test/mod: add checks for loop module

Add some basic checks for the loop module, including the new
`loop_for_fd` helper method.

test/mod/test_loop.py

@@ -0,0 +1,66 @@
+#
+# Test for the loop.py
+#
+
+import contextlib
+import os
+
+
+from tempfile import TemporaryDirectory
+
+import pytest
+
+from osbuild import loop
+
+from ..test import TestBase
+
+
+@pytest.fixture(name="tempdir")
+def tempdir_fixture():
+    with TemporaryDirectory(prefix="loop-") as tmp:
+        yield tmp
+
+
+@pytest.mark.skipif(not TestBase.can_bind_mount(), reason="root only")
+def test_basic(tempdir):
+
+    path = os.path.join(tempdir, "test.img")
+    ctl = loop.LoopControl()
+
+    assert ctl
+
+    with pytest.raises(ValueError):
+        ctl.loop_for_fd(-1)
+
+    lo, f = None, None
+    try:
+        f = open(path, "wb+")
+        f.truncate(1024)
+        f.flush()
+        lo = ctl.loop_for_fd(f.fileno())
+
+        assert lo
+        assert lo.devname
+
+    finally:
+        if lo:
+            with contextlib.suppress(OSError):
+                lo.clear_fd()
+            lo.close()
+        if f:
+            f.close()
+
+        ctl.close()
+
+    # closing must be a no-op on a closed LoopControl
+    ctl.close()
+
+    # check we raise exceptions on methods that require
+    # an open LoopControl
+
+    for fn in (ctl.add, ctl.remove, ctl.get_unbound):
+        with pytest.raises(RuntimeError):
+            fn()
+
+    with pytest.raises(RuntimeError):
+        ctl.loop_for_fd(0)