qapi: Reject multiple and empty feature descriptions

The parser recognizes only the first "Features:" line.  Any subsequent
ones are treated as ordinary text, as visible in test case
doc-duplicate-features.  Recognize "Features:" lines anywhere.  A
second one is an error.

A 'Features:' line without any features is useless, but not an error.
Make it an error.  This makes detecting a second "Features:" line
easier.

qapi/run-state.json actually has an instance of this since commit
fe17522d854 (qapi: Remove deprecated 'singlestep' member of
StatusInfo).  Clean it up.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240216145841.2099240-16-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

qapi/run-state.json

@@ -108,8 +108,6 @@
 #
 # @status: the virtual machine @RunState
 #
-# Features:
-#
 # Since: 0.14
 #
 ##

scripts/qapi/parser.py

@@ -507,7 +507,10 @@ def get_doc(self) -> 'QAPIDoc':
                 if line is None:
                     break
                 # Non-blank line, first of a section
-                if line == 'Features:' and not doc.features:
+                if line == 'Features:':
+                    if doc.features:
+                        raise QAPIParseError(
+                            self, "duplicated 'Features:' line")
                     self.accept(False)
                     line = self.get_doc_line()
                     while line == '':
@@ -520,6 +523,9 @@ def get_doc(self) -> 'QAPIDoc':
                         if text:
                             doc.append_line(text)
                         line = self.get_doc_indented(doc)
+                    if not doc.features:
+                        raise QAPIParseError(
+                            self, 'feature descriptions expected')
                     no_more_args = True
                 elif match := self._match_at_name_colon(line):
                     # description

tests/qapi-schema/doc-duplicate-features.err

@@ -0,0 +1 @@
+doc-duplicate-features.json:9:1: duplicated 'Features:' line

tests/qapi-schema/doc-duplicate-features.out

@@ -1,22 +0,0 @@
-module ./builtin
-object q_empty
-enum QType
-    prefix QTYPE
-    member none
-    member qnull
-    member qnum
-    member qstring
-    member qdict
-    member qlist
-    member qbool
-module doc-duplicate-features.json
-command foo None -> None
-    gen=True success_response=True boxed=False oob=False preconfig=False
-    feature feat
-doc symbol=foo
-    body=
-
-    feature=feat
-mumble
-    section=None
-Features:

tests/qapi-schema/doc-empty-features.err

@@ -0,0 +1 @@
+doc-empty-features.json:8:1: feature descriptions expected

tests/qapi-schema/doc-empty-features.out

@@ -1,17 +0,0 @@
-module ./builtin
-object q_empty
-enum QType
-    prefix QTYPE
-    member none
-    member qnull
-    member qnum
-    member qstring
-    member qdict
-    member qlist
-    member qbool
-module doc-empty-features.json
-command foo None -> None
-    gen=True success_response=True boxed=False oob=False preconfig=False
-doc symbol=foo
-    body=
-not a description