quilt3: simplify write_yaml() a bit (#3877)

api/python/quilt3/util.py

@@ -298,15 +298,12 @@ def write_yaml(data, yaml_path, keep_backup=False):
     try:
         if path.exists():
             path.rename(backup_path)
-        if not path.parent.exists():
-            path.parent.mkdir(parents=True)
+        path.parent.mkdir(parents=True, exist_ok=True)
         with path.open('w') as config_file:
             yaml.dump(data, config_file)
     except Exception:     # intentionally wide catch -- reraised immediately.
         if backup_path.exists():
-            if path.exists():
-                path.unlink()
-            backup_path.rename(path)
+            backup_path.replace(path)
         raise
 
     if backup_path.exists() and not keep_backup:

api/python/tests/test_util.py

@@ -35,6 +35,21 @@ def test_write_yaml(tmpdir):
     assert fname.read_text('utf-8') == 'testing: bar\n'
 
 
+@pytest.mark.parametrize("keep_backup", [False, True])
+def test_write_yaml_exception(tmp_path, keep_backup):
+    fname = tmp_path / "some_file.yml"
+    fname.write_text("42")
+
+    exc = Exception("test exception")
+    with mock.patch("quilt3.util.yaml.dump", side_effect=exc):
+        with pytest.raises(Exception) as exc_info:
+            util.write_yaml("test", fname)
+
+    assert exc_info.value == exc
+    assert fname.read_text("utf-8") == "42"
+    assert list(tmp_path.iterdir()) == [fname]
+
+
 def test_read_yaml(tmpdir):
     # Read a string
     parsed_string = util.read_yaml(TEST_YAML)