[Functional] Add logging to the check test file function

In the functional tests module which is testing WSGI Server, there
is used function to assert that file created by the WSGI service is
created and has got correct size.
This function wasn't logging anything so in case of test failure
there was no way to check why it really failed.

This patch adds some log messages to the check so it will be clear
if file wasn't created at all or if size was not as expected.

Change-Id: Ib610b8513b4ea888a540873b26c3f205ac575b17
Related-Bug: #1886956

neutron/tests/functional/test_server.py

@@ -23,6 +23,7 @@ from unittest import mock
 import httplib2
 from neutron_lib import worker as neutron_worker
 from oslo_config import cfg
+from oslo_log import log
 import psutil
 
 from neutron.common import utils
@@ -32,6 +33,8 @@ from neutron.tests.functional import base
 from neutron import wsgi
 
 
+LOG = log.getLogger(__name__)
+
 CONF = cfg.CONF
 
 # Those messages will be written to temporary file each time
@@ -155,12 +158,19 @@ class TestNeutronServer(base.BaseLoggingTestCase):
         # Wait for temp file to be created and its size reaching the expected
         # value
         expected_size = len(expected_msg)
-        condition = lambda: (os.path.isfile(self.temp_file) and
-                             os.stat(self.temp_file).st_size ==
-                             expected_size)
+
+        def is_temp_file_ok():
+            LOG.debug("Checking file %s", self.temp_file)
+            if not os.path.isfile(self.temp_file):
+                LOG.debug("File %s not exists.", self.temp_file)
+                return False
+            temp_file_size = os.stat(self.temp_file).st_size
+            LOG.debug("Size of file %s is %s. Expected size: %s",
+                      self.temp_file, temp_file_size, expected_size)
+            return temp_file_size == expected_size
 
         try:
-            utils.wait_until_true(condition, timeout=5, sleep=1)
+            utils.wait_until_true(is_temp_file_ok, timeout=5, sleep=1)
         except utils.TimerTimeout:
             if not os.path.isfile(self.temp_file):
                 raise RuntimeError(