util: add ensure_dir_exists parameter to write_file (#443)

This allows us to disable the `ensure_dir` call when it isn't
appropriate.

cloudinit/util.py

@@ -1800,7 +1800,15 @@ def chmod(path, mode):
             os.chmod(path, real_mode)
 
 
-def write_file(filename, content, mode=0o644, omode="wb", preserve_mode=False):
+def write_file(
+    filename,
+    content,
+    mode=0o644,
+    omode="wb",
+    preserve_mode=False,
+    *,
+    ensure_dir_exists=True
+):
     """
     Writes a file with the given content and sets the file mode as specified.
     Restores the SELinux context if possible.
@@ -1811,6 +1819,9 @@ def write_file(filename, content, mode=0o644, omode="wb", preserve_mode=False):
     @param omode: The open mode used when opening the file (w, wb, a, etc.)
     @param preserve_mode: If True and `filename` exists, preserve `filename`s
                           current mode instead of applying `mode`.
+    @param ensure_dir_exists: If True (the default), ensure that the directory
+                              containing `filename` exists before writing to
+                              the file.
     """
 
     if preserve_mode:
@@ -1820,7 +1831,8 @@ def write_file(filename, content, mode=0o644, omode="wb", preserve_mode=False):
         except OSError:
             pass
 
-    ensure_dir(os.path.dirname(filename))
+    if ensure_dir_exists:
+        ensure_dir(os.path.dirname(filename))
     if 'b' in omode.lower():
         content = encode_text(content)
         write_type = 'bytes'

tests/unittests/test_util.py

@@ -98,6 +98,17 @@ def test_dir_is_created_if_required(self):
         self.assertTrue(os.path.isdir(dirname))
         self.assertTrue(os.path.isfile(path))
 
+    def test_dir_is_not_created_if_ensure_dir_false(self):
+        """Verify directories are not created if ensure_dir_exists is False."""
+        dirname = os.path.join(self.tmp, "subdir")
+        path = os.path.join(dirname, "NewFile.txt")
+        contents = "Hey there"
+
+        with self.assertRaises(FileNotFoundError):
+            util.write_file(path, contents, ensure_dir_exists=False)
+
+        self.assertFalse(os.path.isdir(dirname))
+
     def test_explicit_mode(self):
         """Verify explicit file mode works properly."""
         path = os.path.join(self.tmp, "NewFile.txt")