Always handle faulthandler stderr even if already enabled

It seems the code that would not install pytest's faulthandler support
if it was already enabled is not really needed at all, and even detrimental
when using `python -X dev -m pytest` to run Python in "dev" mode.

Also simplified the plugin by removing the hook class, now the hooks
will always be active so there's no need to delay the hook definitions anymore.

Fix #8258

testing/test_faulthandler.py

@@ -94,7 +94,7 @@ def test_cancel_timeout_on_hook(monkeypatch, hook_name) -> None:
     to timeout before entering pdb (pytest-dev/pytest-faulthandler#12) or any
     other interactive exception (pytest-dev/pytest-faulthandler#14)."""
     import faulthandler
-    from _pytest.faulthandler import FaultHandlerHooks
+    from _pytest import faulthandler as faulthandler_plugin
 
     called = []
 
@@ -104,19 +104,18 @@ def test_cancel_timeout_on_hook(monkeypatch, hook_name) -> None:
 
     # call our hook explicitly, we can trust that pytest will call the hook
     # for us at the appropriate moment
-    hook_func = getattr(FaultHandlerHooks, hook_name)
-    hook_func(self=None)
+    hook_func = getattr(faulthandler_plugin, hook_name)
+    hook_func()
     assert called == [1]
 
 
-@pytest.mark.parametrize("faulthandler_timeout", [0, 2])
-def test_already_initialized(faulthandler_timeout: int, pytester: Pytester) -> None:
-    """Test for faulthandler being initialized earlier than pytest (#6575)."""
+def test_already_initialized_crash(pytester: Pytester) -> None:
+    """Even if faulthandler is already initialized, we still dump tracebacks on crashes (#8258)."""
     pytester.makepyfile(
         """
         def test():
             import faulthandler
-            assert faulthandler.is_enabled()
+            faulthandler._sigabrt()
     """
     )
     result = pytester.run(
@@ -125,22 +124,14 @@ def test_already_initialized(faulthandler_timeout: int, pytester: Pytester) -> N
         "faulthandler",
         "-mpytest",
         pytester.path,
-        "-o",
-        f"faulthandler_timeout={faulthandler_timeout}",
     )
-    # ensure warning is emitted if faulthandler_timeout is configured
-    warning_line = "*faulthandler.py*faulthandler module enabled before*"
-    if faulthandler_timeout > 0:
-        result.stdout.fnmatch_lines(warning_line)
-    else:
-        result.stdout.no_fnmatch_line(warning_line)
-    result.stdout.fnmatch_lines("*1 passed*")
-    assert result.ret == 0
+    result.stderr.fnmatch_lines(["*Fatal Python error*"])
+    assert result.ret != 0
 
 
 def test_get_stderr_fileno_invalid_fd() -> None:
     """Test for faulthandler being able to handle invalid file descriptors for stderr (#8249)."""
-    from _pytest.faulthandler import FaultHandlerHooks
+    from _pytest.faulthandler import get_stderr_fileno
 
     class StdErrWrapper(io.StringIO):
         """
@@ -159,4 +150,4 @@ def test_get_stderr_fileno_invalid_fd() -> None:
 
         # Even when the stderr wrapper signals an invalid file descriptor,
         # ``_get_stderr_fileno()`` should return the real one.
-        assert FaultHandlerHooks._get_stderr_fileno() == 2
+        assert get_stderr_fileno() == 2