From cb21fad4da3c90955a04671c112631c14657fd46 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rapha=C3=ABl=20Hertzog?= <hertzog@debian.org>
Date: Wed, 25 Nov 2015 12:54:52 +0100
Subject: Fix #25761 -- re-raised exceptions need __cause__.__traceback__

When Django re-raises an exception, it sets the __cause__ attribute even
in Python 2, thus mimicking Python's 3 behaviour for "raise Foo from Bar".
However Python 3 also ensures that all exceptions have a __traceback__
attribute and thus the "traceback2" Python 2 module (backport of Python
3's "traceback" module) relies on the fact that whenever you have a
__cause__ attribute, the recorded exception also has a __traceback__
attribute.

This is breaking at least testtools which is using traceback2 (see
https://github.com/testing-cabal/testtools/issues/162).

This commit fixes this inconsistency by ensuring that Django sets
the __traceback__ attribute on any exception stored in a __cause__
attribute of a re-raised exception.

Patch-Name: fix-25761-add-traceback-attribute.patch
---
 django/db/migrations/loader.py | 2 ++
 django/db/utils.py             | 2 ++
 django/utils/timezone.py       | 2 ++
 docs/ref/exceptions.txt        | 4 +++-
 4 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/django/db/migrations/loader.py b/django/db/migrations/loader.py
index bbd60a6..8939537 100644
--- a/django/db/migrations/loader.py
+++ b/django/db/migrations/loader.py
@@ -287,6 +287,8 @@ class MigrationLoader(object):
                         ),
                         missing)
                     exc_value.__cause__ = exc
+                    if not hasattr(exc, '__traceback__'):
+                        exc.__traceback__ = sys.exc_info()[2]
                     six.reraise(NodeNotFoundError, exc_value, sys.exc_info()[2])
             raise exc
 
diff --git a/django/db/utils.py b/django/db/utils.py
index ccfcfd4..24020dc 100644
--- a/django/db/utils.py
+++ b/django/db/utils.py
@@ -91,6 +91,8 @@ class DatabaseErrorWrapper(object):
             if issubclass(exc_type, db_exc_type):
                 dj_exc_value = dj_exc_type(*exc_value.args)
                 dj_exc_value.__cause__ = exc_value
+                if not hasattr(exc_value, '__traceback__'):
+                    exc_value.__traceback__ = traceback
                 # Only set the 'errors_occurred' flag for errors that may make
                 # the connection unusable.
                 if dj_exc_type not in (DataError, IntegrityError):
diff --git a/django/utils/timezone.py b/django/utils/timezone.py
index dbda90c..f64576d 100644
--- a/django/utils/timezone.py
+++ b/django/utils/timezone.py
@@ -146,6 +146,8 @@ class LocalTimezone(ReferenceLocalTimezone):
             exc_value = exc_type(
                 "Unsupported value: %r. You should install pytz." % dt)
             exc_value.__cause__ = exc
+            if not hasattr(exc, '__traceback__'):
+                exc.__traceback__ = sys.exc_info()[2]
             six.reraise(exc_type, exc_value, sys.exc_info()[2])
 
 utc = pytz.utc if pytz else UTC()
diff --git a/docs/ref/exceptions.txt b/docs/ref/exceptions.txt
index 46a0c81..a16d352 100644
--- a/docs/ref/exceptions.txt
+++ b/docs/ref/exceptions.txt
@@ -201,7 +201,9 @@ As per :pep:`3134`, a ``__cause__`` attribute is set with the original
 (underlying) database exception, allowing access to any additional
 information provided. (Note that this attribute is available under
 both Python 2 and Python 3, although :pep:`3134` normally only applies
-to Python 3.)
+to Python 3. With Python 2, Django will also record the traceback in the
+``__traceback__`` attribute of the exception made available via
+``__cause__``.)
 
 .. exception:: models.ProtectedError