer.

>  
>  { 'struct': 'UserDefUnionBase',
>    'base': 'UserDefZero',
> diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
> index 2462fc0..0f3d89b 100644
> --- a/scripts/qapi/common.py
> +++ b/scripts/qapi/common.py
> @@ -779,13 +779,6 @@ def check_union(expr, info):
>                                     "enum '%s'"
>                                     % (key, enum_define['enum']))
>  
> -    # If discriminator is user-defined, ensure all values are covered
> -    if enum_define:
> -        for value in enum_define['data']:
> -            if value not in members.keys():
> -                raise QAPISemError(info, "Union '%s' data missing '%s' branch"
> -                                   % (name, value))
> -
>  
>  def check_alternate(expr, info):
>      name = expr['alternate']
> @@ -1644,6 +1637,10 @@ class QAPISchema(object):
>          if tag_name:
>              variants = [self._make_variant(key, value)
>                          for (key, value) in data.items()]
> +            # branches that are not explicitly covered get an empty type
> +            variants += [self._make_variant(key, 'q_empty')
> +                         for key in discriminator_find_enum_define(expr)['data']

Long line, flagged by pycodestyle --diff.

> +                         if key not in data.keys()]
>              members = []
>          else:
>              variants = [self._make_simple_variant(key, value, info)

The way you desugar the union is simple enough, but it uses
discriminator_find_enum_define() outside check_exprs(), which I'd rather
avoid.  Not something you could be reasonably expected to know.

We could desugar in QAPISchemaObjectTypeVariants.check(), where we have
self.tag_member.type.values.

   @@ -1346,10 +1346,17 @@ class QAPISchemaObjectTypeVariants(object):
                v.set_owner(name)

        def check(self, schema, seen):
   -        if not self.tag_member:    # flat union
   +        if not self.tag_member: # flat union
                self.tag_member = seen[c_name(self._tag_name)]
                assert self._tag_name == self.tag_member.name
            assert isinstance(self.tag_member.type, QAPISchemaEnumType)
   +        if self._tag_name:      # flat union
   +            cases = set([v.name for v in self.variants])
   +            for val in self.tag_member.type.values:
   +                if val.name not in cases:
   +                    v = QAPISchemaObjectTypeVariant(val.name, 'q_empty')
   +                    v.set_owner(self.tag_member.owner)
   +                    self.variants.append(v)
            for v in self.variants:
                v.check(schema)
                # Union names must match enum values; alternate names are

Not so simple anymore.

Simpler: desugar in check_union(), i.e. replace your two hunks by just

   @@ -783,8 +783,7 @@ def check_union(expr, info):
        if enum_define:
            for value in enum_define['data']:
                if value not in members.keys():
   -                raise QAPISemError(info, "Union '%s' data missing '%s' branch"
   -                                   % (name, value))
   +                members[value] = 'q_empty'


    def check_alternate(expr, info):

What do you think?

> diff --git a/scripts/qapi/types.py b/scripts/qapi/types.py
> index 64d9c0f..1fb2b6d 100644
> --- a/scripts/qapi/types.py
> +++ b/scripts/qapi/types.py
> @@ -124,7 +124,9 @@ def gen_variants(variants):
>  ''',
>                  c_name=c_name(variants.tag_member.name))
>  
> -    for var in variants.variants:
> +    # filter out the empty types
> +    for var in filter(lambda var: var.type.name != 'q_empty',
> +                      variants.variants):

The comment's position suggests it tells what the loop does, but that's
not the case.  Let's do it the old-fashioned obvious way instead:

       for var in variants.variants:
           if var.type.name == 'q_empty':
               continue

Aside: we need to filter out q_empty, because we don't generate a C type
for it.  Perhaps generating one would be simpler, but that's beyond the
scope of this patch.

>          ret += mcgen('''
>          %(c_type)s %(c_name)s;
>  ''',
> diff --git a/scripts/qapi/visit.py b/scripts/qapi/visit.py
> index 5d72d89..96bd32c 100644
> --- a/scripts/qapi/visit.py
> +++ b/scripts/qapi/visit.py
> @@ -81,14 +81,23 @@ void visit_type_%(c_name)s_members(Visitor *v, %(c_name)s *obj, Error **errp)
>                       c_name=c_name(variants.tag_member.name))
>  
>          for var in variants.variants:
> -            ret += mcgen('''
> +            case_str = c_enum_const(variants.tag_member.type.name,
> +                                    var.name,
> +                                    variants.tag_member.type.prefix)
> +            if var.type.name == 'q_empty':
> +                # valid variant and nothing to do
> +                ret += mcgen('''
> +    case %(case)s:
> +        break;
> +''',
> +                             case=case_str)
> +            else:
> +                ret += mcgen('''

Aside: we need this, because we don't generate a visitor for q_empty.

>      case %(case)s:
>          visit_type_%(c_type)s_members(v, &obj->u.%(c_name)s, &err);
>          break;
>  ''',
> -                         case=c_enum_const(variants.tag_member.type.name,
> -                                           var.name,
> -                                           variants.tag_member.type.prefix),
> +                         case=case_str,
>                           c_type=var.type.c_name(), c_name=c_name(var.name))

Indentation's off, flagged by pycodestyle --diff.

>  
>          ret += mcgen('''
> diff --git a/tests/Makefile.include b/tests/Makefile.include
> index bb08e37..afdd0db 100644
> --- a/tests/Makefile.include
> +++ b/tests/Makefile.include
> @@ -496,7 +496,6 @@ qapi-schema += flat-union-base-any.json
>  qapi-schema += flat-union-base-union.json
>  qapi-schema += flat-union-clash-member.json
>  qapi-schema += flat-union-empty.json
> -qapi-schema += flat-union-incomplete-branch.json
>  qapi-schema += flat-union-inline.json
>  qapi-schema += flat-union-int-branch.json
>  qapi-schema += flat-union-invalid-branch-key.json
> diff --git a/tests/qapi-schema/flat-union-incomplete-branch.err b/tests/qapi-schema/flat-union-incomplete-branch.err
> deleted file mode 100644
> index e826bf0..0000000
> --- a/tests/qapi-schema/flat-union-incomplete-branch.err
> +++ /dev/null
> @@ -1 +0,0 @@
> -tests/qapi-schema/flat-union-incomplete-branch.json:6: Union 'TestUnion' data missing 'value2' branch
> diff --git a/tests/qapi-schema/flat-union-incomplete-branch.exit b/tests/qapi-schema/flat-union-incomplete-branch.exit
> deleted file mode 100644
> index d00491f..0000000
> --- a/tests/qapi-schema/flat-union-incomplete-branch.exit
> +++ /dev/null
> @@ -1 +0,0 @@
> -1
> diff --git a/tests/qapi-schema/flat-union-incomplete-branch.out b/tests/qapi-schema/flat-union-incomplete-branch.out
> deleted file mode 100644
> index e69de29..0000000
> diff --git a/tests/qapi-schema/qapi-schema-test.out b/tests/qapi-schema/qapi-schema-test.out
> index 542a19c..0dbcdaf 100644
> --- a/tests/qapi-schema/qapi-schema-test.out
> +++ b/tests/qapi-schema/qapi-schema-test.out
> @@ -23,7 +23,7 @@ object UserDefOne
>      base UserDefZero
>      member string: str optional=False
>      member enum1: EnumOne optional=True
> -enum EnumOne ['value1', 'value2', 'value3']
> +enum EnumOne ['value1', 'value2', 'value3', 'value4']
>  object UserDefZero
>      member integer: int optional=False
>  object UserDefTwoDictDict
> @@ -52,6 +52,7 @@ object UserDefFlatUnion
>      case value1: UserDefA
>      case value2: UserDefB
>      case value3: UserDefB
> +    case value4: q_empty
>  object UserDefUnionBase
>      base UserDefZero
>      member string: str optional=False
Anton Nefedov June 14, 2018, 9:01 a.m. | #2
On 14/6/2018 10:19 AM, Markus Armbruster wrote:
> Anton Nefedov <anton.nefedov@virtuozzo.com> writes:
> 
>> It often happens that just a few discriminator values imply extra data in
>> a flat union. Existing checks did not make possible to leave other values
>> uncovered. Such cases had to be worked around by either stating a dummy
>> (empty) type or introducing another (subset) discriminator enumeration.
>>
>> Both options create redundant entities in qapi files for little profit.
>>
>> With this patch it is not necessary anymore to add designated union
>> fields for every possible value of a discriminator enumeration.
>>
>> Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
>> Reviewed-by: Eric Blake <eblake@redhat.com>
>> ---
>>   docs/devel/qapi-code-gen.txt                        |  8 +++++---
>>   tests/qapi-schema/flat-union-incomplete-branch.json |  9 ---------
>>   tests/qapi-schema/qapi-schema-test.json             |  4 ++--
>>   scripts/qapi/common.py                              | 11 ++++-------
>>   scripts/qapi/types.py                               |  4 +++-
>>   scripts/qapi/visit.py                               | 17 +++++++++++++----
>>   tests/Makefile.include                              |  1 -
>>   tests/qapi-schema/flat-union-incomplete-branch.err  |  1 -
>>   tests/qapi-schema/flat-union-incomplete-branch.exit |  1 -
>>   tests/qapi-schema/flat-union-incomplete-branch.out  |  0
>>   tests/qapi-schema/qapi-schema-test.out              |  3 ++-
>>   11 files changed, 29 insertions(+), 30 deletions(-)
>>   delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.json
>>   delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.err
>>   delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.exit
>>   delete mode 100644 tests/qapi-schema/flat-union-incomplete-branch.out
>>
>> diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
>> index 1366228..88a70e4 100644
>> --- a/docs/devel/qapi-code-gen.txt
>> +++ b/docs/devel/qapi-code-gen.txt
>> @@ -496,9 +496,11 @@ Resulting in these JSON objects:
>>   
>>   Notice that in a flat union, the discriminator name is controlled by
>>   the user, but because it must map to a base member with enum type, the
>> -code generator can ensure that branches exist for all values of the
>> -enum (although the order of the keys need not match the declaration of
>> -the enum).  In the resulting generated C data types, a flat union is
>> +code generator ensures that branches match the existing values of the
>> +enum. The order of the keys need not match the declaration of the enum.
>> +The keys need not cover all possible enum values. Omitted enum values
>> +are still valid branches that add no additional members to the data type.
>> +In the resulting generated C data types, a flat union is
>>   represented as a struct with the base members included directly, and
>>   then a union of structures for each branch of the struct.
>>   
>> diff --git a/tests/qapi-schema/flat-union-incomplete-branch.json b/tests/qapi-schema/flat-union-incomplete-branch.json
>> deleted file mode 100644
>> index 25a411b..0000000
>> --- a/tests/qapi-schema/flat-union-incomplete-branch.json
>> +++ /dev/null
>> @@ -1,9 +0,0 @@
>> -# we require all branches of the union to be covered
>> -{ 'enum': 'TestEnum',
>> -  'data': [ 'value1', 'value2' ] }
>> -{ 'struct': 'TestTypeA',
>> -  'data': { 'string': 'str' } }
>> -{ 'union': 'TestUnion',
>> -  'base': { 'type': 'TestEnum' },
>> -  'discriminator': 'type',
>> -  'data': { 'value1': 'TestTypeA' } }
>> diff --git a/tests/qapi-schema/qapi-schema-test.json b/tests/qapi-schema/qapi-schema-test.json
>> index 46c7282..881b53b 100644
>> --- a/tests/qapi-schema/qapi-schema-test.json
>> +++ b/tests/qapi-schema/qapi-schema-test.json
>> @@ -39,7 +39,7 @@
>>               '*enum1': 'EnumOne' } }   # intentional forward reference
>>   
>>   { 'enum': 'EnumOne',
>> -  'data': [ 'value1', 'value2', 'value3' ] }
>> +  'data': [ 'value1', 'value2', 'value3', 'value4' ] }
>>   
>>   { 'struct': 'UserDefZero',
>>     'data': { 'integer': 'int' } }
>> @@ -76,7 +76,7 @@
>>     'discriminator': 'enum1',
>>     'data': { 'value1' : 'UserDefA',
>>               'value2' : 'UserDefB',
>> -            'value3' : 'UserDefB' } }
>> +            'value3' : 'UserDefB' } } # skip 'value4'
> 
> Perhaps
> 
>                 'value3' : 'UserDefB'
>                  # 'value4' defaults to empty
>               } }
> 
> would be clearer.
> 

Sure

>>   
>>   { 'struct': 'UserDefUnionBase',
>>     'base': 'UserDefZero',
>> diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
>> index 2462fc0..0f3d89b 100644
>> --- a/scripts/qapi/common.py
>> +++ b/scripts/qapi/common.py
>> @@ -779,13 +779,6 @@ def check_union(expr, info):
>>                                      "enum '%s'"
>>                                      % (key, enum_define['enum']))
>>   
>> -    # If discriminator is user-defined, ensure all values are covered
>> -    if enum_define:
>> -        for value in enum_define['data']:
>> -            if value not in members.keys():
>> -                raise QAPISemError(info, "Union '%s' data missing '%s' branch"
>> -                                   % (name, value))
>> -
>>   
>>   def check_alternate(expr, info):
>>       name = expr['alternate']
>> @@ -1644,6 +1637,10 @@ class QAPISchema(object):
>>           if tag_name:
>>               variants = [self._make_variant(key, value)
>>                           for (key, value) in data.items()]
>> +            # branches that are not explicitly covered get an empty type
>> +            variants += [self._make_variant(key, 'q_empty')
>> +                         for key in discriminator_find_enum_define(expr)['data']
> 
> Long line, flagged by pycodestyle --diff.
> 

Done.

>> +                         if key not in data.keys()]
>>               members = []
>>           else:
>>               variants = [self._make_simple_variant(key, value, info)
> 
> The way you desugar the union is simple enough, but it uses
> discriminator_find_enum_define() outside check_exprs(), which I'd rather
> avoid.  Not something you could be reasonably expected to know.
> 
> We could desugar in QAPISchemaObjectTypeVariants.check(), where we have
> self.tag_member.type.values.
> 
>     @@ -1346,10 +1346,17 @@ class QAPISchemaObjectTypeVariants(object):
>                  v.set_owner(name)
> 
>          def check(self, schema, seen):
>     -        if not self.tag_member:    # flat union
>     +        if not self.tag_member: # flat union
>                  self.tag_member = seen[c_name(self._tag_name)]
>                  assert self._tag_name == self.tag_member.name
>              assert isinstance(self.tag_member.type, QAPISchemaEnumType)
>     +        if self._tag_name:      # flat union
>     +            cases = set([v.name for v in self.variants])
>     +            for val in self.tag_member.type.values:
>     +                if val.name not in cases:
>     +                    v = QAPISchemaObjectTypeVariant(val.name, 'q_empty')
>     +                    v.set_owner(self.tag_member.owner)
>     +                    self.variants.append(v)
>              for v in self.variants:
>                  v.check(schema)
>                  # Union names must match enum values; alternate names are
> 
> Not so simple anymore.
> 
> Simpler: desugar in check_union(), i.e. replace your two hunks by just
> 
>     @@ -783,8 +783,7 @@ def check_union(expr, info):
>          if enum_define:
>              for value in enum_define['data']:
>                  if value not in members.keys():
>     -                raise QAPISemError(info, "Union '%s' data missing '%s' branch"
>     -                                   % (name, value))
>     +                members[value] = 'q_empty'
> 
> 
>      def check_alternate(expr, info):
> 
> What do you think?
> 

it didn't feel right aesthetically to normalize by modifying the parsed
expressions especially at check_exprs() stage. But maybe using
discriminator_find_enum_define() is more sloppy, I can't quite see

>> diff --git a/scripts/qapi/types.py b/scripts/qapi/types.py
>> index 64d9c0f..1fb2b6d 100644
>> --- a/scripts/qapi/types.py
>> +++ b/scripts/qapi/types.py
>> @@ -124,7 +124,9 @@ def gen_variants(variants):
>>   ''',
>>                   c_name=c_name(variants.tag_member.name))
>>   
>> -    for var in variants.variants:
>> +    # filter out the empty types
>> +    for var in filter(lambda var: var.type.name != 'q_empty',
>> +                      variants.variants):
> 
> The comment's position suggests it tells what the loop does, but that's
> not the case.  Let's do it the old-fashioned obvious way instead:
> 
>         for var in variants.variants:
>             if var.type.name == 'q_empty':
>                 continue
> 

Done.

> Aside: we need to filter out q_empty, because we don't generate a C type
> for it.  Perhaps generating one would be simpler, but that's beyond the
> scope of this patch.
> 
>>           ret += mcgen('''
>>           %(c_type)s %(c_name)s;
>>   ''',
>> diff --git a/scripts/qapi/visit.py b/scripts/qapi/visit.py
>> index 5d72d89..96bd32c 100644
>> --- a/scripts/qapi/visit.py
>> +++ b/scripts/qapi/visit.py
>> @@ -81,14 +81,23 @@ void visit_type_%(c_name)s_members(Visitor *v, %(c_name)s *obj, Error **errp)
>>                        c_name=c_name(variants.tag_member.name))
>>   
>>           for var in variants.variants:
>> -            ret += mcgen('''
>> +            case_str = c_enum_const(variants.tag_member.type.name,
>> +                                    var.name,
>> +                                    variants.tag_member.type.prefix)
>> +            if var.type.name == 'q_empty':
>> +                # valid variant and nothing to do
>> +                ret += mcgen('''
>> +    case %(case)s:
>> +        break;
>> +''',
>> +                             case=case_str)
>> +            else:
>> +                ret += mcgen('''
> 
> Aside: we need this, because we don't generate a visitor for q_empty.
> 
>>       case %(case)s:
>>           visit_type_%(c_type)s_members(v, &obj->u.%(c_name)s, &err);
>>           break;
>>   ''',
>> -                         case=c_enum_const(variants.tag_member.type.name,
>> -                                           var.name,
>> -                                           variants.tag_member.type.prefix),
>> +                         case=case_str,
>>                            c_type=var.type.c_name(), c_name=c_name(var.name))
> 
> Indentation's off, flagged by pycodestyle --diff.
> 

Done.

>>   
>>           ret += mcgen('''
>> diff --git a/tests/Makefile.include b/tests/Makefile.include
>> index bb08e37..afdd0db 100644
>> --- a/tests/Makefile.include
>> +++ b/tests/Makefile.include
>> @@ -496,7 +496,6 @@ qapi-schema += flat-union-base-any.json
>>   qapi-schema += flat-union-base-union.json
>>   qapi-schema += flat-union-clash-member.json
>>   qapi-schema += flat-union-empty.json
>> -qapi-schema += flat-union-incomplete-branch.json
>>   qapi-schema += flat-union-inline.json
>>   qapi-schema += flat-union-int-branch.json
>>   qapi-schema += flat-union-invalid-branch-key.json
>> diff --git a/tests/qapi-schema/flat-union-incomplete-branch.err b/tests/qapi-schema/flat-union-incomplete-branch.err
>> deleted file mode 100644
>> index e826bf0..0000000
>> --- a/tests/qapi-schema/flat-union-incomplete-branch.err
>> +++ /dev/null
>> @@ -1 +0,0 @@
>> -tests/qapi-schema/flat-union-incomplete-branch.json:6: Union 'TestUnion' data missing 'value2' branch
>> diff --git a/tests/qapi-schema/flat-union-incomplete-branch.exit b/tests/qapi-schema/flat-union-incomplete-branch.exit
>> deleted file mode 100644
>> index d00491f..0000000
>> --- a/tests/qapi-schema/flat-union-incomplete-branch.exit
>> +++ /dev/null
>> @@ -1 +0,0 @@
>> -1
>> diff --git a/tests/qapi-schema/flat-union-incomplete-branch.out b/tests/qapi-schema/flat-union-incomplete-branch.out
>> deleted file mode 100644
>> index e69de29..0000000
>> diff --git a/tests/qapi-schema/qapi-schema-test.out b/tests/qapi-schema/qapi-schema-test.out
>> index 542a19c..0dbcdaf 100644
>> --- a/tests/qapi-schema/qapi-schema-test.out
>> +++ b/tests/qapi-schema/qapi-schema-test.out
>> @@ -23,7 +23,7 @@ object UserDefOne
>>       base UserDefZero
>>       member string: str optional=False
>>       member enum1: EnumOne optional=True
>> -enum EnumOne ['value1', 'value2', 'value3']
>> +enum EnumOne ['value1', 'value2', 'value3', 'value4']
>>   object UserDefZero
>>       member integer: int optional=False
>>   object UserDefTwoDictDict
>> @@ -52,6 +52,7 @@ object UserDefFlatUnion
>>       case value1: UserDefA
>>       case value2: UserDefB
>>       case value3: UserDefB
>> +    case value4: q_empty
>>   object UserDefUnionBase
>>       base UserDefZero
>>       member string: str optional=False
Markus Armbruster June 15, 2018, 1:38 p.m. | #3
Anton Nefedov <anton.nefedov@virtuozzo.com> writes:

> On 14/6/2018 10:19 AM, Markus Armbruster wrote:
>> Anton Nefedov <anton.nefedov@virtuozzo.com> writes:
>>
>>> It often happens that just a few discriminator values imply extra data in
>>> a flat union. Existing checks did not make possible to leave other values
>>> uncovered. Such cases had to be worked around by either stating a dummy
>>> (empty) type or introducing another (subset) discriminator enumeration.
>>>
>>> Both options create redundant entities in qapi files for little profit.
>>>
>>> With this patch it is not necessary anymore to add designated union
>>> fields for every possible value of a discriminator enumeration.
>>>
>>> Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
>>> Reviewed-by: Eric Blake <eblake@redhat.com>
[...]
>>> diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
>>> index 2462fc0..0f3d89b 100644
>>> --- a/scripts/qapi/common.py
>>> +++ b/scripts/qapi/common.py
>>> @@ -779,13 +779,6 @@ def check_union(expr, info):
>>>                                      "enum '%s'"
>>>                                      % (key, enum_define['enum']))
>>>   -    # If discriminator is user-defined, ensure all values are
>>> covered
>>> -    if enum_define:
>>> -        for value in enum_define['data']:
>>> -            if value not in members.keys():
>>> -                raise QAPISemError(info, "Union '%s' data missing '%s' branch"
>>> -                                   % (name, value))
>>> -
>>>     def check_alternate(expr, info):
>>>       name = expr['alternate']
>>> @@ -1644,6 +1637,10 @@ class QAPISchema(object):
>>>           if tag_name:
>>>               variants = [self._make_variant(key, value)
>>>                           for (key, value) in data.items()]
>>> +            # branches that are not explicitly covered get an empty type
>>> +            variants += [self._make_variant(key, 'q_empty')
>>> +                         for key in discriminator_find_enum_define(expr)['data']
>>
>> Long line, flagged by pycodestyle --diff.
>>
>
> Done.
>
>>> +                         if key not in data.keys()]
>>>               members = []
>>>           else:
>>>               variants = [self._make_simple_variant(key, value, info)
>>
>> The way you desugar the union is simple enough, but it uses
>> discriminator_find_enum_define() outside check_exprs(), which I'd rather
>> avoid.  Not something you could be reasonably expected to know.
>>
>> We could desugar in QAPISchemaObjectTypeVariants.check(), where we have
>> self.tag_member.type.values.
>>
>>     @@ -1346,10 +1346,17 @@ class QAPISchemaObjectTypeVariants(object):
>>                  v.set_owner(name)
>>
>>          def check(self, schema, seen):
>>     -        if not self.tag_member:    # flat union
>>     +        if not self.tag_member: # flat union
>>                  self.tag_member = seen[c_name(self._tag_name)]
>>                  assert self._tag_name == self.tag_member.name
>>              assert isinstance(self.tag_member.type, QAPISchemaEnumType)
>>     +        if self._tag_name:      # flat union
>>     +            cases = set([v.name for v in self.variants])
>>     +            for val in self.tag_member.type.values:
>>     +                if val.name not in cases:
>>     +                    v = QAPISchemaObjectTypeVariant(val.name, 'q_empty')
>>     +                    v.set_owner(self.tag_member.owner)
>>     +                    self.variants.append(v)
>>              for v in self.variants:
>>                  v.check(schema)
>>                  # Union names must match enum values; alternate names are
>>
>> Not so simple anymore.
>>
>> Simpler: desugar in check_union(), i.e. replace your two hunks by just
>>
>>     @@ -783,8 +783,7 @@ def check_union(expr, info):
>>          if enum_define:
>>              for value in enum_define['data']:
>>                  if value not in members.keys():
>>     -                raise QAPISemError(info, "Union '%s' data missing '%s' branch"
>>     -                                   % (name, value))
>>     +                members[value] = 'q_empty'
>>
>>
>>      def check_alternate(expr, info):
>>
>> What do you think?
>>
>
> it didn't feel right aesthetically to normalize by modifying the parsed
> expressions especially at check_exprs() stage. But maybe using
> discriminator_find_enum_define() is more sloppy, I can't quite see

The issue isn't sloppiness, it's adding to our technical debt.  To
explain this, I need to relate a bit of history.

In the beginning, we did everything with syntax trees.  Commit
ac88219a6c7 (Sep 2015) spliced in an intermediate representation.  Its
commit message explains:

    The QAPI code generators work with a syntax tree (nested dictionaries)
    plus a few symbol tables (also dictionaries) on the side.

    They have clearly outgrown these simple data structures.  There's lots
    of rummaging around in dictionaries, and information is recomputed on
    the fly.  For the work I'm going to do, I want more clearly defined
    and more convenient interfaces.
    
    Going forward, I also want less coupling between the back-ends and the
    syntax tree, to make messing with the syntax easier.

We accepted some technical debt back then.  Same commit message:

    Shortcut: the semantic analysis still relies on existing check_exprs()
    to do the actual semantic checking.  All this code needs to move into
    the classes.  Mark as TODO.

It's this one:

    #
    # Semantic analysis of schema expressions
    # TODO fold into QAPISchema
    # TODO catching name collisions in generated code would be nice
    #

check_exprs() is the only entry point into the code under this TODO.
Your patch adds another one.  I don't like that.

Nobody expects you to deduce this from the code by yourself :)

Now, I agree hacking up the syntax tree isn't particularly clean,
either.  Perhaps doing it in QAPISchemaObjectTypeVariants.check() is
best after all.  In case you feel unsure about making that change, I can
take your solution, and address its technical debt issue in a follow-up
patch.

[...]

Patch

diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
index 1366228..88a70e4 100644
--- a/docs/devel/qapi-code-gen.txt
+++ b/docs/devel/qapi-code-gen.txt
@@ -496,9 +496,11 @@  Resulting in these JSON objects:
 
 Notice that in a flat union, the discriminator name is controlled by
 the user, but because it must map to a base member with enum type, the
-code generator can ensure that branches exist for all values of the
-enum (although the order of the keys need not match the declaration of
-the enum).  In the resulting generated C data types, a flat union is
+code generator ensures that branches match the existing values of the
+enum. The order of the keys need not match the declaration of the enum.
+The keys need not cover all possible enum values. Omitted enum values
+are still valid branches that add no additional members to the data type.
+In the resulting generated C data types, a flat union is
 represented as a struct with the base members included directly, and
 then a union of structures for each branch of the struct.
 
diff --git a/tests/qapi-schema/flat-union-incomplete-branch.json b/tests/qapi-schema/flat-union-incomplete-branch.json
deleted file mode 100644
index 25a411b..0000000
--- a/tests/qapi-schema/flat-union-incomplete-branch.json
+++ /dev/null
@@ -1,9 +0,0 @@ 
-# we require all branches of the union to be covered
-{ 'enum': 'TestEnum',
-  'data': [ 'value1', 'value2' ] }
-{ 'struct': 'TestTypeA',
-  'data': { 'string': 'str' } }
-{ 'union': 'TestUnion',
-  'base': { 'type': 'TestEnum' },
-  'discriminator': 'type',
-  'data': { 'value1': 'TestTypeA' } }
diff --git a/tests/qapi-schema/qapi-schema-test.json b/tests/qapi-schema/qapi-schema-test.json
index 46c7282..881b53b 100644
--- a/tests/qapi-schema/qapi-schema-test.json
+++ b/tests/qapi-schema/qapi-schema-test.json
@@ -39,7 +39,7 @@ 
             '*enum1': 'EnumOne' } }   # intentional forward reference
 
 { 'enum': 'EnumOne',
-  'data': [ 'value1', 'value2', 'value3' ] }
+  'data': [ 'value1', 'value2', 'value3', 'value4' ] }
 
 { 'struct': 'UserDefZero',
   'data': { 'integer': 'int' } }
@@ -76,7 +76,7 @@ 
   'discriminator': 'enum1',
   'data': { 'value1' : 'UserDefA',
             'value2' : 'UserDefB',
-            'value3' : 'UserDefB' } }
+            'value3' : 'UserDefB' } } # skip 'value4'
 
 { 'struct': 'UserDefUnionBase',
   'base': 'UserDefZero',
diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
index 2462fc0..0f3d89b 100644
--- a/scripts/qapi/common.py
+++ b/scripts/qapi/common.py
@@ -779,13 +779,6 @@  def check_union(expr, info):
                                    "enum '%s'"
                                    % (key, enum_define['enum']))
 
-    # If discriminator is user-defined, ensure all values are covered
-    if enum_define:
-        for value in enum_define['data']:
-            if value not in members.keys():
-                raise QAPISemError(info, "Union '%s' data missing '%s' branch"
-                                   % (name, value))
-
 
 def check_alternate(expr, info):
     name = expr['alternate']
@@ -1644,6 +1637,10 @@  class QAPISchema(object):
         if tag_name:
             variants = [self._make_variant(key, value)
                         for (key, value) in data.items()]
+            # branches that are not explicitly covered get an empty type
+            variants += [self._make_variant(key, 'q_empty')
+                         for key in discriminator_find_enum_define(expr)['data']
+                         if key not in data.keys()]
             members = []
         else:
             variants = [self._make_simple_variant(key, value, info)
diff --git a/scripts/qapi/types.py b/scripts/qapi/types.py
index 64d9c0f..1fb2b6d 100644
--- a/scripts/qapi/types.py
+++ b/scripts/qapi/types.py
@@ -124,7 +124,9 @@  def gen_variants(variants):
 ''',
                 c_name=c_name(variants.tag_member.name))
 
-    for var in variants.variants:
+    # filter out the empty types
+    for var in filter(lambda var: var.type.name != 'q_empty',
+                      variants.variants):
         ret += mcgen('''
         %(c_type)s %(c_name)s;
 ''',
diff --git a/scripts/qapi/visit.py b/scripts/qapi/visit.py
index 5d72d89..96bd32c 100644
--- a/scripts/qapi/visit.py
+++ b/scripts/qapi/visit.py
@@ -81,14 +81,23 @@  void visit_type_%(c_name)s_members(Visitor *v, %(c_name)s *obj, Error **errp)
                      c_name=c_name(variants.tag_member.name))
 
         for var in variants.variants:
-            ret += mcgen('''
+            case_str = c_enum_const(variants.tag_member.type.name,
+                                    var.name,
+                                    variants.tag_member.type.prefix)
+            if var.type.name == 'q_empty':
+                # valid variant and nothing to do
+                ret += mcgen('''
+    case %(case)s:
+        break;
+''',
+                             case=case_str)
+            else:
+                ret += mcgen('''
     case %(case)s:
         visit_type_%(c_type)s_members(v, &obj->u.%(c_name)s, &err);
         break;
 ''',
-                         case=c_enum_const(variants.tag_member.type.name,
-                                           var.name,
-                                           variants.tag_member.type.prefix),
+                         case=case_str,
                          c_type=var.type.c_name(), c_name=c_name(var.name))
 
         ret += mcgen('''
diff --git a/tests/Makefile.include b/tests/Makefile.include
index bb08e37..afdd0db 100644
--- a/tests/Makefile.include
+++ b/tests/Makefile.include
@@ -496,7 +496,6 @@  qapi-schema += flat-union-base-any.json
 qapi-schema += flat-union-base-union.json
 qapi-schema += flat-union-clash-member.json
 qapi-schema += flat-union-empty.json
-qapi-schema += flat-union-incomplete-branch.json
 qapi-schema += flat-union-inline.json
 qapi-schema += flat-union-int-branch.json
 qapi-schema += flat-union-invalid-branch-key.json
diff --git a/tests/qapi-schema/flat-union-incomplete-branch.err b/tests/qapi-schema/flat-union-incomplete-branch.err
deleted file mode 100644
index e826bf0..0000000
--- a/tests/qapi-schema/flat-union-incomplete-branch.err
+++ /dev/null
@@ -1 +0,0 @@ 
-tests/qapi-schema/flat-union-incomplete-branch.json:6: Union 'TestUnion' data missing 'value2' branch
diff --git a/tests/qapi-schema/flat-union-incomplete-branch.exit b/tests/qapi-schema/flat-union-incomplete-branch.exit
deleted file mode 100644
index d00491f..0000000
--- a/tests/qapi-schema/flat-union-incomplete-branch.exit
+++ /dev/null
@@ -1 +0,0 @@ 
-1
diff --git a/tests/qapi-schema/flat-union-incomplete-branch.out b/tests/qapi-schema/flat-union-incomplete-branch.out
deleted file mode 100644
index e69de29..0000000
diff --git a/tests/qapi-schema/qapi-schema-test.out b/tests/qapi-schema/qapi-schema-test.out
index 542a19c..0dbcdaf 100644
--- a/tests/qapi-schema/qapi-schema-test.out
+++ b/tests/qapi-schema/qapi-schema-test.out
@@ -23,7 +23,7 @@  object UserDefOne
     base UserDefZero
     member string: str optional=False
     member enum1: EnumOne optional=True
-enum EnumOne ['value1', 'value2', 'value3']
+enum EnumOne ['value1', 'value2', 'value3', 'value4']
 object UserDefZero
     member integer: int optional=False
 object UserDefTwoDictDict
@@ -52,6 +52,7 @@  object UserDefFlatUnion
     case value1: UserDefA
     case value2: UserDefB
     case value3: UserDefB
+    case value4: q_empty
 object UserDefUnionBase
     base UserDefZero
     member string: str optional=False