Disables siginterrupt for SIGUSR1 (#1844)

I've recently started using SIGUSR1 to stop old workers on deploy and each
deploy has resulted in all MapReduce tasks dying with
`IOError: [Errno 4] Interrupted system call`. Setting siginterrupt to
False for SIGUSR1 prevents this error for me.

luigi/worker.py

@@ -404,6 +404,7 @@ def __init__(self, scheduler=None, worker_id=None, worker_processes=1, assistant
         if not self._config.no_install_shutdown_handler:
             try:
                 signal.signal(signal.SIGUSR1, self.handle_interrupt)
+                signal.siginterrupt(signal.SIGUSR1, False)
             except AttributeError:
                 pass
 

test/worker_test.py

@@ -176,6 +176,26 @@ def test_stop_getting_new_work(self):
         self.w.run()
         self.assertFalse(d.complete())
 
+    @mock.patch('luigi.worker.signal')
+    def test_signal_interrupt_disabled(self, worker_signal):
+        w = Worker(scheduler=self.sch)
+        worker_signal.signal.assert_called_once_with(worker_signal.SIGUSR1, w.handle_interrupt)
+        worker_signal.siginterrupt.assert_called_once_with(worker_signal.SIGUSR1, False)
+
+    @mock.patch('luigi.worker.signal')
+    def test_signal_interrupt_not_disabled_without_shutdown_handler(self, worker_signal):
+        Worker(scheduler=self.sch, no_install_shutdown_handler=True)
+        self.assertEqual(0, worker_signal.signal.call_count)
+        self.assertEqual(0, worker_signal.siginterrupt.call_count)
+
+    @mock.patch('luigi.worker.signal')
+    def test_signal_interrupt_ok_to_not_exist(self, worker_signal):
+        worker_signal.siginterrupt.side_effect = AttributeError
+        w = Worker(scheduler=self.sch)  # may fail due to the AttributeError
+
+        # we still register the handler when the siginterrupt disable fails
+        worker_signal.signal.assert_called_once_with(worker_signal.SIGUSR1, w.handle_interrupt)
+
     def test_disabled_shutdown_hook(self):
         w = Worker(scheduler=self.sch, keep_alive=True, no_install_shutdown_handler=True)
         with w: