Add a newline when writing the server state file

Before restarting haproxy, the amphora-agent dumps the state of the
servers in a file, so when haproxy reloads, it can recover the operating
state of its backends' members.
But with haproxy 2.4 (in Centos 9 Stream), it fails with a "corrupted
global server state file" error when reading this file. It seems that
most recent haproxy releases have a more strict validation of the format
of the file and ensure that it is terminated with a newline.

This commit adds a newline to the file (all the responses to haproxy
queries are stripped in the amphora-agent). It fixes the issue on Centos
9 Stream (and turns off a warning on Ubuntu).

Story: 2010442
Task: 46873

Change-Id: I00e327e1d94e46aa13a38120df9865ec34eaa593

octavia/amphorae/backends/utils/haproxy_query.py

@@ -160,7 +160,7 @@ class HAProxyQuery(object):
             result = self._query('show servers state')
             # No need for binary mode, the _query converts bytes to ascii.
             with open(state_file_path, 'w', encoding='utf-8') as fh:
-                fh.write(result)
+                fh.write(result + "\n")
             return True
         except Exception as e:
             # Catch any exception - may be socket issue, or write permission

octavia/tests/unit/amphorae/backends/utils/test_haproxy_query.py

@@ -158,7 +158,7 @@ class QueryTestCase(base.TestCase):
 
             self.q.save_state(filename)
 
-            mock_fh.write.assert_called_once_with('DATA')
+            mock_fh.write.assert_called_once_with('DATA\n')
 
     def test_save_state_error(self):
         """save_state() should swallow exceptions"""

releasenotes/notes/fix-corrupted-global-server-state-file-325ab7c62e21ff14.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed a "corrupted global server state file" error in Centos 9 Stream when
+    reloading the state of the servers after restarting haproxy.
+    It also fixed the recovering of the operational state of the servers in
+    haproxy after its restart.