libuv turns 0 duration timers into check watchers so the loop cycles.

CHANGES.rst

@@ -93,7 +93,7 @@ Other Changes
   support in certain tools (setuptools v24.2.1 or newer is required).
   See :issue:`995`.
 
-- pywgi also catches and ignores by default
+- pywsgi also catches and ignores by default
   :const:`errno.WSAECONNABORTED` on Windows. Initial patch in
   :pr:`999` by Jan van Valburg.
 
@@ -218,8 +218,9 @@ libuv
     duration timer callbacks, this can lead the loop to appear to
     hang, as no IO will actually be done.
 
-    In the future, zero duration timers may automatically be changed
-    to check or prepare watchers.
+    To mitigate this issue, ``loop.timer()`` detects attempts to use
+    zero duration timers and turns them into a check watcher. check
+    watchers do not support the ``again`` method.
 
   Again, this is extremely experimental and all of it is subject to
   change.

src/gevent/libuv/loop.py

@@ -466,3 +466,10 @@ class loop(AbstractLoop):
             io_watcher._no_more_watchers = lambda: delitem(io_watchers, fd)
 
         return io_watcher.multiplex(events)
+
+    def timer(self, after, repeat=0.0, ref=True, priority=None):
+        if after <= 0 and repeat <= 0:
+            # Make sure we can spin the loop. See timer.
+            # XXX: Note that this doesn't have a `again` method.
+            return self._watchers.OneShotCheck(self, ref, priority)
+        return super(loop, self).timer(after, repeat, ref, priority)

src/gevent/libuv/watcher.py

@@ -590,18 +590,18 @@ class timer(_base.TimerMixin, watcher):
     # long time, which is not good. So default to not updating the
     # time.
 
-    # Also, newly-added timers of 0 duration can *also* stall the loop, because
-    # they'll be seen to be expired immediately. Updating the time can prevent that,
-    # *if* there was already a timer for a longer duration scheduled.
+    # Also, newly-added timers of 0 duration can *also* stall the
+    # loop, because they'll be seen to be expired immediately.
+    # Updating the time can prevent that, *if* there was already a
+    # timer for a longer duration scheduled.
 
-    # XXX: Have our loop implementation turn 0 duration timers into prepare or
-    # check watchers instead?
+    # To mitigate the above problems, our loop implementation turns
+    # zero duration timers into check watchers instead using OneShotCheck.
+    # This ensures the loop cycles. Of course, the 'again' method does
+    # nothing on them and doesn't exist. In practice that's not an issue.
 
     update_loop_time_on_start = False
 
-    def _update_now(self):
-        self.loop.update()
-
     _again = False
 
     def _watcher_ffi_init(self, args):
@@ -707,3 +707,21 @@ class idle(_base.IdleMixin, watcher):
     # Because libuv doesn't support priorities, idle watchers are
     # potentially quite a bit different than under libev
     pass
+
+
+class check(_base.CheckMixin, watcher):
+    pass
+
+class OneShotCheck(check):
+
+    _watcher_skip_ffi = True
+
+    def start(self, callback, *args):
+        @functools.wraps(callback)
+        def _callback(*args):
+            self.stop()
+            return callback(*args)
+        return check.start(self, _callback, *args)
+
+class prepare(_base.PrepareMixin, watcher):
+    pass

src/gevent/timeout.py

@@ -142,15 +142,21 @@ class Timeout(BaseException):
 
     def start(self):
         """Schedule the timeout."""
-        assert not self.pending, '%r is already started; to restart it, cancel it first' % self
-        if self.seconds is None:  # "fake" timeout (never expires)
+        if self.pending:
+            raise AssertionError('%r is already started; to restart it, cancel it first' % self)
+
+        if self.seconds is None:
+            # "fake" timeout (never expires)
             return
 
         if self.exception is None or self.exception is False or isinstance(self.exception, string_types):
             # timeout that raises self
-            self.timer.start(getcurrent().throw, self)
-        else:  # regular timeout with user-provided exception
-            self.timer.start(getcurrent().throw, self.exception)
+            throws = self
+        else:
+            # regular timeout with user-provided exception
+            throws = self.exception
+
+        self.timer.start(getcurrent().throw, throws)
 
     @classmethod
     def start_new(cls, timeout=None, exception=None, ref=True):

src/greentest/greentest/sysinfo.py

@@ -39,6 +39,7 @@ else:
     DEBUG = False
 
 RUN_LEAKCHECKS = os.getenv('GEVENTTEST_LEAKCHECK')
+RUN_COVERAGE = os.getenv("COVERAGE_PROCESS_START")
 
 # Generally, ignore the portions that are only implemented
 # on particular platforms; they generally contain partial
@@ -50,6 +51,7 @@ if WIN:
 PY2 = None
 PY3 = None
 PY34 = None
+PY35 = None
 PY36 = None
 PY37 = None
 
@@ -61,6 +63,8 @@ if sys.version_info[0] == 3:
     PY3 = True
     if sys.version_info[1] >= 4:
         PY34 = True
+    if sys.version_info[1] >= 5:
+        PY35 = True
     if sys.version_info[1] >= 6:
         PY36 = True
     if sys.version_info[1] >= 7:
@@ -78,6 +82,12 @@ elif sys.version_info[0] == 2:
 
 PYPY3 = PYPY and PY3
 
+PYGTE279 = (
+    sys.version_info[0] == 2
+    and sys.version_info[1] >= 7
+    and sys.version_info[2] >= 9
+)
+
 if WIN:
     NON_APPLICABLE_SUFFIXES.append("posix")
     # This is intimately tied to FileObjectPosix
@@ -110,3 +120,5 @@ from errno import ECONNRESET
 CONN_ABORTED_ERRORS.append(ECONNRESET)
 
 CONN_ABORTED_ERRORS = frozenset(CONN_ABORTED_ERRORS)
+
+RESOLVER_ARES = os.getenv('GEVENT_RESOLVER') == 'ares'

src/greentest/known_failures.py

@@ -6,22 +6,17 @@ import os
 import sys
 import struct
 
+from greentest.sysinfo import RUNNING_ON_APPVEYOR as APPVEYOR
+from greentest.sysinfo import RUNNING_ON_TRAVIS as TRAVIS
+from greentest.sysinfo import RUN_LEAKCHECKS as LEAKTEST
+from greentest.sysinfo import RUN_COVERAGE as COVERAGE
+from greentest.sysinfo import RESOLVER_ARES
 
-APPVEYOR = os.getenv('APPVEYOR')
-TRAVIS = os.getenv('TRAVIS')
-LEAKTEST = os.getenv('GEVENTTEST_LEAKCHECK')
-COVERAGE = os.getenv("COVERAGE_PROCESS_START")
-PYPY = hasattr(sys, 'pypy_version_info')
-PY3 = sys.version_info[0] >= 3
-PY27 = sys.version_info[0] == 2 and sys.version_info[1] == 7
-PY35 = sys.version_info[0] >= 3 and sys.version_info[1] >= 5
-PYGTE279 = (
-    sys.version_info[0] == 2
-    and sys.version_info[1] >= 7
-    and sys.version_info[2] >= 9
-)
-RESOLVER_ARES = os.getenv('GEVENT_RESOLVER') == 'ares'
-LIBUV = os.getenv('GEVENT_CORE_CFFI_ONLY') == 'libuv' # XXX: Formalize this better
+from greentest.sysinfo import PYPY
+from greentest.sysinfo import PY3
+from greentest.sysinfo import PY35
+
+from greentest.sysinfo import LIBUV
 
 
 FAILING_TESTS = [

src/greentest/test__event.py

+from __future__ import absolute_import, print_function, division
 import greentest
 import gevent
 from gevent.event import Event, AsyncResult
@@ -49,7 +50,7 @@ class TestWaitEvent(greentest.GenericWaitTestCase):
             # immediately add the waiter and call it
             o = gevent.wait((event,), timeout=0.01)
             self.assertFalse(event.ready())
-            self.assertFalse(event in o, o)
+            self.assertNotIn(event, o)
 
         gevent.spawn(waiter).join()
 
@@ -87,31 +88,40 @@ class TestAsyncResult(greentest.TestCase):
         self.assertEqual(e.exc_info, ())
         self.assertEqual(e.exception, None)
 
+        class MyException(Exception):
+            pass
+
         def waiter():
-            try:
-                result = e.get()
-                log.append(('received', result))
-            except Exception as ex:
-                log.append(('catched', ex))
+            with self.assertRaises(MyException) as exc:
+                e.get()
+            log.append(('caught', exc.exception))
         gevent.spawn(waiter)
-        obj = Exception()
+        obj = MyException()
         e.set_exception(obj)
         gevent.sleep(0)
-        assert log == [('catched', obj)], log
+        self.assertEqual(log, [('caught', obj)])
 
     def test_set(self):
         event1 = AsyncResult()
         event2 = AsyncResult()
 
+        class MyException(Exception):
+            pass
+
+        timer_exc = MyException('interrupted')
+
         g = gevent.spawn_later(DELAY / 2.0, event1.set, 'hello event1')
-        t = gevent.Timeout.start_new(0, ValueError('interrupted'))
+        t = gevent.Timeout.start_new(0, timer_exc)
         try:
-            try:
-                result = event1.get()
-            except ValueError:
-                X = object()
-                result = gevent.with_timeout(DELAY, event2.get, timeout_value=X)
-                assert result is X, 'Nobody sent anything to event2 yet it received %r' % (result, )
+            with self.assertRaises(MyException) as exc:
+                event1.get()
+            self.assertEqual(timer_exc, exc.exception)
+
+            X = object()
+            result = gevent.with_timeout(DELAY, event2.get, timeout_value=X)
+            self.assertIs(
+                result, X,
+                'Nobody sent anything to event2 yet it received %r' % (result, ))
         finally:
             t.cancel()
             g.kill()
@@ -131,9 +141,11 @@ class TestAsyncResultAsLinkTarget(greentest.TestCase):
         g.link(s1)
         g.link_value(s2)
         g.link_exception(s3)
-        assert s1.get() == 1
-        assert s2.get() == 1
-        assert gevent.with_timeout(DELAY, s3.get, timeout_value=X) is X
+        self.assertEqual(s1.get(), 1)
+        self.assertEqual(s2.get(), 1)
+        X = object()
+        result = gevent.with_timeout(DELAY, s3.get, timeout_value=X)
+        self.assertIs(result, X)
 
     def test_set_exception(self):
         def func():
@@ -144,7 +156,9 @@ class TestAsyncResultAsLinkTarget(greentest.TestCase):
         g.link_value(s2)
         g.link_exception(s3)
         self.assertRaises(greentest.ExpectedException, s1.get)
-        assert gevent.with_timeout(DELAY, s2.get, timeout_value=X) is X
+        X = object()
+        result = gevent.with_timeout(DELAY, s2.get, timeout_value=X)
+        self.assertIs(result, X)
         self.assertRaises(greentest.ExpectedException, s3.get)
 
 
@@ -216,7 +230,5 @@ class TestWait_count2(TestWait):
     count = 2
 
 
-X = object()
-
 if __name__ == '__main__':
     greentest.main()