qapi: Remove null from schema language

We represent the parse tree as OrderedDict.  We fetch optional dict
members with .get().  So far, so good.

We represent null literals as None.  .get() returns None both for
"absent" and for "present, value is the null literal".  Uh-oh.

Test features-if-invalid exposes this bug: "'if': null" is
misinterpreted as absent "if".

We added null to the schema language to "allow [...] an explicit
default value" (commit e53188ada5 "qapi: Allow true, false and null in
schema json", v2.4.0).  Hasn't happened; null is still unused except
as generic invalid value in tests/.

To fix, we'd have to replace .get() by something more careful, or
represent null differently.  Feasible, but we got more and bigger fish
to fry right now.  Remove the null literal from the schema language.
Replace null in tests by another invalid value.

Test features-if-invalid now behaves as it should.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-10-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

docs/devel/qapi-code-gen.txt

@@ -52,7 +52,7 @@ Differences:
 * Strings are restricted to printable ASCII, and escape sequences to
   just '\\'.
 
-* Numbers are not supported.
+* Numbers and null are not supported.
 
 A second layer of syntax defines the sequences of JSON texts that are
 a correctly structured QAPI schema.  We provide a grammar for this
@@ -67,7 +67,7 @@ syntax in an EBNF-like notation:
   expression A separated by ,
 * Grouping: expression ( A ) matches expression A
 * JSON's structural characters are terminals: { } [ ] : ,
-* JSON's literal names are terminals: false true null
+* JSON's literal names are terminals: false true
 * String literals enclosed in 'single quotes' are terminal, and match
   this JSON string, with a leading '*' stripped off
 * When JSON object member's name starts with '*', the member is

scripts/qapi/common.py

@@ -548,10 +548,6 @@ def accept(self, skip_comment=True):
                 self.val = False
                 self.cursor += 4
                 return
-            elif self.src.startswith('null', self.pos):
-                self.val = None
-                self.cursor += 3
-                return
             elif self.tok == '\n':
                 if self.cursor == len(self.src):
                     self.tok = None

tests/qapi-schema/features-if-invalid.err

@@ -0,0 +1 @@
+tests/qapi-schema/features-if-invalid.json:2: 'if' condition must be a string or a list of strings

tests/qapi-schema/features-if-invalid.exit

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

tests/qapi-schema/features-if-invalid.json

@@ -1,5 +1,4 @@
 # Cover feature with invalid 'if'
-# FIXME not rejected, misinterpreted as unconditional
 { 'struct': 'Stru',
   'data': {},
-  'features': [{'name': 'f', 'if': null }] }
+  'features': [{'name': 'f', 'if': false }] }

tests/qapi-schema/features-if-invalid.out

@@ -1,14 +0,0 @@
-module None
-object q_empty
-enum QType
-    prefix QTYPE
-    member none
-    member qnull
-    member qnum
-    member qstring
-    member qdict
-    member qlist
-    member qbool
-module features-if-invalid.json
-object Stru
-    feature f

tests/qapi-schema/pragma-name-case-whitelist-crap.json

@@ -1,3 +1,3 @@
 # 'name-case-whitelist' must be list of strings
 
-{ 'pragma': { 'name-case-whitelist': null } }
+{ 'pragma': { 'name-case-whitelist': false } }