Make libev follow libuv and not update timers by default.

Make Timeout._start_new_or_dummy stop forcing the issue.

The failing test_telnetlib was fixed by avoiding a zero-duration timer. On libuv zero duration timers can block the event loop.

CHANGES.rst

@@ -197,6 +197,15 @@ libuv
 
     libev has also been changed to follow this behaviour.
 
+  - Timers of zero duration do not necessarily cause the event loop to
+    cycle, as they do in libev. Instead, they may be called
+    immediately. If zero duration timers are added from other zero
+    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.
+
   Again, this is extremely experimental and all of it is subject to
   change.
 

src/gevent/libuv/watcher.py

@@ -589,6 +589,14 @@ class timer(_base.TimerMixin, watcher):
     # This can lead to us being stuck running timers for a terribly
     # 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.
+
+    # XXX: Have our loop implementation turn 0 duration timers into prepare or
+    # check watchers instead?
+
     update_loop_time_on_start = False
 
     def _update_now(self):

src/gevent/select.py

@@ -150,9 +150,16 @@ def select(rlist, wlist, xlist, timeout=None): # pylint:disable=unused-argument
             # Ignore interrupted syscalls
             raise
 
-    if sel_results[0] or sel_results[1] or sel_results[2]:
+    if sel_results[0] or sel_results[1] or sel_results[2] or (timeout is not None and timeout == 0):
         # If we actually had stuff ready, go ahead and return it. No need
         # to go through the trouble of doing our own stuff.
+
+        # Likewise, if the timeout is 0, we already did a 0 timeout
+        # select and we don't need to do it again. Note that in libuv,
+        # zero duration timers may be called immediately, without
+        # cycling the event loop at all. 2.7/test_telnetlib.py "hangs"
+        # calling zero-duration timers if we go to the loop here.
+
         # However, because this is typically a place where scheduling switches
         # can occur, we need to make sure that's still the case; otherwise a single
         # consumer could monopolize the thread. (shows up in test_ftplib.)

src/gevent/timeout.py

@@ -13,7 +13,7 @@ module add timeouts to arbitrary code.
     If a blocking function is called or an intense calculation is ongoing during
     which no switches occur, :class:`Timeout` is powerless.
 """
-
+from __future__ import absolute_import, print_function, division
 from gevent._compat import string_types
 from gevent.hub import getcurrent, _NONE, get_hub
 
@@ -152,7 +152,7 @@ class Timeout(BaseException):
             self.timer.start(getcurrent().throw, self.exception)
 
     @classmethod
-    def start_new(cls, timeout=None, exception=None, ref=True, _update=None):
+    def start_new(cls, timeout=None, exception=None, ref=True):
         """Create a started :class:`Timeout`.
 
         This is a shortcut, the exact action depends on *timeout*'s type:
@@ -169,8 +169,6 @@ class Timeout(BaseException):
                 timeout.start()
             return timeout
         timeout = cls(timeout, exception, ref=ref)
-        if _update:
-            get_hub().loop.update()
         timeout.start()
         return timeout
 
@@ -189,10 +187,7 @@ class Timeout(BaseException):
         # under PyPy in synthetic benchmarks it makes no difference.
         if timeout is None:
             return _FakeTimer
-        # If we don't update the time here (and the timer watcher doesn't),
-        # as under libuv, then certain tests hang, notably the monkey-patched test_telnetlib
-        # in test_read_eager_A. libev does not demonstrate this behaviour.
-        return Timeout.start_new(timeout, exception, _update=True)
+        return Timeout.start_new(timeout, exception)
 
     @property
     def pending(self):

src/greentest/greentest.py

@@ -578,12 +578,10 @@ class TestCase(TestCaseMetaClass("NewBase", (BaseTestCase,), {})):
         econtext, ekind, evalue = error
         if kind is not None:
             self.assertIsInstance(kind, type)
-            try:
-                assert issubclass(ekind, kind), error
-            except TypeError as e:
-                # Seen on PyPy on Windows
-                print("TYPE ERROR", e, ekind, kind, type(kind))
-                raise
+            self.assertIsNotNone(
+                ekind,
+                "Error must not be none %r" % (error,))
+            assert issubclass(ekind, kind), error
         if value is not None:
             if isinstance(value, str):
                 self.assertEqual(str(evalue), value)