log meaningful error message on download exception

This changes:
    "IOError: [Errno 28] No space left on device"
to read (for example):
    "Error writing to
/var/lib/instances/_base/89e3812ae38b76b8f658e56a75f584de3bfb6941.part:
IOError: [Errno 28] No space left on device"

Co-Authored-By: Michael Still (mikal@stillhq.com)
Change-Id: I86305d100513b7f9c8cf50f5432524ec45460617
Closes-Bug: 1431631

nova/image/glance.py

@@ -28,6 +28,7 @@ import glanceclient.exc
 from oslo_config import cfg
 from oslo_log import log as logging
 from oslo_serialization import jsonutils
+from oslo_utils import excutils
 from oslo_utils import netutils
 from oslo_utils import timeutils
 import six
@@ -364,6 +365,10 @@ class GlanceImageService(object):
             try:
                 for chunk in image_chunks:
                     data.write(chunk)
+            except Exception as ex:
+                with excutils.save_and_reraise_exception():
+                    LOG.error(_LE("Error writing to %(path)s: %(exception)s"),
+                              {'path': dst_path, 'exception': ex})
             finally:
                 if close_file:
                     data.close()

nova/tests/unit/image/test_glance.py

@@ -15,6 +15,7 @@
 
 
 import datetime
+import StringIO
 
 import glanceclient.exc
 import mock
@@ -500,6 +501,26 @@ class TestDownloadNoDirectUri(test.NoDBTestCase):
         )
         self.assertFalse(data.close.called)
 
+    @mock.patch('__builtin__.open')
+    @mock.patch('nova.image.glance.GlanceImageService.show')
+    def test_download_data_dest_path_write_fails(self, show_mock, open_mock):
+        client = mock.MagicMock()
+        client.call.return_value = [1, 2, 3]
+        ctx = mock.sentinel.ctx
+        service = glance.GlanceImageService(client)
+
+        # NOTE(mikal): data is a file like object, which in our case always
+        # raises an exception when we attempt to write to the file.
+        class FakeDiskException(Exception):
+            pass
+
+        class Exceptionator(StringIO.StringIO):
+            def write(self, _):
+                raise FakeDiskException('Disk full!')
+
+        self.assertRaises(FakeDiskException, service.download, ctx,
+                          mock.sentinel.image_id, data=Exceptionator())
+
     @mock.patch('nova.image.glance.GlanceImageService._get_transfer_module')
     @mock.patch('nova.image.glance.GlanceImageService.show')
     def test_download_direct_file_uri(self, show_mock, get_tran_mock):