Add an option to the workflow config to force use of compat mode

cylc/flow/config.py

@@ -217,7 +217,8 @@ def __init__(
         run_dir: Optional[str] = None,
         log_dir: Optional[str] = None,
         work_dir: Optional[str] = None,
-        share_dir: Optional[str] = None
+        share_dir: Optional[str] = None,
+        force_compat_mode: bool = False,
     ) -> None:
         """
         Initialize the workflow config object.
@@ -226,8 +227,13 @@ def __init__(
             workflow: workflow ID
             fpath: workflow config file path
             options: CLI options
+            force_compat_mode:
+                If True, forces Cylc to use compatibility mode
+                overriding compatibility mode checks.
+                See https://github.com/cylc/cylc-rose/issues/319
+
         """
-        check_deprecation(Path(fpath))
+        check_deprecation(Path(fpath), force_compat_mode=force_compat_mode)
         self.mem_log = mem_log_func
         if self.mem_log is None:
             self.mem_log = lambda x: None

cylc/flow/workflow_files.py

@@ -779,17 +779,27 @@
     return title
 
 
-def check_deprecation(path, warn=True):
+def check_deprecation(path, warn=True, force_compat_mode=False):
     """Warn and turn on back-compat flag if Cylc 7 suite.rc detected.
 
     Path can point to config file or parent directory (i.e. workflow name).
+
+    Args:
+        warn:
+            If True, then a warning will be logged when compatibility
+            mode is activated.
+        force_compat_mode:
+            If True, forces Cylc to use compatibility mode
+            overriding compatibility mode checks.
+            See https://github.com/cylc/cylc-rose/issues/319
     """
     if (
         # Don't want to log if it's already been set True.
         not cylc.flow.flags.cylc7_back_compat
         and (
             path.resolve().name == WorkflowFiles.SUITE_RC
             or (path / WorkflowFiles.SUITE_RC).is_file()
+            or force_compat_mode
         )
     ):
         cylc.flow.flags.cylc7_back_compat = True

tests/unit/test_config.py

@@ -29,6 +29,7 @@
 from cylc.flow.cycling import loader
 from cylc.flow.cycling.loader import INTEGER_CYCLING_TYPE, ISO8601_CYCLING_TYPE
 from cylc.flow.exceptions import (
+    GraphParseError,
     PointParsingError,
     InputError,
     WorkflowConfigError,
@@ -1675,3 +1676,20 @@ def test_cylc_env_at_parsing(
             assert var in cylc_env
         else:
             assert var not in cylc_env
+
+
+def test_force_workflow_compat_mode(tmp_path):
+    fpath = (tmp_path / 'flow.cylc')
+    from textwrap import dedent
+    fpath.write_text(dedent("""
+        [scheduler]
+            allow implicit tasks = true
+        [scheduling]
+            [[graph]]
+                R1 = a:succeeded | a:failed => b
+    """))
+    # It fails without compat mode:
+    with pytest.raises(GraphParseError, match='Opposite outputs'):
+        WorkflowConfig('foo', str(fpath), {})
+    # It succeeds with compat mode:
+    WorkflowConfig('foo', str(fpath), {}, force_compat_mode=True)