bpo-36820: Break unnecessary cycle in socket.py, codeop.py and dyld.py (GH-13135)

Break cycle generated when saving an exception in socket.py, codeop.py and dyld.py as they keep alive not only the exception but user objects through the ``__traceback__`` attribute. https://bugs.python.org/issue36820 Automerge-Triggered-By: @pablogsal (cherry picked from commit b64334c) Co-authored-by: Mario Corchero <mcorcherojim@bloomberg.net>

Author: miss-islington

Lib/codeop.py

@@ -93,10 +93,13 @@ def _maybe_compile(compiler, source, filename, symbol):
  except SyntaxError as e:
  err2 = e
 
- if code:
- return code
- if not code1 and repr(err1) == repr(err2):
- raise err1
+ try:
+ if code:
+ return code
+ if not code1 and repr(err1) == repr(err2):
+ raise err1
+ finally:
+ err1 = err2 = None
 
 def _compile(source, filename, symbol):
  return compile(source, filename, symbol, PyCF_DONT_IMPLY_DEDENT)

Lib/ctypes/macholib/dyld.py

@@ -149,6 +149,8 @@ def framework_find(fn, executable_path=None, env=None):
  return dyld_find(fn, executable_path=executable_path, env=env)
  except ValueError:
  raise error
+ finally:
+ error = None
 
 def test_dyld_find():
  env = {}

Lib/socket.py

@@ -724,7 +724,11 @@ def create_connection(address, timeout=_GLOBAL_DEFAULT_TIMEOUT,
  sock.close()
 
  if err is not None:
- raise err
+ try:
+ raise err
+ finally:
+ # Break explicitly a reference cycle
+ err = None
  else:
  raise error("getaddrinfo returns an empty list")
 

Misc/NEWS.d/next/Library/2019-05-06-15-34-17.bpo-36820.Eh5mIB.rst

@@ -0,0 +1,3 @@
+Break cycle generated when saving an exception in socket.py, codeop.py and
+dyld.py as they keep alive not only the exception but user objects through
+the ``__traceback__`` attribute. Patch by Mario Corchero.