qapi: Reject alternates that can't work with keyval_parse()

Alternates are sum types like unions, but use the JSON type on the
wire / QType in QObject instead of an explicit tag.  That's why we
require alternate members to have distinct QTypes.

The recently introduced keyval_parse() (commit d454dbe) can only
produce string scalars.  The qobject_input_visitor_new_keyval() input
visitor mostly hides the difference, so code using a QObject input
visitor doesn't have to care whether its input was parsed from JSON or
KEY=VALUE,...  The difference leaks for alternates, as noted in commit
0ee9ae7: a non-string, non-enum scalar alternate value can't currently
be expressed.

In part, this is just our insufficiently sophisticated implementation.
Consider alternate type 'GuestFileWhence'.  It has an integer member
and a 'QGASeek' member.  The latter is an enumeration with values
'set', 'cur', 'end'.  The meaning of b=set, b=cur, b=end, b=0, b=1 and
so forth is perfectly obvious.  However, our current implementation
falls apart at run time for b=0, b=1, and so forth.  Fixable, but not
today; add a test case and a TODO comment.

Now consider an alternate type with a string and an integer member.
What's the meaning of a=42?  Is it the string "42" or the integer 42?
Whichever meaning you pick makes the other inexpressible.  This isn't
just an implementation problem, it's fundamental.  Our current
implementation will pick string.

So far, we haven't needed such alternates.  To make sure we stop and
think before we add one that cannot sanely work with keyval_parse(),
let's require alternate members to have sufficiently distinct
representation in KEY=VALUE,... syntax:

* A string member clashes with any other scalar member

* An enumeration member clashes with bool members when it has value
  'on' or 'off'.

* An enumeration member clashes with numeric members when it has a
  value that starts with '-', '+', or a decimal digit.  This is a
  rather lazy approximation of the actual number syntax accepted by
  the visitor.

  Note that enumeration values starting with '-' and '+' are rejected
  elsewhere already, but better safe than sorry.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1495471335-23707-5-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

scripts/qapi.py

@@ -812,11 +812,26 @@ def check_alternate(expr, info):
         if not qtype:
             raise QAPISemError(info, "Alternate '%s' member '%s' cannot use "
                                "type '%s'" % (name, key, value))
-        if qtype in types_seen:
+        conflicting = set([qtype])
+        if qtype == 'QTYPE_QSTRING':
+            enum_expr = enum_types.get(value)
+            if enum_expr:
+                for v in enum_expr['data']:
+                    if v in ['on', 'off']:
+                        conflicting.add('QTYPE_QBOOL')
+                    if re.match(r'[-+0-9.]', v): # lazy, could be tightened
+                        conflicting.add('QTYPE_QINT')
+                        conflicting.add('QTYPE_QFLOAT')
+            else:
+                conflicting.add('QTYPE_QINT')
+                conflicting.add('QTYPE_QFLOAT')
+                conflicting.add('QTYPE_QBOOL')
+        if conflicting & set(types_seen):
             raise QAPISemError(info, "Alternate '%s' member '%s' can't "
                                "be distinguished from member '%s'"
                                % (name, key, types_seen[qtype]))
-        types_seen[qtype] = key
+        for qt in conflicting:
+            types_seen[qt] = key
 
 
 def check_enum(expr, info):

tests/Makefile.include

@@ -342,6 +342,8 @@ qapi-schema += alternate-array.json
 qapi-schema += alternate-base.json
 qapi-schema += alternate-clash.json
 qapi-schema += alternate-conflict-dict.json
+qapi-schema += alternate-conflict-enum-bool.json
+qapi-schema += alternate-conflict-enum-int.json
 qapi-schema += alternate-conflict-string.json
 qapi-schema += alternate-empty.json
 qapi-schema += alternate-nested.json

tests/qapi-schema/alternate-conflict-dict.json

@@ -1,4 +1,4 @@
-# we reject alternates with multiple object branches
+# alternate branches of object type conflict with each other
 { 'struct': 'One',
   'data': { 'name': 'str' } }
 { 'struct': 'Two',

tests/qapi-schema/alternate-conflict-enum-bool.err

@@ -0,0 +1 @@
+tests/qapi-schema/alternate-conflict-enum-bool.json:4: Alternate 'Alt' member 'two' can't be distinguished from member 'one'

tests/qapi-schema/alternate-conflict-enum-bool.exit

@@ -0,0 +1 @@
+1

tests/qapi-schema/alternate-conflict-enum-bool.json

@@ -0,0 +1,6 @@
+# alternate branch of 'enum' type that conflicts with bool
+{ 'enum': 'Enum',
+  'data': [ 'aus', 'off' ] }
+{ 'alternate': 'Alt',
+  'data': { 'one': 'Enum',
+            'two': 'bool' } }

tests/qapi-schema/alternate-conflict-enum-int.err

@@ -0,0 +1 @@
+tests/qapi-schema/alternate-conflict-enum-int.json:4: Alternate 'Alt' member 'two' can't be distinguished from member 'one'

tests/qapi-schema/alternate-conflict-enum-int.exit

@@ -0,0 +1 @@
+1

tests/qapi-schema/alternate-conflict-enum-int.json

@@ -0,0 +1,6 @@
+# alternate branches of 'enum' type that conflicts with numbers
+{ 'enum': 'Enum',
+  'data': [ '1', '2', '3' ] }
+{ 'alternate': 'Alt',
+  'data': { 'one': 'Enum',
+            'two': 'int' } }

tests/qapi-schema/alternate-conflict-string.err

@@ -1 +1 @@
-tests/qapi-schema/alternate-conflict-string.json:4: Alternate 'Alt' member 'two' can't be distinguished from member 'one'
+tests/qapi-schema/alternate-conflict-string.json:2: Alternate 'Alt' member 'two' can't be distinguished from member 'one'

tests/qapi-schema/alternate-conflict-string.json

@@ -1,6 +1,4 @@
-# we reject alternates with multiple string-like branches
-{ 'enum': 'Enum',
-  'data': [ 'hello', 'world' ] }
+# alternate branches of 'str' type conflict with all scalar types
 { 'alternate': 'Alt',
   'data': { 'one': 'str',
-            'two': 'Enum' } }
+            'two': 'int' } }

tests/qapi-schema/qapi-schema-test.json

@@ -101,12 +101,14 @@
 # for testing use of 'number' within alternates
 { 'alternate': 'AltEnumBool', 'data': { 'e': 'EnumOne', 'b': 'bool' } }
 { 'alternate': 'AltEnumNum', 'data': { 'e': 'EnumOne', 'n': 'number' } }
-{ 'alternate': 'AltNumStr', 'data': { 'n': 'number', 's': 'str' } }
 { 'alternate': 'AltNumEnum', 'data': { 'n': 'number', 'e': 'EnumOne' } }
 { 'alternate': 'AltEnumInt', 'data': { 'e': 'EnumOne', 'i': 'int' } }
 { 'alternate': 'AltIntNum', 'data': { 'i': 'int', 'n': 'number' } }
 { 'alternate': 'AltNumInt', 'data': { 'n': 'number', 'i': 'int' } }
 
+# for testing use of 'str' within alternates
+{ 'alternate': 'AltStrObj', 'data': { 's': 'str', 'o': 'TestStruct' } }
+
 # for testing native lists
 { 'union': 'UserDefNativeListUnion',
   'data': { 'integer': ['int'],

tests/qapi-schema/qapi-schema-test.out

@@ -22,10 +22,10 @@ alternate AltNumInt
     tag type
     case n: number
     case i: int
-alternate AltNumStr
+alternate AltStrObj
     tag type
-    case n: number
     case s: str
+    case o: TestStruct
 event EVENT_A None
    boxed=False
 event EVENT_B None

tests/test-keyval.c

@@ -614,22 +614,26 @@ static void test_keyval_visit_alternate(void)
     Error *err = NULL;
     Visitor *v;
     QDict *qdict;
-    AltNumStr *ans;
+    AltStrObj *aso;
     AltNumInt *ani;
+    AltEnumBool *aeb;
 
     /*
      * Can't do scalar alternate variants other than string.  You get
      * the string variant if there is one, else an error.
+     * TODO make it work for unambiguous cases like AltEnumBool below
      */
-    qdict = keyval_parse("a=1,b=2", NULL, &error_abort);
+    qdict = keyval_parse("a=1,b=2,c=on", NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     QDECREF(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
-    visit_type_AltNumStr(v, "a", &ans, &error_abort);
-    g_assert_cmpint(ans->type, ==, QTYPE_QSTRING);
-    g_assert_cmpstr(ans->u.s, ==, "1");
-    qapi_free_AltNumStr(ans);
-    visit_type_AltNumInt(v, "a", &ani, &err);
+    visit_type_AltStrObj(v, "a", &aso, &error_abort);
+    g_assert_cmpint(aso->type, ==, QTYPE_QSTRING);
+    g_assert_cmpstr(aso->u.s, ==, "1");
+    qapi_free_AltStrObj(aso);
+    visit_type_AltNumInt(v, "b", &ani, &err);
+    error_free_or_abort(&err);
+    visit_type_AltEnumBool(v, "c", &aeb, &err);
     error_free_or_abort(&err);
     visit_end_struct(v, NULL);
     visit_free(v);

util/keyval.c

@@ -65,11 +65,11 @@
  * denote numbers, true, false or null.  The special QObject input
  * visitor returned by qobject_input_visitor_new_keyval() mostly hides
  * this by automatically converting strings to the type the visitor
- * expects.  Breaks down for alternate types and type 'any', where the
- * visitor's expectation isn't clear.  Code visiting such types needs
- * to do the conversion itself, but only when using this keyval
- * visitor.  Awkward.  Alternate types without a string member don't
- * work at all.
+ * expects.  Breaks down for type 'any', where the visitor's
+ * expectation isn't clear.  Code visiting 'any' needs to do the
+ * conversion itself, but only when using this keyval visitor.
+ * Awkward.  Note that we carefully restrict alternate types to avoid
+ * similar ambiguity.
  *
  * Additional syntax for use with an implied key:
  *