[Windows] Fix ErrorHandler tests, by disabling signal error handling (#6868)

This PR is a part of the effort to remove the last broken unit tests in certbot codebase for Windows, as described in #6850.

It solves the problems associated to ErrorHandler in Windows (enlighted by tests errors) by ... wipping out the problem: no signal is handled by ErrorHandler on Windows. See the relevant inline comment in certbot.error_handler for explanation and sources.

certbot/error_handler.py

@@ -19,14 +19,30 @@ logger = logging.getLogger(__name__)
 # potentially occur from inside Python. Signals such as SIGILL were not
 # included as they could be a sign of something devious and we should terminate
 # immediately.
-_SIGNALS = [signal.SIGTERM]
 if os.name != "nt":
+    _SIGNALS = [signal.SIGTERM]
     for signal_code in [signal.SIGHUP, signal.SIGQUIT,
                         signal.SIGXCPU, signal.SIGXFSZ]:
         # Adding only those signals that their default action is not Ignore.
         # This is platform-dependent, so we check it dynamically.
         if signal.getsignal(signal_code) != signal.SIG_IGN:
             _SIGNALS.append(signal_code)
+else:
+    # POSIX signals are not implemented natively in Windows, but emulated from the C runtime.
+    # As consumed by CPython, most of handlers on theses signals are useless, in particular
+    # SIGTERM: for instance, os.kill(pid, signal.SIGTERM) will call TerminateProcess, that stops
+    # immediately the process without calling the attached handler. Besides, non-POSIX signals
+    # (CTRL_C_EVENT and CTRL_BREAK_EVENT) are implemented in a console context to handle the
+    # CTRL+C event to a process launched from the console. Only CTRL_C_EVENT has a reliable
+    # behavior in fact, and maps to the handler to SIGINT. However in this case, a
+    # KeyboardInterrupt is raised, that will be handled by ErrorHandler through the context manager
+    # protocol. Finally, no signal on Windows is electable to be handled using ErrorHandler.
+    #
+    # Refs: https://stackoverflow.com/a/35792192, https://maruel.ca/post/python_windows_signal,
+    # https://docs.python.org/2/library/os.html#os.kill,
+    # https://www.reddit.com/r/Python/comments/1dsblt/windows_command_line_automation_ctrlc_question
+    _SIGNALS = []
+
 
 class ErrorHandler(object):
     """Context manager for running code that must be cleaned up on failure.

certbot/tests/error_handler_test.py

@@ -10,7 +10,6 @@ import mock
 from acme.magic_typing import Callable, Dict, Union
 # pylint: enable=unused-import, no-name-in-module
 
-import certbot.tests.util as test_util
 
 def get_signals(signums):
     """Get the handlers for an iterable of signums."""
@@ -66,9 +65,9 @@ class ErrorHandlerTest(unittest.TestCase):
         self.init_func.assert_called_once_with(*self.init_args,
                                                **self.init_kwargs)
 
-    # On Windows, this test kills pytest itself !
-    @test_util.broken_on_windows
     def test_context_manager_with_signal(self):
+        if not self.signals:
+            self.skipTest(reason='Signals cannot be handled on Windows.')
         init_signals = get_signals(self.signals)
         with signal_receiver(self.signals) as signals_received:
             with self.handler:
@@ -98,9 +97,9 @@ class ErrorHandlerTest(unittest.TestCase):
                                                **self.init_kwargs)
         bad_func.assert_called_once_with()
 
-    # On Windows, this test kills pytest itself !
-    @test_util.broken_on_windows
     def test_bad_recovery_with_signal(self):
+        if not self.signals:
+            self.skipTest(reason='Signals cannot be handled on Windows.')
         sig1 = self.signals[0]
         sig2 = self.signals[-1]
         bad_func = mock.MagicMock(side_effect=lambda: send_signal(sig1))
@@ -149,10 +148,9 @@ class ExitHandlerTest(ErrorHandlerTest):
                                                **self.init_kwargs)
         func.assert_called_once_with()
 
-    # On Windows, this test kills pytest itself !
-    @test_util.broken_on_windows
     def test_bad_recovery_with_signal(self):
         super(ExitHandlerTest, self).test_bad_recovery_with_signal()
 
+
 if __name__ == "__main__":
     unittest.main()  # pragma: no cover