Avoid logging traceback when detach device not found

We use the oslo.utils save_and_reraise_exception context manager in our
detach device code and catch specific exceptions that mean 'not found'
and raise DeviceNotFound instead. When we do that, the
save_and_reraise_exception context manager logs an ERROR traceback of
the original exception, for informational purposes. This is misleading
when trying to debug other issues, as it makes it look like the caught
exception caused a problem.

This passes the reraise=False keyword arg to the context manager and
sets the 'reraise' attribute to True only if we are not going to raise
a different exception.

Related-Bug: #1836212

Change-Id: Icce1e31fe3ebcbf9e4897bbfa57b7f3d1fba67a3

Author: melwitt

nova/tests/unit/virt/libvirt/test_guest.py

@@ -327,6 +327,11 @@ def test_detach_device_with_retry_operation_failed(self, mock_detach):
             inc_sleep_time=.01, max_retry_count=3)
         # Some time later, we can do the wait/retry to ensure detach
         self.assertRaises(exception.DeviceNotFound, retry_detach)
+        # Check that the save_and_reraise_exception context manager didn't log
+        # a traceback when the libvirtError was caught and DeviceNotFound was
+        # raised.
+        self.assertNotIn('Original exception being dropped',
+                         self.stdlog.logger.output)
 
     @mock.patch.object(libvirt_guest.Guest, "detach_device")
     def test_detach_device_with_retry_operation_internal(self, mock_detach):

nova/virt/libvirt/guest.py

@@ -403,7 +403,7 @@ def _try_detach_device(conf, persistent=False, live=False):
                               device, persistent, live)
 
             except libvirt.libvirtError as ex:
-                with excutils.save_and_reraise_exception():
+                with excutils.save_and_reraise_exception(reraise=False) as ctx:
                     errcode = ex.get_error_code()
                     if errcode in (libvirt.VIR_ERR_OPERATION_FAILED,
                                    libvirt.VIR_ERR_INTERNAL_ERROR):
@@ -420,6 +420,10 @@ def _try_detach_device(conf, persistent=False, live=False):
                             # detach fails because the device is not found
                             raise exception.DeviceNotFound(
                                 device=alternative_device_name)
+                    # Re-raise the original exception if we're not raising
+                    # DeviceNotFound instead. This will avoid logging of a
+                    # "Original exception being dropped" traceback.
+                    ctx.reraise = True
 
         conf = get_device_conf_func(device)
         if conf is None: