Log original dropped exception when a new exception occurs

If a exception is caught while processing a previous exception, make sure
to log it so it doesn't silently get discarded

Change-Id: Ic887db9c2592229970737daf5dd9732b2258877b

4 files changed, 18 insertions, 10 deletions

diff --git a/nova/compute/manager.py b/nova/compute/manager.py
index f133ee133..054e682a5 100644
--- a/nova/compute/manager.py
+++ b/nova/compute/manager.py
@@ -470,7 +470,7 @@ class ComputeManager(manager.SchedulerDependentManager):
             # be fixed once we have no-db-messaging
             pass
         except:
-            with utils.original_exception_raised():
+            with utils.save_and_reraise_exception():
                 _deallocate_network()
 
     def _get_instance_volume_bdms(self, context, instance_id):
diff --git a/nova/exception.py b/nova/exception.py
index 998fece1e..872f532f9 100644
--- a/nova/exception.py
+++ b/nova/exception.py
@@ -107,6 +107,8 @@ def wrap_exception(notifier=None, publisher_id=None, event_type=None,
     # TODO(sandy): Find a way to import nova.notifier.api so we don't have
     # to pass it in as a parameter. Otherwise we get a cyclic import of
     # nova.notifier.api -> nova.utils -> nova.exception :(
+    # TODO(johannes): Also, it would be nice to use
+    # utils.save_and_reraise_exception() without an import loop
     def inner(f):
         def wrapped(*args, **kw):
             try:
diff --git a/nova/utils.py b/nova/utils.py
index ad585ba1c..baa47102c 100644
--- a/nova/utils.py
+++ b/nova/utils.py
@@ -979,21 +979,27 @@ def generate_glance_url():
 
 
 @contextlib.contextmanager
-def original_exception_raised():
-    """Run some code, then re-raise the original exception.
+def save_and_reraise_exception():
+    """Save current exception, run some code and then re-raise.
 
-    This is needed because when Eventlet switches greenthreads, it clears the
-    exception context. This means if exception handler code blocks, we'll lose
-    the helpful exception traceback information.
+    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 and 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.
+    then re-raise the original exception. If another exception occurs, the
+    saved exception is logged and the new exception is reraised.
     """
     type_, value, traceback = sys.exc_info()
     try:
         yield
-    finally:
-        raise type_, value, traceback
+    except:
+        LOG.exception(_('Original exception being dropped'),
+                      exc_info=(type_, value, traceback))
+        raise
+    raise type_, value, traceback
 
 
 def make_dev_path(dev, partition=None, base='/dev'):
diff --git a/nova/virt/xenapi/vm_utils.py b/nova/virt/xenapi/vm_utils.py
index 7142d0457..5b08a3c69 100644
--- a/nova/virt/xenapi/vm_utils.py
+++ b/nova/virt/xenapi/vm_utils.py
@@ -523,7 +523,7 @@ w
             cls.create_vbd(session, vm_ref, vdi_ref, userdevice,
                            bootable=False)
         except:
-            with utils.original_exception_raised():
+            with utils.save_and_reraise_exception():
                 cls.destroy_vdi(session, vdi_ref)
 
     @classmethod