Add test and tweak logic to permit symlinks elsewhere for flow.cylc file

cylc/flow/workflow_files.py

@@ -1792,9 +1792,13 @@ def is_forbidden(flow_file: Path) -> bool:
             return True
         return False
     link = flow_file.resolve()
-    if Path(link).parent == flow_file.parent:  # link points within dir
+    if link.parent == flow_file.parent:
+        # link points within dir (permitted)
         return False
-    return True
+    # link points elsewhere, check that suite.rc does not also exist in dir
+    if flow_file.parent.joinpath(WorkflowFiles.SUITE_RC).exists():
+        return True
+    return False
 
 
 def detect_flow_exists(

tests/unit/test_workflow_files.py

@@ -1846,14 +1846,6 @@ def test_detect_both_flow_and_suite(tmp_path):
             WorkflowFiles.SUITE_RC, False, WorkflowFilesError, None,
             id="flow.cylc symlinked to non-existent suite.rc"
         ),
-        pytest.param(
-            'other-path', True, None, WorkflowFiles.SUITE_RC,
-            id="flow.cylc symlinked to other file, suite.rc exists"
-        ),
-        pytest.param(
-            'other-path', False, WorkflowFilesError, None,
-            id="flow.cylc symlinked to other file, no suite.rc"
-        ),
         pytest.param(
             None, True, None, WorkflowFiles.SUITE_RC,
             id="No flow.cylc, suite.rc exists"
@@ -1864,17 +1856,11 @@ def test_detect_both_flow_and_suite(tmp_path):
         ),
     ]
 )
-@pytest.mark.parametrize(
-    'symlink_suiterc_arg',
-    [pytest.param(False, id="symlink_suiterc=False "),
-     pytest.param(True, id="symlink_suiterc=True ")]
-)
 def test_check_flow_file_symlink(
     flow_file_target: Optional[str],
     suiterc_exists: bool,
     err: Optional[Type[Exception]],
     expected_file: Optional[str],
-    symlink_suiterc_arg: bool,
     tmp_path: Path, caplog: pytest.LogCaptureFixture
 ) -> None:
     """Test check_flow_file() when flow.cylc is a symlink or doesn't exist.
@@ -1887,8 +1873,6 @@ def test_check_flow_file_symlink(
         expected_file: Which file's path should get returned, when
             symlink_suiterc_arg is FALSE (otherwise it will always be
             flow.cylc, assuming no exception occurred).
-        symlink_suiterc_arg: Value of the symlink_suiterc arg passed to
-            check_flow_file().
     """
     flow_file = tmp_path.joinpath(WorkflowFiles.FLOW_FILE)
     suiterc = tmp_path.joinpath(WorkflowFiles.SUITE_RC)
@@ -1898,25 +1882,14 @@ def test_check_flow_file_symlink(
     if flow_file_target:
         flow_file.symlink_to(flow_file_target)
 
-    caplog.set_level(logging.INFO, CYLC_LOG)
-    log_msg = ""
-
     if err:
         with pytest.raises(err):
-            check_flow_file(tmp_path, symlink_suiterc_arg)
+            check_flow_file(tmp_path)
     else:
         assert expected_file is not None  # otherwise test is wrong
-        result = check_flow_file(tmp_path, symlink_suiterc_arg)
-        if symlink_suiterc_arg is True:
-            assert flow_file.samefile(suiterc)
-            expected_file = WorkflowFiles.FLOW_FILE
-            if flow_file_target != WorkflowFiles.SUITE_RC:
-                log_msg = "Symlink created: flow.cylc -> suite.rc"
+        result = check_flow_file(tmp_path)
         assert result == tmp_path.joinpath(expected_file)
 
-    if log_msg:
-        assert log_msg in caplog.messages
-
 
 def test_detect_both_flow_and_suite_symlinked(tmp_path):
     """Test flow.cylc symlinked to suite.rc together in dir is permitted."""
@@ -1935,6 +1908,8 @@ def test_flow_symlinked_elsewhere_and_suite_present(tmp_path: Path):
     run_dir.mkdir(exist_ok=True)
     flow_file = (run_dir / WorkflowFiles.FLOW_FILE)
     flow_file.symlink_to(suite_file)
+    forbidden_external = is_forbidden(flow_file)
+    assert forbidden_external is False
     (run_dir / WorkflowFiles.SUITE_RC).touch()
     forbidden = is_forbidden(flow_file)
     assert forbidden is True
@@ -1951,7 +1926,6 @@ def test_flow_symlinked_elsewhere_and_suite_present(tmp_path: Path):
 
 def test_is_forbidden_symlink_returns_false_for_non_symlink(tmp_path):
     """Test sending a non symlink path is not marked as forbidden"""
-    Path(tmp_path)
     flow_file = (tmp_path / WorkflowFiles.FLOW_FILE)
     flow_file.touch()
     forbidden = is_forbidden(Path(flow_file))