udevadm settle should handle non-udev system gracefully (#1806)

Not all Linux systems use udev (and therefore udevadm). Specifically
an Alpine Linux system may have either udev (via "eudev" package)
or mdev, or mdevd installed.

Change the udev_settle function to check for the presence of the
udevadm binary before calling it - if it is not present then
silently exit the function.

This change will enable cloud-init to run on Alpine systems using
mdev/mdevd rather than udev.

2 files changed, 29 insertions, 10 deletions

diff --git a/cloudinit/util.py b/cloudinit/util.py
index 1fd2c81b..078ce1c2 100644
--- a/cloudinit/util.py
+++ b/cloudinit/util.py
@@ -2934,6 +2934,10 @@ def mount_is_read_write(mount_point):
 
 def udevadm_settle(exists=None, timeout=None):
     """Invoke udevadm settle with optional exists and timeout parameters"""
+    if not subp.which("udevadm"):
+        # a distro, such as Alpine, may not have udev installed if
+        # it relies on a udev alternative such as mdev/mdevd.
+        return
     settle_cmd = ["udevadm", "settle"]
     if exists:
         # skip the settle if the requested path already exists
diff --git a/tests/unittests/test_util.py b/tests/unittests/test_util.py
index 6dc827c1..0c2735ae 100644
--- a/tests/unittests/test_util.py
+++ b/tests/unittests/test_util.py
@@ -822,46 +822,60 @@ class TestBlkid(CiTestCase):
         )
 
 
+@mock.patch("cloudinit.subp.which")
 @mock.patch("cloudinit.subp.subp")
 class TestUdevadmSettle(CiTestCase):
-    def test_with_no_params(self, m_subp):
+    def test_with_no_params(self, m_which, m_subp):
         """called with no parameters."""
+        m_which.side_effect = lambda m: m in ("udevadm",)
         util.udevadm_settle()
         m_subp.called_once_with(mock.call(["udevadm", "settle"]))
 
-    def test_with_exists_and_not_exists(self, m_subp):
+    def test_udevadm_not_present(self, m_which, m_subp):
+        """where udevadm program does not exist should not invoke subp."""
+        m_which.side_effect = lambda m: m in ("",)
+        util.udevadm_settle()
+        m_subp.called_once_with(["which", "udevadm"])
+
+    def test_with_exists_and_not_exists(self, m_which, m_subp):
         """with exists=file where file does not exist should invoke subp."""
+        m_which.side_effect = lambda m: m in ("udevadm",)
         mydev = self.tmp_path("mydev")
         util.udevadm_settle(exists=mydev)
         m_subp.called_once_with(
             ["udevadm", "settle", "--exit-if-exists=%s" % mydev]
         )
 
-    def test_with_exists_and_file_exists(self, m_subp):
-        """with exists=file where file does exist should not invoke subp."""
+    def test_with_exists_and_file_exists(self, m_which, m_subp):
+        """with exists=file where file does exist should only invoke subp
+        once for 'which' call."""
+        m_which.side_effect = lambda m: m in ("udevadm",)
         mydev = self.tmp_path("mydev")
         util.write_file(mydev, "foo\n")
         util.udevadm_settle(exists=mydev)
-        self.assertIsNone(m_subp.call_args)
+        m_subp.called_once_with(["which", "udevadm"])
 
-    def test_with_timeout_int(self, m_subp):
+    def test_with_timeout_int(self, m_which, m_subp):
         """timeout can be an integer."""
+        m_which.side_effect = lambda m: m in ("udevadm",)
         timeout = 9
         util.udevadm_settle(timeout=timeout)
         m_subp.called_once_with(
             ["udevadm", "settle", "--timeout=%s" % timeout]
         )
 
-    def test_with_timeout_string(self, m_subp):
+    def test_with_timeout_string(self, m_which, m_subp):
         """timeout can be a string."""
+        m_which.side_effect = lambda m: m in ("udevadm",)
         timeout = "555"
         util.udevadm_settle(timeout=timeout)
-        m_subp.assert_called_once_with(
+        m_subp.called_once_with(
             ["udevadm", "settle", "--timeout=%s" % timeout]
         )
 
-    def test_with_exists_and_timeout(self, m_subp):
+    def test_with_exists_and_timeout(self, m_which, m_subp):
         """test call with both exists and timeout."""
+        m_which.side_effect = lambda m: m in ("udevadm",)
         mydev = self.tmp_path("mydev")
         timeout = "3"
         util.udevadm_settle(exists=mydev)
@@ -874,7 +888,8 @@ class TestUdevadmSettle(CiTestCase):
             ]
         )
 
-    def test_subp_exception_raises_to_caller(self, m_subp):
+    def test_subp_exception_raises_to_caller(self, m_which, m_subp):
+        m_which.side_effect = lambda m: m in ("udevadm",)
         m_subp.side_effect = subp.ProcessExecutionError("BOOM")
         self.assertRaises(subp.ProcessExecutionError, util.udevadm_settle)