raise exception ComputeHostNotFound if host is not found

When performing a live migration,I passed a wrong host parameter,
and then I got a wrong result like this:
"Compute service of host-1 is unavailable at this time".
I thought the compute service stopped, but the fact was I passed
a host id instead of a host name,therefore it could not find the host.

In order to distinguish the error "host not found" from the error
"service not available", I think we should raise a different exception
ComputeHostNotFound instead of ComputeServiceUnavailable.

This patch we will return more accurate error msg for live migration.

Closes-Bug: #1538837
Change-Id: I6ad377147070f85b9b1d5a1d1be459e890e02bcc

nova/conductor/manager.py

@@ -320,6 +320,7 @@ class ComputeTaskManager(base.Base):
         try:
             task.execute()
         except (exception.NoValidHost,
+                exception.ComputeHostNotFound,
                 exception.ComputeServiceUnavailable,
                 exception.InvalidHypervisorType,
                 exception.InvalidCPUInfo,

nova/conductor/tasks/live_migrate.py

@@ -77,16 +77,13 @@ class LiveMigrationTask(base.TaskBase):
         if self.instance.power_state not in (power_state.RUNNING,
                                              power_state.PAUSED):
             raise exception.InstanceInvalidState(
-                    instance_uuid = self.instance.uuid,
-                    attr = 'power_state',
-                    state = self.instance.power_state,
-                    method = 'live migrate')
+                    instance_uuid=self.instance.uuid,
+                    attr='power_state',
+                    state=self.instance.power_state,
+                    method='live migrate')
 
     def _check_host_is_up(self, host):
-        try:
-            service = objects.Service.get_by_compute_host(self.context, host)
-        except exception.NotFound:
-            raise exception.ComputeServiceUnavailable(host=host)
+        service = objects.Service.get_by_compute_host(self.context, host)
 
         if not self.servicegroup_api.service_is_up(service):
             raise exception.ComputeServiceUnavailable(host=host)

nova/exception.py

@@ -145,7 +145,7 @@ class NovaException(Exception):
                 # log the issue and the kwargs
                 LOG.exception(_LE('Exception in string format operation'))
                 for name, value in six.iteritems(kwargs):
-                    LOG.error("%s: %s" % (name, value))    # noqa
+                    LOG.error("%s: %s" % (name, value))  # noqa
 
                 if CONF.fatal_exception_format_errors:
                     six.reraise(*exc_info)

nova/tests/unit/conductor/tasks/test_live_migrate.py

@@ -138,10 +138,11 @@ class LiveMigrationTaskTestCase(test.NoDBTestCase):
         self.mox.StubOutWithMock(objects.Service, 'get_by_compute_host')
 
         objects.Service.get_by_compute_host(
-            self.context, "host").AndRaise(exception.NotFound)
+            self.context, "host").AndRaise(
+            exception.ComputeHostNotFound(host='host'))
 
         self.mox.ReplayAll()
-        self.assertRaises(exception.ComputeServiceUnavailable,
+        self.assertRaises(exception.ComputeHostNotFound,
             self.task._check_host_is_up, "host")
 
     def test_check_requested_destination(self):
@@ -187,10 +188,11 @@ class LiveMigrationTaskTestCase(test.NoDBTestCase):
         self.mox.StubOutWithMock(objects.Service, 'get_by_compute_host')
 
         objects.Service.get_by_compute_host(
-            self.context, self.destination).AndRaise(exception.NotFound)
+            self.context, self.destination).AndRaise(
+            exception.ComputeHostNotFound(host='host'))
 
         self.mox.ReplayAll()
-        self.assertRaises(exception.ComputeServiceUnavailable,
+        self.assertRaises(exception.ComputeHostNotFound,
                           self.task._check_requested_destination)
 
     def test_check_requested_destination_fails_with_not_enough_memory(self):