qapi: Add documentation format validation

Add explicit validation for QAPI documentation formatting rules:

1. Lines must not exceed 70 columns in width (including '# ' prefix)
2. Sentences must be separated by two spaces

Example sections and literal :: blocks (seldom case) are excluded, we
don't require them to be <= 70, that would be too restrictive. Anyway,
they share common 80-columns recommendations (not requirements).

Add two simple tests, illustrating the change.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20251031183129.246814-1-vsementsov@yandex-team.ru>

The detection of example and literal blocks isn't quite correct, but
it works well enough, and we can improve on top.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Comments, error messages, and test file names tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/parser.py

@@ -108,6 +108,11 @@ class QAPISchemaParser:
         self.exprs: List[QAPIExpression] = []
         self.docs: List[QAPIDoc] = []
 
+        # State for tracking qmp-example blocks and simple
+        # :: literal blocks.
+        self._literal_mode = False
+        self._literal_mode_indent = 0
+
         # Showtime!
         self._parse()
 
@@ -423,12 +428,55 @@ class QAPISchemaParser:
             if self.val != '##':
                 raise QAPIParseError(
                     self, "junk after '##' at end of documentation comment")
+            self._literal_mode = False
             return None
         if self.val == '#':
             return ''
         if self.val[1] != ' ':
             raise QAPIParseError(self, "missing space after #")
-        return self.val[2:].rstrip()
+
+        line = self.val[2:].rstrip()
+
+        if re.match(r'(\.\. +qmp-example)? *::$', line):
+            self._literal_mode = True
+            self._literal_mode_indent = 0
+        elif self._literal_mode and line:
+            indent = re.match(r'^ *', line).end()
+            if self._literal_mode_indent == 0:
+                self._literal_mode_indent = indent
+            elif indent < self._literal_mode_indent:
+                # ReST directives stop at decreasing indentation
+                self._literal_mode = False
+
+        if not self._literal_mode:
+            self._validate_doc_line_format(line)
+
+        return line
+
+    def _validate_doc_line_format(self, line: str) -> None:
+        """
+        Validate documentation format rules for a single line:
+        1. Lines should not exceed 70 characters
+        2. Sentences should be separated by two spaces
+        """
+        full_line_length = len(line) + 2  # "# " = 2 characters
+        if full_line_length > 70:
+            # Skip URL lines - they can't be broken
+            if re.match(r' *(https?|ftp)://[^ ]*$', line):
+                pass
+            else:
+                raise QAPIParseError(
+                    self, "documentation line longer than 70 characters")
+
+        single_space_pattern = r'(\be\.g\.|^ *\d\.|([.!?])) [A-Z0-9(]'
+        for m in list(re.finditer(single_space_pattern, line)):
+            if not m.group(2):
+                continue
+            # HACK so the error message points to the offending spot
+            self.pos = self.line_pos + 2 + m.start(2) + 1
+            raise QAPIParseError(
+                self, "Use two spaces between sentences\n"
+                "If this not the end of a sentence, please report a bug.")
 
     @staticmethod
     def _match_at_name_colon(string: str) -> Optional[Match[str]]:

tests/qapi-schema/doc-bad-space-between-sentences.err

@@ -0,0 +1,2 @@
+doc-bad-space-between-sentences.json:4:47: Use two spaces between sentences
+If this not the end of a sentence, please report a bug.

tests/qapi-schema/doc-bad-space-between-sentences.json

@@ -0,0 +1,6 @@
+##
+# @foo:
+#
+# Sentences should be separated by two spaces. But here is only one.
+##
+{ 'command': 'foo' }

tests/qapi-schema/doc-long-line.err

@@ -0,0 +1 @@
+doc-long-line.json:4:1: documentation line longer than 70 characters

tests/qapi-schema/doc-long-line.json

@@ -0,0 +1,6 @@
+##
+# @foo:
+#
+# This line has exactly 71 chars, including the leading hash and space.
+##
+{ 'command': 'foo' }

tests/qapi-schema/meson.build

@@ -61,6 +61,7 @@ schemas = [
   'doc-bad-event-arg.json',
   'doc-bad-feature.json',
   'doc-bad-indent.json',
+  'doc-bad-space-between-sentences.json',
   'doc-bad-symbol.json',
   'doc-bad-union-member.json',
   'doc-before-include.json',
@@ -81,6 +82,7 @@ schemas = [
   'doc-invalid-return2.json',
   'doc-invalid-section.json',
   'doc-invalid-start.json',
+  'doc-long-line.json',
   'doc-missing-colon.json',
   'doc-missing-expr.json',
   'doc-missing-space.json',