bpo-40528: Implement a metadata system for ASDL Generator (GH-20193)

ASDL Generator was lack of proper annotation related to generated
module. This patch implements a MetadataVisitor that produces a
metadata object to pass to other visitors that are visiting that
same module. For the inital patch, it dynamically retrieves int
sequences (like cmpop), that was previously hardcoded. It offers
an interface that is easy to extend.

Parser/asdl_c.py

@@ -4,6 +4,7 @@
 import os
 import sys
 import textwrap
+import types
 
 from argparse import ArgumentParser
 from contextlib import contextmanager
@@ -100,11 +101,12 @@ def asdl_of(name, obj):
 class EmitVisitor(asdl.VisitorBase):
     """Visit that emits lines"""
 
-    def __init__(self, file):
+    def __init__(self, file, metadata = None):
         self.file = file
         self.identifiers = set()
         self.singletons = set()
         self.types = set()
+        self._metadata = metadata
         super(EmitVisitor, self).__init__()
 
     def emit_identifier(self, name):
@@ -127,6 +129,42 @@ class EmitVisitor(asdl.VisitorBase):
                 line = (" " * TABSIZE * depth) + line
             self.file.write(line + "\n")
 
+    @property
+    def metadata(self):
+        if self._metadata is None:
+            raise ValueError(
+                "%s was expecting to be annnotated with metadata"
+                % type(self).__name__
+            )
+        return self._metadata
+
+    @metadata.setter
+    def metadata(self, value):
+        self._metadata = value
+
+class MetadataVisitor(asdl.VisitorBase):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        # Metadata:
+        #    - simple_sums: Tracks the list of compound type
+        #                   names where all the constructors
+        #                   belonging to that type lack of any
+        #                   fields.
+        self.metadata = types.SimpleNamespace(
+            simple_sums=set()
+        )
+
+    def visitModule(self, mod):
+        for dfn in mod.dfns:
+            self.visit(dfn)
+
+    def visitType(self, type):
+        self.visit(type.value, type.name)
+
+    def visitSum(self, sum, name):
+        if is_simple(sum):
+            self.metadata.simple_sums.add(name)
 
 class TypeDefVisitor(EmitVisitor):
     def visitModule(self, mod):
@@ -244,7 +282,7 @@ class StructVisitor(EmitVisitor):
         ctype = get_c_type(field.type)
         name = field.name
         if field.seq:
-            if field.type == 'cmpop':
+            if field.type in self.metadata.simple_sums:
                 self.emit("asdl_int_seq *%(name)s;" % locals(), depth)
             else:
                 _type = field.type
@@ -304,7 +342,7 @@ class PrototypeVisitor(EmitVisitor):
                 name = f.name
             # XXX should extend get_c_type() to handle this
             if f.seq:
-                if f.type == 'cmpop':
+                if f.type in self.metadata.simple_sums:
                     ctype = "asdl_int_seq *"
                 else:
                     ctype = f"asdl_{f.type}_seq *"
@@ -549,16 +587,11 @@ class Obj2ModVisitor(PickleVisitor):
             ctype = get_c_type(field.type)
             self.emit("%s %s;" % (ctype, field.name), depth)
 
-    def isSimpleSum(self, field):
-        # XXX can the members of this list be determined automatically?
-        return field.type in ('expr_context', 'boolop', 'operator',
-                              'unaryop', 'cmpop')
-
     def isNumeric(self, field):
         return get_c_type(field.type) in ("int", "bool")
 
     def isSimpleType(self, field):
-        return self.isSimpleSum(field) or self.isNumeric(field)
+        return field.type in self.metadata.simple_sums or self.isNumeric(field)
 
     def visitField(self, field, name, sum=None, prod=None, depth=0):
         ctype = get_c_type(field.type)
@@ -1282,18 +1315,23 @@ class ObjVisitor(PickleVisitor):
 
     def set(self, field, value, depth):
         if field.seq:
-            # XXX should really check for is_simple, but that requires a symbol table
+            if field.type in self.metadata.simple_sums:
-            if field.type == "cmpop":
                 # While the sequence elements are stored as void*,
-                # ast2obj_cmpop expects an enum
+                # simple sums expects an enum
                 self.emit("{", depth)
                 self.emit("Py_ssize_t i, n = asdl_seq_LEN(%s);" % value, depth+1)
                 self.emit("value = PyList_New(n);", depth+1)
                 self.emit("if (!value) goto failed;", depth+1)
                 self.emit("for(i = 0; i < n; i++)", depth+1)
                 # This cannot fail, so no need for error handling
-                self.emit("PyList_SET_ITEM(value, i, ast2obj_cmpop(state, (cmpop_ty)asdl_seq_GET(%s, i)));" % value,
+                self.emit(
-                          depth+2, reflow=False)
+                    "PyList_SET_ITEM(value, i, ast2obj_{0}(state, ({0}_ty)asdl_seq_GET({1}, i)));".format(
+                        field.type,
+                        value
+                    ),
+                    depth + 2,
+                    reflow=False,
+                )
                 self.emit("}", depth)
             else:
                 self.emit("value = ast2obj_list(state, (asdl_seq*)%s, ast2obj_%s);" % (value, field.type), depth)
@@ -1362,11 +1400,13 @@ int PyAST_Check(PyObject* obj)
 """
 
 class ChainOfVisitors:
-    def __init__(self, *visitors):
+    def __init__(self, *visitors, metadata = None):
         self.visitors = visitors
+        self.metadata = metadata
 
     def visit(self, object):
         for v in self.visitors:
+            v.metadata = self.metadata
             v.visit(object)
             v.emit("", 0)
 
@@ -1468,7 +1508,7 @@ def generate_module_def(mod, f, internal_h):
     f.write('    return 1;\n')
     f.write('};\n\n')
 
-def write_header(mod, f):
+def write_header(mod, metadata, f):
     f.write(textwrap.dedent("""
         #ifndef Py_INTERNAL_AST_H
         #define Py_INTERNAL_AST_H
@@ -1483,12 +1523,19 @@ def write_header(mod, f):
         #include "pycore_asdl.h"
 
     """).lstrip())
-    c = ChainOfVisitors(TypeDefVisitor(f),
+
-                        SequenceDefVisitor(f),
+    c = ChainOfVisitors(
-                        StructVisitor(f))
+        TypeDefVisitor(f),
+        SequenceDefVisitor(f),
+        StructVisitor(f),
+        metadata=metadata
+    )
     c.visit(mod)
+
     f.write("// Note: these macros affect function definitions, not only call sites.\n")
-    PrototypeVisitor(f).visit(mod)
+    prototype_visitor = PrototypeVisitor(f, metadata=metadata)
+    prototype_visitor.visit(mod)
+
     f.write(textwrap.dedent("""
 
         PyObject* PyAST_mod2obj(mod_ty t);
@@ -1535,8 +1582,7 @@ def write_internal_h_footer(mod, f):
         #endif /* !Py_INTERNAL_AST_STATE_H */
     """), file=f)
 
-
+def write_source(mod, metadata, f, internal_h_file):
-def write_source(mod, f, internal_h_file):
     generate_module_def(mod, f, internal_h_file)
 
     v = ChainOfVisitors(
@@ -1549,6 +1595,7 @@ def write_source(mod, f, internal_h_file):
         Obj2ModVisitor(f),
         ASTModuleVisitor(f),
         PartingShots(f),
+        metadata=metadata
     )
     v.visit(mod)
 
@@ -1561,6 +1608,10 @@ def main(input_filename, c_filename, h_filename, internal_h_filename, dump_modul
     if not asdl.check(mod):
         sys.exit(1)
 
+    metadata_visitor = MetadataVisitor()
+    metadata_visitor.visit(mod)
+    metadata = metadata_visitor.metadata
+
     with c_filename.open("w") as c_file, \
          h_filename.open("w") as h_file, \
          internal_h_filename.open("w") as internal_h_file:
@@ -1569,8 +1620,8 @@ def main(input_filename, c_filename, h_filename, internal_h_filename, dump_modul
         internal_h_file.write(auto_gen_msg)
 
         write_internal_h_header(mod, internal_h_file)
-        write_source(mod, c_file, internal_h_file)
+        write_source(mod, metadata, c_file, internal_h_file)
-        write_header(mod, h_file)
+        write_header(mod, metadata, h_file)
         write_internal_h_footer(mod, internal_h_file)
 
     print(f"{c_filename}, {h_filename}, {internal_h_filename} regenerated.")