Fix certificate copying

The problem with default shutil.copytree() is that it doesn't "update"
existing directories.  So I was wrong in adf58f0, we can't
pre-create the dest-directory.  More, the certificate directory is being
updated over and over with subsequent calls, and we want to keep
updating the dest directory.  For this to work correctly, we need to use
'shutil.copytree(dirs_exist_ok=True)' which is though not in Python 3.6.

Therefore we try the 'shutil.copytree' first (works for Python standard
library 3.7+), and we fallback to the deprecated 'distutils' copy_tree()
method to stay compatible with Python 3.6.

Partly-reverts: adf58f0
Fixes: #1094

Author: praiskup

mock/py/mockbuild/file_util.py

@@ -3,10 +3,21 @@
 import errno
 import os
 import os.path
+import shutil
 import stat
 import subprocess
 import time
 
+# TODO: Remove with copy_or_update_tree() removal
+try:
+    deprecated_copy_tree = None
+    deprecated_copy_tree_error = None
+    # pylint: disable=deprecated-module
+    from distutils.dir_util import copy_tree as deprecated_copy_tree   # type: ignore
+    from distutils.errors import DistutilsFileError as deprecated_copy_tree_error  # type: ignore
+except ImportError:
+    pass
+
 from . import exception
 from .trace_decorator import getLog, traceLog
 
@@ -114,3 +125,20 @@ def find_non_nfs_dir():
         if not get_fs_type(d).startswith('nfs'):
             return d
     raise exception.Error('Cannot find non-NFS directory in: %s' % dirs)
+
+
+def copy_or_update_tree(src, dest):
+    """
+    Copy directory src into the dest directory.  This method won't be needed
+    when we stop supporting Python 3.6 (RHEL 8) where shutil.copytree doesn't
+    support the dirs_exist_ok= option.
+    """
+    try:
+        shutil.copytree(src, dest, dirs_exist_ok=True)
+    except TypeError:  # no dirs_exist_ok= support!
+        try:
+            # RHEL 8 only
+            deprecated_copy_tree(src, dest)
+        except deprecated_copy_tree_error as err:
+            # Caller(s) expect us to raise OSError.
+            raise OSError("Can't copy using distutils's copy_tree()") from err

mock/py/mockbuild/package_manager.py

@@ -341,9 +341,8 @@ def copy_certs(self):
         cert_paths = ["/etc/pki/ca-trust", "/usr/share/pki/ca-trust-source"]
         for cert_path in cert_paths:
             pki_dir = self.buildroot.make_chroot_path(cert_path)
-            file_util.mkdirIfAbsent(pki_dir)
             try:
-                shutil.copytree(cert_path, pki_dir)
+                file_util.copy_or_update_tree(cert_path, pki_dir)
             except OSError as err:
                 warning = "Couldn't copy %s certs from host: %s"
                 self.buildroot.root_log.debug(warning, cert_path, str(err))