Warn when cmdline_main raises SystemExit

changelog/8986.bugfix.rst

@@ -0,0 +1,2 @@
+Warn when a plugin raises :class:`SystemExit` from the
+:hook:`pytest_cmdline_main` hook.

src/_pytest/config/__init__.py

@@ -234,7 +234,15 @@ def _main(
             return ExitCode.USAGE_ERROR
 
         try:
-            ret: ExitCode | int = config.hook.pytest_cmdline_main(config=config)
+            try:
+                ret: ExitCode | int = config.hook.pytest_cmdline_main(config=config)
+            except SystemExit:
+                warnings.warn(
+                    PytestConfigWarning(
+                        "A plugin raised SystemExit from the pytest_cmdline_main hook"
+                    )
+                )
+                raise
             try:
                 return ExitCode(ret)
             except ValueError:

testing/test_config.py

@@ -32,6 +32,7 @@
 from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pathlib import absolutepath
 from _pytest.pytester import Pytester
+from _pytest.warning_types import PytestConfigWarning
 from _pytest.warning_types import PytestDeprecationWarning
 import pytest
 
@@ -852,6 +853,23 @@ def test_absolute_win32_path(self, pytester: Pytester) -> None:
 
 
 class TestConfigAPI:
+    @pytest.mark.parametrize("code", [0, 42])
+    def test_cmdline_main_system_exit(self, code: int) -> None:
+        class Plugin:
+            def pytest_cmdline_main(self, config: Config) -> None:
+                raise SystemExit(code)
+
+        with (
+            pytest.warns(
+                PytestConfigWarning,
+                match="plugin raised SystemExit from the pytest_cmdline_main hook",
+            ),
+            pytest.raises(SystemExit) as exc_info,
+        ):
+            pytest.main([], plugins=[Plugin()])
+
+        assert exc_info.value.code == code
+
     def test_config_trace(self, pytester: Pytester) -> None:
         config = pytester.parseconfig()
         values: list[str] = []