Respect the preset permissions of config files

It's sometimes desirable to specify the permissions or ownership of the
generated config file instead of setting the default 644 root:root.

If the file is already present, os-apply-config will just update the
contents and leave everything else alone.

Closes-Bug: #1246266
Change-Id: Idd41ec276dd04f517760c1dbd97c75e376840b41

os_apply_config/apply_config.py

@@ -71,11 +71,17 @@ def print_key(
 
 def write_file(path, contents):
     logger.info("writing %s", path)
+    if os.path.exists(path):
+        stat = os.stat(path)
+        mode, uid, gid = stat.st_mode, stat.st_uid, stat.st_gid
+    else:
+        mode, uid, gid = 0o644, -1, -1
     d = os.path.dirname(path)
     os.path.exists(d) or os.makedirs(d)
     with tempfile.NamedTemporaryFile(dir=d, delete=False) as newfile:
         newfile.write(contents)
-        os.chmod(newfile.name, 0o644)
+        os.chmod(newfile.name, mode)
+        os.chown(newfile.name, uid, gid)
         os.rename(newfile.name, path)
 
 # return a map of filenames->filecontents

os_apply_config/tests/test_apply_config.py

@@ -176,6 +176,26 @@ def test_install_config_subhash(self):
             assert os.path.exists(full_path)
             self.assertEqual(open(full_path).read(), contents)
 
+    def test_respect_file_permissions(self):
+        fd, path = tempfile.mkstemp()
+        with os.fdopen(fd, 'w') as t:
+            t.write(json.dumps(CONFIG))
+            t.flush()
+        tmpdir = tempfile.mkdtemp()
+        template = "/etc/keystone/keystone.conf"
+        target_file = os.path.join(tmpdir, template[1:])
+        os.makedirs(os.path.dirname(target_file))
+        # File dosen't exist, use the default mode (644)
+        apply_config.install_config([path], TEMPLATES, tmpdir, False)
+        self.assertEqual(os.stat(target_file).st_mode, 0o100644)
+        self.assertEqual(open(target_file).read(), OUTPUT[template])
+        # Set a different mode:
+        os.chmod(target_file, 0o600)
+        apply_config.install_config([path], TEMPLATES, tmpdir, False)
+        # The permissions should be preserved
+        self.assertEqual(os.stat(target_file).st_mode, 0o100600)
+        self.assertEqual(open(target_file).read(), OUTPUT[template])
+
     def test_build_tree(self):
         self.assertEqual(apply_config.build_tree(
             apply_config.template_paths(TEMPLATES), CONFIG), OUTPUT)