Python issue #21645, Tulip issue 192: Rewrite signal handling

Since Python 3.3, the C signal handler writes the signal number into the wakeup
file descriptor and then schedules the Python call using Py_AddPendingCall().

asyncio uses the wakeup file descriptor to wake up the event loop, and relies
on Py_AddPendingCall() to schedule the final callback with call_soon().

If the C signal handler is called in a thread different than the thread of the
event loop, the loop is awaken but Py_AddPendingCall() was not called yet. In
this case, the event loop has nothing to do and go to sleep again.
Py_AddPendingCall() is called while the event loop is sleeping again and so the
final callback is not scheduled immediatly.

This patch changes how asyncio handles signals. Instead of relying on
Py_AddPendingCall() and the wakeup file descriptor, asyncio now only relies on
the wakeup file descriptor. asyncio reads signal numbers from the wakeup file
descriptor to call its signal handler.

Lib/test/test_asyncio/test_proactor_events.py

@@ -435,7 +435,7 @@ class BaseProactorEventLoopTests(test_utils.TestCase):
 
     def test_write_to_self(self):
         self.loop._write_to_self()
-        self.csock.send.assert_called_with(b'x')
+        self.csock.send.assert_called_with(b'\0')
 
     def test_process_events(self):
         self.loop._process_events([])

Lib/test/test_asyncio/test_unix_events.py

@@ -42,7 +42,7 @@ class SelectorEventLoopSignalTests(test_utils.TestCase):
             ValueError, self.loop._check_signal, signal.NSIG + 1)
 
     def test_handle_signal_no_handler(self):
-        self.loop._handle_signal(signal.NSIG + 1, ())
+        self.loop._handle_signal(signal.NSIG + 1)
 
     def test_handle_signal_cancelled_handler(self):
         h = asyncio.Handle(mock.Mock(), (),
@@ -50,7 +50,7 @@ class SelectorEventLoopSignalTests(test_utils.TestCase):
         h.cancel()
         self.loop._signal_handlers[signal.NSIG + 1] = h
         self.loop.remove_signal_handler = mock.Mock()
-        self.loop._handle_signal(signal.NSIG + 1, ())
+        self.loop._handle_signal(signal.NSIG + 1)
         self.loop.remove_signal_handler.assert_called_with(signal.NSIG + 1)
 
     @mock.patch('asyncio.unix_events.signal')