write_files: add support for appending to files.

Add 'append: true' to write_files entries to append 'content' to file
specified by 'path' key.  This modifies the file open mode to append.

2 files changed, 18 insertions, 1 deletions

diff --git a/cloudinit/config/cc_write_files.py b/cloudinit/config/cc_write_files.py
index 31d1db61..0b6546e2 100644
--- a/cloudinit/config/cc_write_files.py
+++ b/cloudinit/config/cc_write_files.py
@@ -49,6 +49,10 @@ binary gzip data can be specified and will be decoded before being written.
             ...
           path: /bin/arch
           permissions: '0555'
+        - content: |
+            15 * * * * root ship_logs
+          path: /etc/crontab
+          append: true
 """
 
 import base64
@@ -113,7 +117,8 @@ def write_files(name, files):
         contents = extract_contents(f_info.get('content', ''), extractions)
         (u, g) = util.extract_usergroup(f_info.get('owner', DEFAULT_OWNER))
         perms = decode_perms(f_info.get('permissions'), DEFAULT_PERMS)
-        util.write_file(path, contents, mode=perms)
+        omode = 'ab' if util.get_cfg_option_bool(f_info, 'append') else 'wb'
+        util.write_file(path, contents, omode=omode, mode=perms)
         util.chownbyname(path, u, g)
 
 
diff --git a/tests/unittests/test_handler/test_handler_write_files.py b/tests/unittests/test_handler/test_handler_write_files.py
index 7fa8fd21..bc8756ca 100644
--- a/tests/unittests/test_handler/test_handler_write_files.py
+++ b/tests/unittests/test_handler/test_handler_write_files.py
@@ -52,6 +52,18 @@ class TestWriteFiles(FilesystemMockingTestCase):
             "test_simple", [{"content": expected, "path": filename}])
         self.assertEqual(util.load_file(filename), expected)
 
+    def test_append(self):
+        self.patchUtils(self.tmp)
+        existing = "hello "
+        added = "world\n"
+        expected = existing + added
+        filename = "/tmp/append.file"
+        util.write_file(filename, existing)
+        write_files(
+            "test_append",
+            [{"content": added, "path": filename, "append": "true"}])
+        self.assertEqual(util.load_file(filename), expected)
+
     def test_yaml_binary(self):
         self.patchUtils(self.tmp)
         data = util.load_yaml(YAML_TEXT)