[PATCH v3 09/13] qapi/parser: add import cycle workaround

[John Snow]

There is a cycle that exists in the QAPI generator: [schema -> expr ->
parser -> schema]. It exists because the QAPIDoc class needs the names
of types defined by the schema module, but the schema module needs to
import both expr.py/parser.py to do its actual parsing.

Ultimately, the layering violation is that parser.py should not have any
knowledge of specifics of the Schema. QAPIDoc performs double-duty here
both as a parser *and* as a finalized object that is part of the schema.

I see three paths here:

(1) Just use the TYPE_CHECKING trick to eliminate the cycle which is only
    present during static analysis.

(2) Don't bother to annotate connect_member() et al, give them 'object'
    or 'Any'. I don't particularly like this, because it diminishes the
    usefulness of type hints for documentation purposes. Still, it's an
    extremely quick fix.

(3) Reimplement doc <--> definition correlation directly in schema.py,
    integrating doc fields directly into QAPISchemaMember and relieving
    the QAPIDoc class of the responsibility. Users of the information
    would instead visit the members first and retrieve their
    documentation instead of the inverse operation -- visiting the
    documentation and retrieving their members.

I prefer (3), but (1) is the easiest way to have my cake (strong type
hints) and eat it too (Not have import cycles). Do (1) for now, but plan
for (3). See also:
https://mypy.readthedocs.io/en/latest/runtime_troubles.html#import-cycles

Signed-off-by: John Snow <jsnow@redhat.com>
---
 scripts/qapi/parser.py | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/scripts/qapi/parser.py b/scripts/qapi/parser.py
index 123fc2f099c..30b1d98df0b 100644
--- a/scripts/qapi/parser.py
+++ b/scripts/qapi/parser.py
@@ -18,6 +18,7 @@
 import os
 import re
 from typing import (
+    TYPE_CHECKING,
     Dict,
     List,
     Optional,
@@ -30,6 +31,12 @@
 from .source import QAPISourceInfo
 
 
+if TYPE_CHECKING:
+    # pylint: disable=cyclic-import
+    # TODO: Remove cycle. [schema -> expr -> parser -> schema]
+    from .schema import QAPISchemaFeature, QAPISchemaMember
+
+
 # Return value alias for get_expr().
 _ExprValue = Union[List[object], Dict[str, object], str, bool]
 
@@ -473,9 +480,9 @@ def append(self, line):
     class ArgSection(Section):
         def __init__(self, parser, name, indent=0):
             super().__init__(parser, name, indent)
-            self.member = None
+            self.member: Optional['QAPISchemaMember'] = None
 
-        def connect(self, member):
+        def connect(self, member: 'QAPISchemaMember') -> None:
             self.member = member
 
     class NullSection(Section):
@@ -750,14 +757,14 @@ def _append_freeform(self, line):
                                  % match.group(1))
         self._section.append(line)
 
-    def connect_member(self, member):
+    def connect_member(self, member: 'QAPISchemaMember') -> None:
         if member.name not in self.args:
             # Undocumented TODO outlaw
             self.args[member.name] = QAPIDoc.ArgSection(self._parser,
                                                         member.name)
         self.args[member.name].connect(member)
 
-    def connect_feature(self, feature):
+    def connect_feature(self, feature: 'QAPISchemaFeature') -> None:
         if feature.name not in self.features:
             raise QAPISemError(feature.info,
                                "feature '%s' lacks documentation"
-- 
2.31.1