bpo-33947: dataclasses no longer can raise RecursionError in repr (GF9916)

The reprlib code was copied here instead of importing reprlib. I'm not sure if we really need to avoid the import, but since I expect dataclasses to be more common that reprlib, it seems wise. Plus, the code is small.

Lib/dataclasses.py

@@ -5,6 +5,9 @@ import types
 import inspect
 import keyword
 import builtins
+import functools
+import _thread
+
 
 __all__ = ['dataclass',
            'field',
@@ -337,6 +340,27 @@ def _tuple_str(obj_name, fields):
     return f'({",".join([f"{obj_name}.{f.name}" for f in fields])},)'
 
 
+# This function's logic is copied from "recursive_repr" function in
+# reprlib module to avoid dependency.
+def _recursive_repr(user_function):
+    # Decorator to make a repr function return "..." for a recursive
+    # call.
+    repr_running = set()
+
+    @functools.wraps(user_function)
+    def wrapper(self):
+        key = id(self), _thread.get_ident()
+        if key in repr_running:
+            return '...'
+        repr_running.add(key)
+        try:
+            result = user_function(self)
+        finally:
+            repr_running.discard(key)
+        return result
+    return wrapper
+
+
 def _create_fn(name, args, body, *, globals=None, locals=None,
                return_type=MISSING):
     # Note that we mutate locals when exec() is called.  Caller
@@ -497,12 +521,13 @@ def _init_fn(fields, frozen, has_post_init, self_name):
 
 
 def _repr_fn(fields):
-    return _create_fn('__repr__',
-                      ('self',),
-                      ['return self.__class__.__qualname__ + f"(' +
-                       ', '.join([f"{f.name}={{self.{f.name}!r}}"
-                                  for f in fields]) +
-                       ')"'])
+    fn = _create_fn('__repr__',
+                    ('self',),
+                    ['return self.__class__.__qualname__ + f"(' +
+                     ', '.join([f"{f.name}={{self.{f.name}!r}}"
+                                for f in fields]) +
+                     ')"'])
+    return _recursive_repr(fn)
 
 
 def _frozen_get_del_attr(cls, fields):