🧪 Test unhandled internal exceptions in workers

This is to ensure that such exceptions don't crash the worker threads
starving the server of the processing resources.

cheroot/test/test_conn.py

@@ -875,6 +875,89 @@ def _trigger_scary_exc(_req, _resp):
         )
 
 
+def test_remains_alive_post_unhandled_exception(
+        mocker,
+        monkeypatch,
+        test_client,
+        testing_server,
+        wsgi_server_thread,
+):
+    """Ensure worker threads are resillient to unhandled exceptions."""
+
+    class ScaryCrash(BaseException):  # noqa: WPS418, WPS431
+        """A simulated crash during HTTP parsing."""
+
+    _orig_read_request_line = (
+        test_client.server_instance.
+        ConnectionClass.RequestHandlerClass.
+        read_request_line
+    )
+
+    def _read_request_line(self):
+        _orig_read_request_line(self)
+        raise ScaryCrash(666)
+
+    monkeypatch.setattr(
+        test_client.server_instance.ConnectionClass.RequestHandlerClass,
+        'read_request_line',
+        _read_request_line,
+    )
+
+    server_connection_close_spy = mocker.spy(
+        test_client.server_instance.ConnectionClass,
+        'close',
+    )
+
+    # NOTE: The initial worker thread count is 10.
+    assert len(testing_server.requests._threads) == 10
+
+    test_client.get_connection().send(b'GET / HTTP/1.1')
+
+    # NOTE: This spy ensure the log entry gets recorded before we're testing
+    # NOTE: them and before server shutdown, preserving their order and making
+    # NOTE: the log entry presence non-flaky.
+    while not server_connection_close_spy.called:  # noqa: WPS328
+        pass
+
+    # NOTE: This checks for whether there's any crashed threads
+    while testing_server.requests.idle < 10:  # noqa: WPS328
+        pass
+    assert len(testing_server.requests._threads) == 10
+    assert all(
+        worker_thread.is_alive()
+        for worker_thread in testing_server.requests._threads
+    )
+    testing_server.interrupt = SystemExit('test requesting shutdown')
+    assert not testing_server.requests._threads
+    wsgi_server_thread.join()  # no extra logs upon server termination
+
+    actual_log_entries = testing_server.error_log.calls[:]
+    testing_server.error_log.calls.clear()  # prevent post-test assertions
+
+    expected_log_entries = (
+        (
+            logging.ERROR,
+            '^Unhandled error while processing an incoming connection '
+            r'ScaryCrash\(666\)$',
+        ),
+        (
+            logging.INFO,
+            '^SystemExit raised: shutting down$',
+        ),
+    )
+
+    assert len(actual_log_entries) == len(expected_log_entries)
+
+    for (  # noqa: WPS352
+            (expected_log_level, expected_msg_regex),
+            (actual_msg, actual_log_level, _tb),
+    ) in zip(expected_log_entries, actual_log_entries):
+        assert expected_log_level == actual_log_level
+        assert _matches_pattern(expected_msg_regex, actual_msg) is not None, (
+            f'{actual_msg !r} does not match {expected_msg_regex !r}'
+        )
+
+
 @pytest.mark.parametrize(
     'timeout_before_headers',
     (