Straighten out the test__threading_2 reference leak.

This should fix the refleaks in general (it was a frame object that was leaking).
Submitting to CI for broad test, and if it is good then we will try backing out the test__util changes too.

Also make it possible for gevent.config to show help.

_setuputils.py

@@ -234,7 +234,6 @@ def cythonize1(ext):
         '.',
     ]
     try:
-        print("XXX: Drop compile_time_env")
         new_ext = cythonize(
             [ext],
             include_path=standard_include_paths,

src/gevent/_compat.h

@@ -8,6 +8,11 @@
 
 #include <Python.h>
 
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-function"
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -31,7 +36,7 @@ extern "C" {
    need to increment the refcount before returning, not just to match the
    official functions, but to match what Cython expects an API like this to
    return. Otherwise we get crashes. */
-static void* PyFrame_GetBack(PyFrameObject* frame)
+static PyObject* PyFrame_GetBack(PyFrameObject* frame)
 {
     PyObject* result = (PyObject*)((PyFrameObject*)frame)->f_back;
     Py_XINCREF(result);
@@ -41,13 +46,38 @@ static void* PyFrame_GetBack(PyFrameObject* frame)
 static PyObject* PyFrame_GetCode(PyFrameObject* frame)
 {
     PyObject* result = (PyObject*)((PyFrameObject*)frame)->f_code;
-    Py_XINCREF(result);
+    /* There is always code!  */
+    Py_INCREF(result);
     return result;
 }
 #endif /* support 3.8 and below. */
 #endif
 
+/**
+   Unlike PyFrame_GetBack, which can return NULL,
+   this method is guaranteed to return a new reference to an object.
+
+   The object is either a frame object or None.
+
+   This is necessary to help Cython deal correctly with reference counting.
+   (There are other ways of dealing with this having to do with exactly how
+   variables/return types are declared IIRC, but this is the most
+   straightforward. Still, it is critical that the cython declaration of
+   this function use ``object`` as its return type.)
+ */
+static PyObject* Gevent_PyFrame_GetBack(PyFrameObject* frame)
+{
+    PyObject* back = (PyObject*)PyFrame_GetBack(frame);
+    if (back) {
+        return back;
+    }
+    Py_RETURN_NONE;
+}
+
 #ifdef __cplusplus
 }
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
 #endif
 #endif /* _COMPAT_H */

src/gevent/_config.py

@@ -6,6 +6,11 @@ This should be used as ``from gevent import config``. That variable
 is an object of :class:`Config`.
 
 .. versionadded:: 1.3a2
+
+.. versionchanged:: NEXT
+   Invoking this module like ``python -m gevent._config`` will
+   print a help message about available configuration properties.
+   This is handy to quickly look for environment variables.
 """
 
 from __future__ import print_function, absolute_import, division
@@ -206,6 +211,12 @@ class Config(object):
     def __dir__(self):
         return list(self.settings)
 
+    def print_help(self):
+        for k, v in self.settings.items():
+            print(k)
+            print(textwrap.indent(v.__doc__.lstrip(), ' ' * 4))
+            print()
+
 
 class ImportableSetting(object):
 
@@ -473,7 +484,9 @@ class TrackGreenletTree(BoolSettingMixin, Setting):
     Setting this to a false value will make spawning `Greenlet`
     objects and using `spawn_raw` faster, but the
     ``spawning_greenlet``, ``spawn_tree_locals`` and ``spawning_stack``
-    will not be captured.
+    will not be captured. Setting this to a false value can also
+    reduce memory usage because capturing the stack captures
+    some information about Python frames.
 
     .. versionadded:: 1.3b1
     """
@@ -717,3 +730,7 @@ try:
     Loop().get()
 except ImportError: # pragma: no cover
     pass
+
+
+if __name__ == '__main__':
+    config.print_help()

src/gevent/_gevent_cgreenlet.pxd

@@ -57,19 +57,13 @@ ctypedef object CodeType
 cdef extern from "_compat.h":
     int PyFrame_GetLineNumber(FrameType frame)
     CodeType PyFrame_GetCode(FrameType frame)
-    void* PyFrame_GetBack(FrameType frame)
+    object Gevent_PyFrame_GetBack(FrameType frame)
     ctypedef class types.FrameType [object PyFrameObject]:
         pass
 
 @cython.nonecheck(False)
-cdef inline FrameType get_f_back(FrameType frame):
-    # We cannot just call the original version, because it
-    # can return NULL even when there is no exception set. That confuses
-    # Cython and CPython. We need to manually check for NULL here.
-    f_back = PyFrame_GetBack(frame)
-    if f_back != NULL:
-        return <FrameType>f_back
-
+cdef inline object get_f_back(FrameType frame):
+    return Gevent_PyFrame_GetBack(frame)
 
 cdef void _init()
 

src/gevent/tests/test__threading_2.py

@@ -413,7 +413,6 @@ class ThreadTests(unittest.TestCase):
 
     if not hasattr(sys, 'pypy_version_info'):
         def test_no_refcycle_through_target(self):
-            self.skipTest('WARNING: LEAKING.Debugging.')
             class RunSelfFunction(object):
                 def __init__(self, should_raise):
                     # The links in this refcycle from Thread back to self