cc_mounts: correctly fallback to dd if fallocate fails (#585)

`create_swap()` was previously catching and not re-raising the
ProcessExecutionError that indicated swap creation failure; this meant
that the fallback logic could never be triggered.

This commit adds the required re-raise (as well as removing a duplicated
log message).

LP: #1897099

2 files changed, 37 insertions, 4 deletions

diff --git a/cloudinit/config/cc_mounts.py b/cloudinit/config/cc_mounts.py
index 54f2f878..c22d1698 100644
--- a/cloudinit/config/cc_mounts.py
+++ b/cloudinit/config/cc_mounts.py
@@ -255,8 +255,9 @@ def create_swapfile(fname: str, size: str) -> None:
         try:
             subp.subp(cmd, capture=True)
         except subp.ProcessExecutionError as e:
-            LOG.warning(errmsg, fname, size, method, e)
+            LOG.info(errmsg, fname, size, method, e)
             util.del_file(fname)
+            raise
 
     swap_dir = os.path.dirname(fname)
     util.ensure_dir(swap_dir)
@@ -269,9 +270,8 @@ def create_swapfile(fname: str, size: str) -> None:
     else:
         try:
             create_swap(fname, size, "fallocate")
-        except subp.ProcessExecutionError as e:
-            LOG.warning(errmsg, fname, size, "dd", e)
-            LOG.warning("Will attempt with dd.")
+        except subp.ProcessExecutionError:
+            LOG.info("fallocate swap creation failed, will attempt with dd")
             create_swap(fname, size, "dd")
 
     if os.path.exists(fname):
diff --git a/cloudinit/config/tests/test_mounts.py b/cloudinit/config/tests/test_mounts.py
index 764a33e3..56510fd6 100644
--- a/cloudinit/config/tests/test_mounts.py
+++ b/cloudinit/config/tests/test_mounts.py
@@ -4,6 +4,7 @@ from unittest import mock
 import pytest
 
 from cloudinit.config.cc_mounts import create_swapfile
+from cloudinit.subp import ProcessExecutionError
 
 
 M_PATH = 'cloudinit.config.cc_mounts.'
@@ -26,3 +27,35 @@ class TestCreateSwapfile:
 
         create_swapfile(fname, '')
         assert mock.call(['mkswap', fname]) in m_subp.call_args_list
+
+    @mock.patch(M_PATH + "util.get_mount_info")
+    @mock.patch(M_PATH + "subp.subp")
+    def test_fallback_from_fallocate_to_dd(
+        self, m_subp, m_get_mount_info, caplog, tmpdir
+    ):
+        swap_file = tmpdir.join("swap-file")
+        fname = str(swap_file)
+
+        def subp_side_effect(cmd, *args, **kwargs):
+            # Mock fallocate failing, to initiate fallback
+            if cmd[0] == "fallocate":
+                raise ProcessExecutionError()
+
+        m_subp.side_effect = subp_side_effect
+        # Use ext4 so both fallocate and dd are valid swap creation methods
+        m_get_mount_info.return_value = (mock.ANY, "ext4")
+
+        create_swapfile(fname, "")
+
+        cmds = [args[0][0] for args, _kwargs in m_subp.call_args_list]
+        assert "fallocate" in cmds, "fallocate was not called"
+        assert "dd" in cmds, "fallocate failure did not fallback to dd"
+
+        assert cmds.index("dd") > cmds.index(
+            "fallocate"
+        ), "dd ran before fallocate"
+
+        assert mock.call(["mkswap", fname]) in m_subp.call_args_list
+
+        msg = "fallocate swap creation failed, will attempt with dd"
+        assert msg in caplog.text