Catch FileExistsError when creating /etc/corosync dir.

Hooks are expected to be idempotent, if the install hook for whatever
reason needs to be re-run and the /etc/corosync directory already exists,
because for example it was created in a previous run, the exception
FileExistsError will be raised, this change captures the exception and
moves on.

Change-Id: If43a5c95bb59c9cca7f1a975214a9f013ad6f4d6
Closes-Bug: #1971762
(cherry picked from commit 715d31e09f)

hooks/hooks.py

@@ -153,7 +153,10 @@ DEPRECATED_TRANSPORT_VALUES = {"multicast": "udp", "unicast": "udpu"}
 def install():
     # LP:1874719 Configure a corosync.conf file to avoid a spurious node1 to
     # be created in the cluster.
-    os.mkdir('/etc/corosync', mode=0o755)
+    try:
+        os.mkdir('/etc/corosync', mode=0o755)
+    except FileExistsError:
+        pass
     if emit_corosync_conf():
         log('Installed initial corosync.conf file', level=INFO)
     else:

unit_tests/test_hacluster_hooks.py

@@ -402,6 +402,15 @@ class TestHooks(test_utils.CharmTestCase):
         apt_install.assert_called_once_with(expected_pkgs, fatal=True)
         setup_ocf_files.assert_called_once_with()
 
+        mkdir.reset_mock()
+
+        def raise_():
+            raise FileExistsError()
+
+        mkdir.side_effect = lambda p, mode: raise_()
+        hooks.install()
+        mkdir.assert_called_once_with('/etc/corosync', mode=0o755)
+
     @mock.patch('pcmk.set_property')
     @mock.patch.object(hooks, 'is_stonith_configured')
     @mock.patch.object(hooks, 'configure_stonith')