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.

1 files changed, 5 insertions, 1 deletions

diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index d79c0801b24..cd1a75aa78e 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -94,12 +94,16 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
         self._internal_fds += 1
         self.add_reader(self._ssock.fileno(), self._read_from_self)
 
+    def _process_self_data(self, data):
+        pass
+
     def _read_from_self(self):
         while True:
             try:
                 data = self._ssock.recv(4096)
                 if not data:
                     break
+                self._process_self_data(data)
             except InterruptedError:
                 continue
             except BlockingIOError:
@@ -114,7 +118,7 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
         csock = self._csock
         if csock is not None:
             try:
-                csock.send(b'x')
+                csock.send(b'\0')
             except OSError:
                 pass