Add validation for empty file system backup on Undercloud

This patch works as an addition to the exclude path
when executing an Undercloud backup.

In this case if we exclude all files, there is no need to execute
the task to create the FS backup.

Change-Id: Icd25a8d579151015c9cdf56861d2be933848a723
Closes-Bug: 1767385

tripleo_common/actions/undercloud.py

@@ -161,14 +161,18 @@ class CreateFileSystemBackup(base.Action):
         """ % (self.outfile, separated_string, self.outfile)
 
         proc_failed = False
-        try:
-            subprocess.check_call(script, shell=True)
-        except subprocess.CalledProcessError:
-            proc_failed = True
-            msg = 'File system backup failed'
-            os.remove(self.outfile)
+        if self.sources_path:
+            try:
+                subprocess.check_call(script, shell=True)
+            except subprocess.CalledProcessError:
+                proc_failed = True
+                msg = 'File system backup failed'
+                os.remove(self.outfile)
+            else:
+                msg = ('File system backup created succesfully at: %s'
+                       % self.outfile)
         else:
-            msg = 'File system backup created succesfully at: ' + self.outfile
+            msg = 'File system backup has no files to backup'
 
         if proc_failed:
             # Delete failed backup here

tripleo_common/tests/actions/test_undercloud.py

@@ -101,6 +101,9 @@ class CreateFileSystemBackupTest(base.TestCase):
         self.fsback = undercloud.CreateFileSystemBackup(
             '/home/stack/,/etc/hosts',
             '/var/tmp/undercloud-backup-ef9b_H')
+        self.fsemptyback = undercloud.CreateFileSystemBackup(
+            '',
+            '/var/tmp/undercloud-backup-ef9b_H')
 
     @mock.patch('tripleo_common.actions.base.TripleOAction.get_object_client')
     @mock.patch('subprocess.check_call')
@@ -120,6 +123,16 @@ class CreateFileSystemBackupTest(base.TestCase):
                          '\n        ')
         mock_check_call.assert_called_once_with(assert_string, shell=True)
 
+    @mock.patch('tripleo_common.actions.base.TripleOAction.get_object_client')
+    @mock.patch('subprocess.check_call')
+    def test_create_empty_file_system_backup(
+            self,
+            mock_check_call,
+            mock_get_object_client):
+        self.fsemptyback.logger = mock.Mock()
+        self.fsemptyback.run(mock_get_object_client)
+        mock_check_call.assert_not_called()
+
 
 class CreateBackupDirTest(base.TestCase):