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 b64334cb93)

Co-authored-by: Mario Corchero <mcorcherojim@bloomberg.net>

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

@@ -804,7 +804,11 @@ 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.