Add save_and_reraise_exception().

This patch adds save_and_reraise_exception() to a new module,
openstack.common.excutils.  This is based on the function of the same
name from nova.utils.  It is used by nova.rpc, so something equivalent
is needed in openstack-common before nova.rpc can be moved here.

Change-Id: Icec574a081837e0f2d0548a5759d21b0352dbee6

openstack/common/excutils.py

@@ -0,0 +1,49 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2011 OpenStack LLC.
+# Copyright 2012, Red Hat, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+"""
+Exception related utilities.
+"""
+
+import contextlib
+import logging
+import sys
+import traceback
+
+
+@contextlib.contextmanager
+def save_and_reraise_exception():
+    """Save current exception, run some code and then re-raise.
+
+    In some cases the exception context can be cleared, resulting in None
+    being attempted to be reraised after an exception handler is run. This
+    can happen when eventlet switches greenthreads or when running an
+    exception handler, code raises and catches an exception. In both
+    cases the exception context will be cleared.
+
+    To work around this, we save the exception state, run handler code, and
+    then re-raise the original exception. If another exception occurs, the
+    saved exception is logged and the new exception is reraised.
+    """
+    type_, value, tb = sys.exc_info()
+    try:
+        yield
+    except Exception:
+        logging.error('Original exception being dropped: %s' %
+                (traceback.format_exception(type_, value, tb)))
+        raise
+    raise type_, value, tb

tests/unit/test_excutils.py

@@ -0,0 +1,50 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2012, Red Hat, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import unittest
+
+from openstack.common import excutils
+
+
+class SaveAndReraiseTest(unittest.TestCase):
+
+    def test_save_and_reraise_exception(self):
+        e = None
+        msg = 'foo'
+        try:
+            try:
+                raise Exception(msg)
+            except:
+                with excutils.save_and_reraise_exception():
+                    pass
+        except Exception, _e:
+            e = _e
+
+        self.assertEqual(str(e), msg)
+
+    def test_save_and_reraise_exception_dropped(self):
+        e = None
+        msg = 'second exception'
+        try:
+            try:
+                raise Exception('dropped')
+            except:
+                with excutils.save_and_reraise_exception():
+                    raise Exception(msg)
+        except Exception, _e:
+            e = _e
+
+        self.assertEqual(str(e), msg)