Find greenlets in Cython code, keeping the GIL.

Mostly (hopefully) fixes #1302

CHANGES.rst

@@ -37,6 +37,11 @@
   implementations. Place the required package or module on `sys.path`
   first.
 
+- Reduce the chances that using the blocking monitor functionality
+  could result in apparently random ``SystemError:
+  Objects/tupleobject.c: bad argument to internal function``. Reported
+  in :issue:`1302` by Ulrich Petri.
+
 1.3.7 (2018-10-12)
 ==================
 

src/gevent/__greenlet_primitives.pxd

 cimport cython
 
 # This file must not cimport anything from gevent.
-
+cdef get_objects
 cdef wref
 
 cdef BlockingSwitchOutError
@@ -42,3 +42,6 @@ cdef class SwitchOutGreenletWithLoop(TrackedRawGreenlet):
 
     cpdef switch(self)
     cpdef switch_out(self)
+
+
+cpdef list get_reachable_greenlets()

src/gevent/_greenlet_primitives.py

@@ -11,6 +11,7 @@ from __future__ import division
 from __future__ import print_function
 
 from weakref import ref as wref
+from gc import get_objects
 
 from greenlet import greenlet
 
@@ -65,6 +66,21 @@ class SwitchOutGreenletWithLoop(TrackedRawGreenlet):
     def switch_out(self):
         raise BlockingSwitchOutError('Impossible to call blocking function in the event loop callback')
 
+
+def get_reachable_greenlets():
+    # We compile this loop with Cython so that it's faster, and so that
+    # the GIL isn't dropped at unpredictable times during the loop.
+    # Dropping the GIL could lead to accessing partly constructed objects
+    # in undefined states (particularly, tuples). This helps close a hole
+    # where a `SystemError: Objects/tupleobject.c bad argument to internal function`
+    # could get raised. (Note that this probably doesn't completely close the hole,
+    # if other threads have dropped the GIL, but hopefully the speed makes that
+    # more rare.) See https://github.com/gevent/gevent/issues/1302
+    return [
+        x for x in get_objects()
+        if isinstance(x, greenlet) and not getattr(x, 'greenlet_tree_is_ignored', False)
+    ]
+
 def _init():
     greenlet_init() # pylint:disable=undefined-variable
 

src/gevent/util.py

@@ -6,13 +6,11 @@ Low-level utilities.
 from __future__ import absolute_import, print_function, division
 
 import functools
-import gc
 import pprint
 import sys
 import traceback
 
 from greenlet import getcurrent
-from greenlet import greenlet as RawGreenlet
 
 from gevent._compat import PYPY
 from gevent._compat import thread_mod_name
@@ -454,20 +452,16 @@ class GreenletTree(object):
 
     @classmethod
     def _forest(cls):
+        from gevent._greenlet_primitives import get_reachable_greenlets
         main_greenlet = cls._root_greenlet(getcurrent())
 
         trees = {}
         roots = {}
         current_tree = roots[main_greenlet] = trees[main_greenlet] = cls(main_greenlet)
 
+        glets = get_reachable_greenlets()
 
-
-        for ob in gc.get_objects():
-            if not isinstance(ob, RawGreenlet):
-                continue
-            if getattr(ob, 'greenlet_tree_is_ignored', False):
-                continue
-
+        for ob in glets:
             spawn_parent = cls.__spawning_parent(ob)
 
             if spawn_parent is None: