Merge lp:~cjwatson/storm/py3-traceback-reference-cycles into lp:storm

[Colin Watson]

Avoid traceback reference cycles when wrapping exceptions.

`wrap_exceptions` stored the original exception's traceback in a local variable, but the traceback in turn contains a reference to that frame, creating a reference cycle. The situation is worse on Python 3, because the original exception also has the associated traceback as a `__traceback__` attribute, producing more complicated reference cycles. Delete these local variables before returning to avoid leaking memory over time.

[Ioana Lasc (ilasc)]

review: Approve

=== modified file 'NEWS'
--- NEWS	2021-04-19 09:52:52 +0000
+++ NEWS	2021-05-13 23:27:28 +0000
@@ -1,3 +1,11 @@
+0.26
+====
+
+Bug fixes
+---------
+
+- Avoid traceback reference cycles when wrapping exceptions.
+
 0.25 (2021-04-19)
 =================
 
 
=== modified file 'storm/exceptions.py'
--- storm/exceptions.py	2019-06-26 07:16:54 +0000
+++ storm/exceptions.py	2021-05-13 23:27:28 +0000
@@ -178,8 +178,12 @@
                 # As close to "raise wrapped.with_traceback(tb) from e" as
                 # we can manage, but without causing syntax errors on
                 # various versions of Python.
-                if six.PY2:
-                    six.reraise(wrapped, None, tb)
-                else:
-                    six.raise_from(wrapped.with_traceback(tb), e)
+                try:
+                    if six.PY2:
+                        six.reraise(wrapped, None, tb)
+                    else:
+                        six.raise_from(wrapped.with_traceback(tb), e)
+                finally:
+                    # Avoid traceback reference cycles.
+                    del wrapped, tb
         raise