Add cross-cell resize tests for _poll_unconfirmed_resizes

This adds two tests and updates the cross-cell resize docs to
show that _poll_unconfirmed_resizes can work if the cells are
able to "up-call" to the API DB to confirm the resize. Since
lots of deployments still enable up-calls we don't explicitly
block _poll_unconfirmed_resizes from processing cross-cell
migrations. The other test shows that _poll_unconfirmed_resizes
fails if up-calls are disabled.

Part of blueprint cross-cell-resize

Change-Id: I39e8159f3e734a1219e1a44434d6360572620424

1 files changed, 5 insertions, 0 deletions

diff --git a/doc/source/admin/configuration/cross-cell-resize.rst b/doc/source/admin/configuration/cross-cell-resize.rst
index 5b30c4f47c..b768d7bc8b 100644
--- a/doc/source/admin/configuration/cross-cell-resize.rst
+++ b/doc/source/admin/configuration/cross-cell-resize.rst
@@ -251,6 +251,11 @@ Other limitations:
   source and destination compute host are not on shared storage or when
   shelve offloading and unshelving a server with a config drive. If necessary,
   the resized server can be rebuilt to regain the personality files.
+* The ``_poll_unconfirmed_resizes`` periodic task, which can be
+  :oslo.config:option:`configured <resize_confirm_window>` to automatically
+  confirm pending resizes on the target host, *might* not support cross-cell
+  resizes because doing so would require an :ref:`up-call <upcall>` to the
+  API to confirm the resize and cleanup the source cell database.
 
 .. _personality files: https://docs.openstack.org/api-guide/compute/server_concepts.html#server-personality
 .. _evacuating: https://docs.openstack.org/api-ref/compute/#evacuate-server-evacuate-action