fix: keep local coverage central without xdist workers

src/pytest_cov/plugin.py

@@ -232,7 +232,15 @@ def __init__(self, options: argparse.Namespace, pluginmanager, start=True, no_co
         self.options = options
         self._wrote_heading = False
 
-        is_dist = getattr(options, 'numprocesses', False) or getattr(options, 'distload', False) or getattr(options, 'dist', 'no') != 'no'
+        # The distribution mode only takes effect when xdist has execution
+        # environments. ``--dist`` may be configured without ``-n``; in that
+        # case pytest still runs in the current process and needs the central
+        # controller to configure subprocess coverage.
+        is_dist = (
+            bool(getattr(options, 'numprocesses', False))
+            or getattr(options, 'distload', False)
+            or (getattr(options, 'dist', 'no') != 'no' and bool(getattr(options, 'tx', ())))
+        )
         if getattr(options, 'no_cov', False):
             self._disabled = True
             return

tests/test_pytest_cov.py

@@ -1112,6 +1112,30 @@ def test_foo():
     assert result.ret == 0
 
 
+def test_dist_option_without_numprocesses_uses_central_controller(monkeypatch):
+    started = []
+    monkeypatch.setattr(
+        pytest_cov.plugin.CovPlugin,
+        'start',
+        lambda self, controller_cls, config=None, nodeid=None: started.append(
+            controller_cls.__name__
+        ),
+    )
+    options = SimpleNamespace(
+        cov_report={},
+        cov_source=['.'],
+        dist='loadgroup',
+        distload=False,
+        no_cov=False,
+        numprocesses=None,
+        tx=[],
+    )
+
+    pytest_cov.plugin.CovPlugin(options, None)
+
+    assert started == ['Central']
+
+
 @pytest.mark.skipif('sys.platform == "win32" and platform.python_implementation() == "PyPy"')
 def test_dist_subprocess_collocated(testdir):
     testdir.makepyprojecttoml(