Let Event have weak references.

Fixes #1211

Note that this can crash on CPython 3.7b4; see https://github.com/cython/cython/issues/2270

CHANGES.rst

@@ -7,7 +7,9 @@
 1.3.1 (unreleased)
 ==================
 
-- Nothing changed yet.
+- Fix weak references to :class:`gevent.event.Event`. Reported in
+  :issue:`1211` by Matias Guijarro. Note that creating weak references
+  to Event objects may crash Python 3.7.
 
 
 1.3.0 (2018-05-11)

src/gevent/_event.pxd

@@ -53,9 +53,9 @@ cdef class _AbstractLinkable:
 
 
 cdef class Event(_AbstractLinkable):
+   cdef object __weakref__
    cdef bint _flag
 
-
 cdef class AsyncResult(_AbstractLinkable):
     cdef readonly _value
     cdef readonly tuple _exc_info

src/gevent/event.py

@@ -157,7 +157,7 @@ class Event(_AbstractLinkable):
         the waiting greenlets being awakened. These details may change in the future.
     """
 
-    __slots__ = ('_flag',)
+    __slots__ = ('_flag', '__weakref__')
 
     def __init__(self):
         _AbstractLinkable.__init__(self)

src/greentest/greentest/__init__.py

@@ -78,6 +78,7 @@ from greentest.skipping import skipOnLibuvOnPyPyOnWin
 from greentest.skipping import skipOnPurePython
 from greentest.skipping import skipWithCExtensions
 from greentest.skipping import skipOnLibuvOnTravisOnCPython27
+from greentest.skipping import skipOnPy37
 
 from greentest.exception import ExpectedException
 

src/greentest/greentest/skipping.py

@@ -41,6 +41,8 @@ skipOnPyPy3OnCI = _do_not_skip
 skipOnPyPy3 = _do_not_skip
 skipOnPyPyOnWindows = _do_not_skip
 
+skipOnPy37 = unittest.skip if sysinfo.PY37 else _do_not_skip
+
 skipOnPurePython = unittest.skip if sysinfo.PURE_PYTHON else _do_not_skip
 skipWithCExtensions = unittest.skip if not sysinfo.PURE_PYTHON else _do_not_skip
 

src/greentest/test__event.py

 from __future__ import absolute_import, print_function, division
 
+import weakref
+
 import gevent
 from gevent.event import Event, AsyncResult
 
@@ -234,6 +236,16 @@ class TestWait_count1(TestWait):
 class TestWait_count2(TestWait):
     count = 2
 
+class TestEventBasics(greentest.TestCase):
+
+    @greentest.skipOnPy37("Cython crashes in this case. "
+                          "See https://github.com/cython/cython/issues/2270")
+    def test_weakref(self):
+        # Event objects should allow weakrefs
+        e = Event()
+        r = weakref.ref(e)
+        self.assertIs(e, r())
+
 
 del AbstractGenericGetTestCase
 del AbstractGenericWaitTestCase