3 files changed, 60 insertions, 2 deletions
diff --git a/lib/sqlalchemy/util/__init__.py b/lib/sqlalchemy/util/__init__.py
index 57bbdca85..3fa06c793 100644
--- a/lib/sqlalchemy/util/__init__.py
+++ b/lib/sqlalchemy/util/__init__.py
@@ -6,7 +6,8 @@
 
 from .compat import callable, cmp, reduce,  \
     threading, py3k, py3k_warning, jython, pypy, cpython, win32, set_types, \
-    pickle, dottedgetter, parse_qsl, namedtuple, next, WeakSet
+    pickle, dottedgetter, parse_qsl, namedtuple, next, WeakSet, reraise, \
+    raise_from_cause
 
 from ._collections import KeyedTuple, ImmutableContainer, immutabledict, \
     Properties, OrderedProperties, ImmutableProperties, OrderedDict, \
@@ -26,7 +27,7 @@ from .langhelpers import iterate_attributes, class_hierarchy, \
     duck_type_collection, assert_arg_type, symbol, dictlike_iteritems,\
     classproperty, set_creation_order, warn_exception, warn, NoneType,\
     constructor_copy, methods_equivalent, chop_traceback, asint,\
-    generic_repr, counter, PluginLoader, hybridmethod
+    generic_repr, counter, PluginLoader, hybridmethod, safe_reraise
 
 from .deprecations import warn_deprecated, warn_pending_deprecation, \
     deprecated, pending_deprecation
diff --git a/lib/sqlalchemy/util/compat.py b/lib/sqlalchemy/util/compat.py
index 2a0f06f8e..033a87cc7 100644
--- a/lib/sqlalchemy/util/compat.py
+++ b/lib/sqlalchemy/util/compat.py
@@ -140,3 +140,28 @@ else:
     def b(s):
         return s
 
+
+if py3k:
+    def reraise(tp, value, tb=None, cause=None):
+        if cause is not None:
+            value.__cause__ = cause
+        if value.__traceback__ is not tb:
+            raise value.with_traceback(tb)
+        raise value
+
+    def raise_from_cause(exception, exc_info):
+        exc_type, exc_value, exc_tb = exc_info
+        reraise(type(exception), exception, tb=exc_tb, cause=exc_value)
+else:
+    exec("def reraise(tp, value, tb=None, cause=None):\n"
+            "    raise tp, value, tb\n")
+
+    def raise_from_cause(exception, exc_info):
+        # not as nice as that of Py3K, but at least preserves
+        # the code line where the issue occurred
+        exc_type, exc_value, exc_tb = exc_info
+        reraise(type(exception), exception, tb=exc_tb)
+
+
+
+
diff --git a/lib/sqlalchemy/util/langhelpers.py b/lib/sqlalchemy/util/langhelpers.py
index e3aed24d8..f6d9164e6 100644
--- a/lib/sqlalchemy/util/langhelpers.py
+++ b/lib/sqlalchemy/util/langhelpers.py
@@ -20,6 +20,7 @@ from .compat import set_types, threading, \
 from functools import update_wrapper
 from .. import exc
 import hashlib
+from . import compat
 
 def md5_hex(x):
     # Py3K
@@ -28,6 +29,37 @@ def md5_hex(x):
     m.update(x)
     return m.hexdigest()
 
+class safe_reraise(object):
+    """Reraise an exception after invoking some
+    handler code.
+
+    Stores the existing exception info before
+    invoking so that it is maintained across a potential
+    coroutine context switch.
+
+    e.g.::
+
+        try:
+            sess.commit()
+        except:
+            with safe_reraise():
+                sess.rollback()
+
+    """
+
+    def __enter__(self):
+        self._exc_info = sys.exc_info()
+
+    def __exit__(self, type_, value, traceback):
+        # see #2703 for notes
+        if type_ is None:
+            exc_type, exc_value, exc_tb = self._exc_info
+            self._exc_info = None   # remove potential circular references
+            compat.reraise(exc_type, exc_value, exc_tb)
+        else:
+            self._exc_info = None   # remove potential circular references
+            compat.reraise(type_, value, traceback)
+
 def decode_slice(slc):
     """decode a slice object as sent to __getitem__.