util/linux: add explicit FS_IMMUTABLE_FL helpers

The FS_IOC_{GET,SET}FLAGS ioctl numbers are not stable across different
architectures. Most of them use the asm-generic versions, but ALPHA and
SPARC in particular use completely different IOC number setups (see the
definition of _IOC, _IOR, _IOW, etc. in the kernel).

This commit moves the helpers for `FS_IMMUTABLE_FL` into
`osbuild/util/` and adds explicit tests. This will make sure that we
catch any ioctl mismatches as soon as possible when we run the osbuild
test-suite on other architectures. Until then, we will have to live with
this mismatch.

test/test_util_linux.py

@@ -0,0 +1,72 @@
+#
+# Tests for the `osbuild.util.linux` module.
+#
+
+
+import os
+import subprocess
+import tempfile
+import unittest
+
+import osbuild.util.linux as linux
+
+
+class TestUtilLinux(unittest.TestCase):
+    def setUp(self):
+        self.vartmpdir = tempfile.TemporaryDirectory(dir="/var/tmp")
+
+    def tearDown(self):
+        self.vartmpdir.cleanup()
+
+    def test_ioctl_get_immutable(self):
+        #
+        # Test the `ioctl_get_immutable()` helper and make sure it works
+        # as intended.
+        #
+
+        with open(f"{self.vartmpdir.name}/immutable", "x") as f:
+            assert not linux.ioctl_get_immutable(f.fileno())
+
+    @unittest.skipUnless(os.geteuid() == 0, "root-only")
+    def test_ioctl_toggle_immutable(self):
+        #
+        # Test the `ioctl_toggle_immutable()` helper and make sure it works
+        # as intended.
+        #
+
+        with open(f"{self.vartmpdir.name}/immutable", "x") as f:
+            # Check the file is mutable by default and if we clear it again.
+            assert not linux.ioctl_get_immutable(f.fileno())
+            linux.ioctl_toggle_immutable(f.fileno(), False)
+            assert not linux.ioctl_get_immutable(f.fileno())
+
+            # Set immutable and check for it. Try again to verify with flag set.
+            linux.ioctl_toggle_immutable(f.fileno(), True)
+            assert linux.ioctl_get_immutable(f.fileno())
+            linux.ioctl_toggle_immutable(f.fileno(), True)
+            assert linux.ioctl_get_immutable(f.fileno())
+
+            # Verify immutable files cannot be unlinked.
+            with self.assertRaises(OSError):
+                os.unlink(f"{self.vartmpdir.name}/immutable")
+
+            # Check again that clearing the flag works.
+            linux.ioctl_toggle_immutable(f.fileno(), False)
+            assert not linux.ioctl_get_immutable(f.fileno())
+
+            # This time, check that we actually set the same flag as `chattr`.
+            subprocess.run(["chattr", "+i",
+                            f"{self.vartmpdir.name}/immutable"], check=True)
+            assert linux.ioctl_get_immutable(f.fileno())
+
+            # Same for clearing it.
+            subprocess.run(["chattr", "-i",
+                            f"{self.vartmpdir.name}/immutable"], check=True)
+            assert not linux.ioctl_get_immutable(f.fileno())
+
+            # Verify we can unlink the file again, once the flag is cleared.
+            os.unlink(f"{self.vartmpdir.name}/immutable")
+
+
+if __name__ == "__main__":
+    unittest.main()