upgrade: Run configuration upgrade under empty ccache collection

Use temporary empty DIR-based ccache collection to prevent upgrade
failures in case KCM: or KEYRING: ccache type is used by default in
krb5.conf and is not available. We don't need any user credentials
during upgrade procedure but kadmin.local would attempt to resolve
default ccache and if that's not available, kadmin.local will fail.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1558818
Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>

ipaserver/install/server/upgrade.py

@@ -11,6 +11,8 @@ import shutil
 import pwd
 import fileinput
 import sys
+import tempfile
+from contextlib import contextmanager
 from augeas import Augeas
 import dns.exception
 from ipalib import api, x509
@@ -1931,6 +1933,31 @@ def upgrade_check(options):
         logger.warning("Upgrade without version check may break your system")
 
 
+@contextmanager
+def empty_ccache():
+    # Create temporary directory and use it as a DIR: ccache collection
+    # instead of whatever is a default in /etc/krb5.conf
+    #
+    # In Fedora 28 KCM: became a default credentials cache collection
+    # but if KCM daemon (part of SSSD) is not running, libkrb5 will fail
+    # to initialize. This causes kadmin.local to fail.
+    # Since we are in upgrade, we cannot kinit anyway (KDC is offline).
+    # Bug https://bugzilla.redhat.com/show_bug.cgi?id=1558818
+    kpath_dir = tempfile.mkdtemp(prefix="upgrade_ccaches",
+                                 dir=paths.IPA_CCACHES)
+    kpath = "DIR:{}".format(kpath_dir)
+    old_path = os.environ.get('KRB5CCNAME')
+    try:
+        os.environ['KRB5CCNAME'] = kpath
+        yield
+    finally:
+        if old_path:
+            os.environ['KRB5CCNAME'] = old_path
+        else:
+            del os.environ['KRB5CCNAME']
+        shutil.rmtree(kpath_dir)
+
+
 def upgrade():
     realm = api.env.realm
     schema_files = [os.path.join(paths.USR_SHARE_IPA_DIR, f) for f
@@ -1955,7 +1982,8 @@ def upgrade():
 
     print('Upgrading IPA services')
     logger.info('Upgrading the configuration of the IPA services')
-    upgrade_configuration()
+    with empty_ccache():
+        upgrade_configuration()
     logger.info('The IPA services were upgraded')
 
     # store new data version after upgrade