qapi: New documentation section tag "Errors"

We use section "Returns" for documenting both success and error
response of commands.

I intend to generate better command success response documentation.
Easier when "Returns" documents just he success response.

Create new section tag "Errors".  The next two commits will move error
response documentation from "Returns" sections to "Errors" sections.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240227113921.236097-4-armbru@redhat.com>

docs/devel/qapi-code-gen.rst

@@ -996,7 +996,8 @@ line "Features:", like this::
 
 A tagged section begins with a paragraph that starts with one of the
 following words: "Note:"/"Notes:", "Since:", "Example:"/"Examples:",
-"Returns:", "TODO:".  It ends with the start of a new section.
+"Returns:", "Errors:", "TODO:".  It ends with the start of a new
+section.
 
 The second and subsequent lines of tagged sections must be indented
 like this::
@@ -1007,6 +1008,9 @@ like this::
  #     Duis aute irure dolor in reprehenderit in voluptate velit esse
  #     cillum dolore eu fugiat nulla pariatur.
 
+"Returns" and "Errors" sections are only valid for commands.  They
+document the success and the error response, respectively.
+
 A "Since: x.y.z" tagged section lists the release that introduced the
 definition.
 

scripts/qapi/parser.py

@@ -543,7 +543,7 @@ class QAPISchemaParser:
                         line = self.get_doc_indented(doc)
                     no_more_args = True
                 elif match := re.match(
-                        r'(Returns|Since|Notes?|Examples?|TODO): *',
+                        r'(Returns|Errors|Since|Notes?|Examples?|TODO): *',
                         line):
                     # tagged section
                     doc.new_tagged_section(self.info, match.group(1))
@@ -639,8 +639,9 @@ class QAPIDoc:
         # dicts mapping parameter/feature names to their description
         self.args: Dict[str, QAPIDoc.ArgSection] = {}
         self.features: Dict[str, QAPIDoc.ArgSection] = {}
-        # a command's "Returns" section
+        # a command's "Returns" and "Errors" section
         self.returns: Optional[QAPIDoc.Section] = None
+        self.errors: Optional[QAPIDoc.Section] = None
         # "Since" section
         self.since: Optional[QAPIDoc.Section] = None
         # sections other than .body, .args, .features
@@ -670,6 +671,11 @@ class QAPIDoc:
                 raise QAPISemError(
                     info, "duplicated '%s' section" % tag)
             self.returns = section
+        elif tag == 'Errors':
+            if self.errors:
+                raise QAPISemError(
+                    info, "duplicated '%s' section" % tag)
+            self.errors = section
         elif tag == 'Since':
             if self.since:
                 raise QAPISemError(
@@ -715,10 +721,15 @@ class QAPIDoc:
         self.features[feature.name].connect(feature)
 
     def check_expr(self, expr: QAPIExpression) -> None:
-        if self.returns and 'command' not in expr:
+        if 'command' not in expr:
+            if self.returns:
                 raise QAPISemError(
                     self.returns.info,
                     "'Returns' section is only valid for commands")
+            if self.errors:
+                raise QAPISemError(
+                    self.returns.info,
+                    "'Errors' section is only valid for commands")
 
     def check(self) -> None:
 

tests/qapi-schema/doc-good.json

@@ -159,6 +159,8 @@
 #
 # Returns: @Object
 #
+# Errors: some
+#
 # TODO: frobnicate
 #
 # Notes:

tests/qapi-schema/doc-good.out

@@ -173,6 +173,8 @@ another feature
 @arg3 is undocumented
     section=Returns
 @Object
+    section=Errors
+some
     section=TODO
 frobnicate
     section=Notes

tests/qapi-schema/doc-good.txt

@@ -206,6 +206,12 @@ Returns
 "Object"
 
 
+Errors
+~~~~~~
+
+some
+
+
 Notes
 ~~~~~