play: preserve colour formatting when detaching

CHANGES.md

@@ -14,6 +14,9 @@ ones in. -->
 
 ### Enhancements
 
+[#5461](https://github.com/cylc/cylc-flow/pull/5461) - Preserve colour
+formatting when starting workflows in distributed mode using `run hosts`.
+
 [#5291](https://github.com/cylc/cylc-flow/pull/5291) - re-implement old-style
 clock triggers as wall_clock xtriggers.
 

cylc/flow/scheduler_cli.py

@@ -386,7 +386,7 @@ def scheduler_cli(options: 'Values', workflow_id_raw: str) -> None:
     _upgrade_database(db_file)
 
     # re-execute on another host if required
-    _distribute(options.host, workflow_id_raw, workflow_id)
+    _distribute(options.host, workflow_id_raw, workflow_id, options.color)
 
     # print the start message
     _print_startup_message(options)
@@ -557,7 +557,7 @@ def _print_startup_message(options):
         LOG.warning(SUITERC_DEPR_MSG)
 
 
-def _distribute(host, workflow_id_raw, workflow_id):
+def _distribute(host, workflow_id_raw, workflow_id, color):
     """Re-invoke this command on a different host if requested.
 
     Args:
@@ -589,6 +589,15 @@ def _distribute(host, workflow_id_raw, workflow_id):
         # Prevent recursive host selection
         cmd.append("--host=localhost")
 
+        # Preserve CLI colour
+        if is_terminal() and color != 'never':
+            # the detached process doesn't pass the is_terminal test
+            # so we have to explicitly tell Cylc to use color
+            cmd.append('--color=always')
+        else:
+            # otherwise set --color=never to make testing easier
+            cmd.append('--color=never')
+
         # Re-invoke the command
         # NOTE: has the potential to raise NoHostsError, however, this will
         # most likely have been raised during host-selection

tests/unit/test_scheduler_cli.py

@@ -21,6 +21,7 @@
 
 from cylc.flow.exceptions import ServiceFileError
 from cylc.flow.scheduler_cli import (
+    _distribute,
     _version_check,
 )
 
@@ -222,3 +223,39 @@ def test_version_check_no_db(tmp_path):
     """It should pass if there is no DB file (e.g. on workflow first start)."""
     db_file = tmp_path / 'db'  # non-existent file
     assert _version_check(db_file, False, False)
+
+
+@pytest.mark.parametrize(
+    'cli_colour, is_terminal, distribute_colour',
+    [
+        ('never', True, '--color=never'),
+        ('auto', True, '--color=always'),
+        ('always', True, '--color=always'),
+        ('never', False, '--color=never'),
+        ('auto', False, '--color=never'),
+        ('always', False, '--color=never'),
+    ]
+)
+def test_distribute_colour(
+    monkeymock,
+    cli_colour,
+    is_terminal,
+    distribute_colour,
+):
+    """It should start detached workflows with the correct --colour option.
+
+    The is_terminal test will fail for detached scheduler processes which means
+    that the colour formatting will be stripped for startup. This includes
+    the Cylc header logo and any warnings/errors raised during config parsing.
+
+    In order to preserver colour formatting we must set the `--colour` arg to
+    `always` when we want the detached process to start in colour mode.
+
+    See https://github.com/cylc/cylc-flow/issues/5159
+    """
+    monkeymock('cylc.flow.scheduler_cli.sys.exit')
+    _is_terminal = monkeymock('cylc.flow.scheduler_cli.is_terminal')
+    _is_terminal.return_value = is_terminal
+    _cylc_server_cmd = monkeymock('cylc.flow.scheduler_cli.cylc_server_cmd')
+    _distribute('myhost', 'foo', 'foo/run1', cli_colour)
+    assert distribute_colour in _cylc_server_cmd.call_args[0][0]