cc_mounts: Fix swapfile not working on btrfs (#4319)

To make a swapfile work on btrfs, we need to create an empty file
and add the "no copy-on-write" attribute before making it a swapfile.

See https://btrfs.readthedocs.io/en/latest/Swapfile.html

Fixes GH-3713
LP: #1884127

cloudinit/config/cc_mounts.py

@@ -317,6 +317,10 @@ def create_swap(fname, size, method):
 
     fstype = util.get_mount_info(swap_dir)[1]
 
+    if fstype == "btrfs":
+        subp.subp(["truncate", "-s", "0", fname])
+        subp.subp(["chattr", "+C", fname])
+
     if (
         fstype == "xfs" and util.kernel_version() < (4, 18)
     ) or fstype == "btrfs":

tests/unittests/config/test_cc_mounts.py

@@ -304,6 +304,8 @@ def test_swap_creation_method_btrfs(
         cc_mounts.handle(None, self.cc, self.mock_cloud, [])
         self.m_subp_subp.assert_has_calls(
             [
+                mock.call(["truncate", "-s", "0", self.swap_path]),
+                mock.call(["chattr", "+C", self.swap_path]),
                 mock.call(
                     [
                         "dd",

tools/.github-cla-signers

@@ -18,6 +18,7 @@ bdrung
 beantaxi
 beezly
 berolinux
+bin456789
 bipinbachhao
 BirknerAlex
 bmhughes