qapi: Disentangle QAPIDoc code

Documentation comments follow a certain structure: First, we have a text
with a general description (called QAPIDoc.body). After this,
descriptions of the arguments follow. Finally, we have a part that
contains various named sections.

The code doesn't show this structure, but just checks various attributes
that indicate indirectly which part is being processed, so it happens to
do the right set of things in the right phase. This is hard to follow,
and adding support for documentation of features would be even harder.

This patch restructures the code so that the three parts are clearly
separated. The code becomes a bit longer, but easier to follow. The
resulting output remains unchanged.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20190606153803.5278-5-armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/common.py

@@ -120,6 +120,24 @@ def __init__(self, name):
         def connect(self, member):
             self.member = member
 
+    class DocPart:
+        """
+        Describes which part of the documentation we're parsing right now.
+
+        Expression documentation blocks consist of
+        * a BODY part: first line naming the expression, plus an
+          optional overview
+        * an ARGS part: description of each argument (for commands and
+          events) or member (for structs, unions and alternates),
+        * a VARIOUS part: optional tagged sections.
+
+        Free-form documentation blocks consist only of a BODY part.
+        """
+        # TODO Make it a subclass of Enum when Python 2 support is removed
+        BODY = 1
+        ARGS = 2
+        VARIOUS = 3
+
     def __init__(self, parser, info):
         # self._parser is used to report errors with QAPIParseError.  The
         # resulting error position depends on the state of the parser.
@@ -135,6 +153,7 @@ def __init__(self, parser, info):
         self.sections = []
         # the current section
         self._section = self.body
+        self._part = QAPIDoc.DocPart.BODY
 
     def has_section(self, name):
         """Return True if we have a section with this name."""
@@ -144,7 +163,27 @@ def has_section(self, name):
         return False
 
     def append(self, line):
-        """Parse a comment line and add it to the documentation."""
+        """
+        Parse a comment line and add it to the documentation.
+
+        The way that the line is dealt with depends on which part of
+        the documentation we're parsing right now:
+
+        BODY means that we're ready to process free-form text into
+        self.body.  A symbol name is only allowed if no other text was
+        parsed yet.  It is interpreted as the symbol name that
+        describes the currently documented object.  On getting the
+        second symbol name, we proceed to ARGS.
+
+        ARGS means that we're parsing the arguments section.  Any
+        symbol name is interpreted as an argument and an ArgSection is
+        created for it.
+
+        VARIOUS is the final part where free-form sections may appear.
+        This includes named sections such as "Return:" as well as
+        unnamed paragraphs.  Symbols are not allowed any more in this
+        part.
+        """
         line = line[1:]
         if not line:
             self._append_freeform(line)
@@ -154,37 +193,85 @@ def append(self, line):
             raise QAPIParseError(self._parser, "Missing space after #")
         line = line[1:]
 
+        if self._part == QAPIDoc.DocPart.BODY:
+            self._append_body_line(line)
+        elif self._part == QAPIDoc.DocPart.ARGS:
+            self._append_args_line(line)
+        elif self._part == QAPIDoc.DocPart.VARIOUS:
+            self._append_various_line(line)
+        else:
+            assert False
+
+    def end_comment(self):
+        self._end_section()
+
+    @staticmethod
+    def _is_section_tag(name):
+        return name in ('Returns:', 'Since:',
+                        # those are often singular or plural
+                        'Note:', 'Notes:',
+                        'Example:', 'Examples:',
+                        'TODO:')
+
+    def _append_body_line(self, line):
+        name = line.split(' ', 1)[0]
         # FIXME not nice: things like '#  @foo:' and '# @foo: ' aren't
         # recognized, and get silently treated as ordinary text
-        if self.symbol:
-            self._append_symbol_line(line)
-        elif not self.body.text and line.startswith('@'):
+        if not self.symbol and not self.body.text and line.startswith('@'):
             if not line.endswith(':'):
                 raise QAPIParseError(self._parser, "Line should end with :")
             self.symbol = line[1:-1]
             # FIXME invalid names other than the empty string aren't flagged
             if not self.symbol:
                 raise QAPIParseError(self._parser, "Invalid name")
+        elif self.symbol:
+            # We already know that we document some symbol
+            if name.startswith('@') and name.endswith(':'):
+                self._part = QAPIDoc.DocPart.ARGS
+                self._append_args_line(line)
+            elif self._is_section_tag(name):
+                self._part = QAPIDoc.DocPart.VARIOUS
+                self._append_various_line(line)
+            else:
+                self._append_freeform(line.strip())
         else:
-            self._append_freeform(line)
+            # This is free-form documentation without a symbol
+            self._append_freeform(line.strip())
 
-    def end_comment(self):
-        self._end_section()
-
-    def _append_symbol_line(self, line):
+    def _append_args_line(self, line):
         name = line.split(' ', 1)[0]
 
         if name.startswith('@') and name.endswith(':'):
             line = line[len(name)+1:]
             self._start_args_section(name[1:-1])
-        elif name in ('Returns:', 'Since:',
-                      # those are often singular or plural
-                      'Note:', 'Notes:',
-                      'Example:', 'Examples:',
-                      'TODO:'):
+        elif self._is_section_tag(name):
+            self._part = QAPIDoc.DocPart.VARIOUS
+            self._append_various_line(line)
+            return
+        elif (self._section.text.endswith('\n\n')
+              and line and not line[0].isspace()):
+            self._start_section()
+            self._part = QAPIDoc.DocPart.VARIOUS
+            self._append_various_line(line)
+            return
+
+        self._append_freeform(line.strip())
+
+    def _append_various_line(self, line):
+        name = line.split(' ', 1)[0]
+
+        if name.startswith('@') and name.endswith(':'):
+            raise QAPIParseError(self._parser,
+                                 "'%s' can't follow '%s' section"
+                                 % (name, self.sections[0].name))
+        elif self._is_section_tag(name):
             line = line[len(name)+1:]
             self._start_section(name[:-1])
 
+        if (not self._section.name or
+                not self._section.name.startswith('Example')):
+            line = line.strip()
+
         self._append_freeform(line)
 
     def _start_args_section(self, name):
@@ -194,10 +281,7 @@ def _start_args_section(self, name):
         if name in self.args:
             raise QAPIParseError(self._parser,
                                  "'%s' parameter name duplicated" % name)
-        if self.sections:
-            raise QAPIParseError(self._parser,
-                                 "'@%s:' can't follow '%s' section"
-                                 % (name, self.sections[0].name))
+        assert not self.sections
         self._end_section()
         self._section = QAPIDoc.ArgSection(name)
         self.args[name] = self._section
@@ -219,13 +303,6 @@ def _end_section(self):
             self._section = None
 
     def _append_freeform(self, line):
-        in_arg = isinstance(self._section, QAPIDoc.ArgSection)
-        if (in_arg and self._section.text.endswith('\n\n')
-                and line and not line[0].isspace()):
-            self._start_section()
-        if (in_arg or not self._section.name
-                or not self._section.name.startswith('Example')):
-            line = line.strip()
         match = re.match(r'(@\S+:)', line)
         if match:
             raise QAPIParseError(self._parser,