golang.pyx: Switch pychan from `class` to `cdef class`

We will need to add C-level attributes to pychan and this requires it to
become cdef class. The class is exported because at least
_golang_test.pyx will also need to have access to those attributes.

If we just do `class pychan` -> `cdef class pychan` e.g. the following
starts to break:

    1.venv/local/lib/python2.7/site-packages/py/_path/local.py:701: in pyimport
        __import__(modname)
    golang/__init__.py:174: in <module>
        from ._golang import    \
    golang/_golang.pyx:455: in init golang._golang
        _pychan_send  = _pychan_send.__func__
    E   AttributeError: 'method_descriptor' object has no attribute '__func__'

and

    golang/_golang.pyx:513: in golang._golang.pyselect
        if im_class(recv) is not pychan:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    f = <built-in method recv of golang._golang.pychan object at 0x7f7055e57cc8>

        def im_class(f):
    >       return f.im_class
    E       AttributeError: 'builtin_function_or_method' object has no attribute 'im_class'

    golang/_pycompat.py:28: AttributeError

This is probably because for `cdef class` methods Cython does not
emulate full method bindings the same way as Python does.  Anyway we can
check which method is passed to pyselect by chanop.__name__ or by
inspecting PyCFunction directly. And not having method binding wrapper
should only remove a bit of overhead.

So we are ok with reworking send/recv chanop detection, and since
this way im_class provided by golang._pycompat becomes unused, it is
also removed.

The timings are probably within noise:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               21.7µs ± 1%  20.0µs ± 1%  -7.60%  (p=0.000 n=10+10)
    chan             9.91µs ± 4%  9.37µs ± 4%  -5.39%  (p=0.000 n=10+10)
    select           19.2µs ± 4%  20.2µs ± 4%  +5.62%  (p=0.001 n=9+8)
    def              58.0ns ± 0%  58.0ns ± 0%    ~     (all equal)
    func_def         44.4µs ± 0%  43.8µs ± 1%  -1.22%  (p=0.000 n=10+10)
    call             63.0ns ± 0%  62.4ns ± 1%  -0.95%  (p=0.011 n=10+10)
    func_call        1.05µs ± 1%  1.06µs ± 1%    ~     (p=0.059 n=10+10)
    try_finally       135ns ± 0%   136ns ± 0%  +0.74%  (p=0.000 n=10+9)
    defer            2.36µs ± 1%  2.28µs ± 1%  -3.59%  (p=0.000 n=10+10)
    workgroup_empty  49.0µs ± 1%  48.2µs ± 1%  -1.63%  (p=0.000 n=10+9)
    workgroup_raise  62.6µs ± 1%  58.9µs ± 1%  -5.96%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               21.7µs ± 1%  20.5µs ± 1%  -5.33%  (p=0.000 n=10+9)
    chan             9.91µs ± 4%  9.72µs ± 5%    ~     (p=0.190 n=10+10)
    select           19.2µs ± 4%  19.5µs ±14%    ~     (p=0.968 n=9+10)
    def              58.0ns ± 0%  58.0ns ± 0%    ~     (all equal)
    func_def         44.4µs ± 0%  45.4µs ± 1%  +2.23%  (p=0.000 n=10+10)
    call             63.0ns ± 0%  64.0ns ± 0%  +1.59%  (p=0.000 n=10+10)
    func_call        1.05µs ± 1%  1.06µs ± 0%  +0.65%  (p=0.002 n=10+10)
    try_finally       135ns ± 0%   137ns ± 0%  +1.48%  (p=0.000 n=10+10)
    defer            2.36µs ± 1%  2.38µs ± 1%  +0.72%  (p=0.006 n=10+10)
    workgroup_empty  49.0µs ± 1%  48.2µs ± 1%  -1.65%  (p=0.000 n=10+10)
    workgroup_raise  62.6µs ± 1%  60.3µs ± 1%  -3.69%  (p=0.000 n=10+10)

golang/_golang.pxd

@@ -63,3 +63,9 @@ cdef extern from "golang/libgolang.h" namespace "golang" nogil:
 
 cdef void topyexc() except *
 cpdef pypanic(arg)
+
+from cython cimport final
+
+@final
+cdef class pychan:
+    cdef dict __dict__

golang/_golang.pyx

 # -*- coding: utf-8 -*-
 # cython: language_level=2
+# cython: binding=False
 # distutils: language = c++
 # distutils: depends = libgolang.h
 #
@@ -35,9 +36,7 @@ from cpython cimport Py_INCREF, Py_DECREF, PY_MAJOR_VERSION
 from cython cimport final
 
 import sys
-import six
 import threading, collections, random
-from golang._pycompat import im_class
 
 # ---- panic ----
 
@@ -253,7 +252,8 @@ def _dequeWaiter(queue):
 
 
 # pychan is Python channel with Go semantic.
-class pychan(object):
+@final
+cdef class pychan:
     # ._cap         channel capacity
     # ._mu          lock
     # ._dataq       deque *: data buffer
@@ -446,16 +446,6 @@ pynilchan = pychan(None)    # TODO -> <chan*>(NULL) after move to Cython
 # pydefault represents default case for pyselect.
 pydefault  = object()
 
-# unbound pychan.{send,recv,recv_}
-_pychan_send  = pychan.send
-_pychan_recv  = pychan.recv
-_pychan_recv_ = pychan.recv_
-if six.PY2:
-    # on py3 class.func gets the func; on py2 - unbound_method(func)
-    _pychan_send  = _pychan_send.__func__
-    _pychan_recv  = _pychan_recv.__func__
-    _pychan_recv_ = _pychan_recv_.__func__
-
 # pyselect executes one ready send or receive channel case.
 #
 # if no case is ready and default case was provided, select chooses default.
@@ -502,9 +492,9 @@ def pyselect(*casev):
         # send
         elif isinstance(case, tuple):
             send, tx = case
-            if im_class(send) is not pychan:
-                pypanic("pyselect: send on non-chan: %r" % (im_class(send),))
-            if send.__func__ is not _pychan_send:
+            if send.__self__.__class__ is not pychan:
+                pypanic("pyselect: send on non-chan: %r" % (send.__self__.__class__,))
+            if send.__name__ != "send":         # XXX better check PyCFunction directly
                 pypanic("pyselect: send expected: %r" % (send,))
 
             ch = send.__self__
@@ -521,11 +511,11 @@ def pyselect(*casev):
         # recv
         else:
             recv = case
-            if im_class(recv) is not pychan:
-                pypanic("pyselect: recv on non-chan: %r" % (im_class(recv),))
-            if recv.__func__ is _pychan_recv:
+            if recv.__self__.__class__ is not pychan:
+                pypanic("pyselect: recv on non-chan: %r" % (recv.__self__.__class__,))
+            if recv.__name__ == "recv":         # XXX better check PyCFunction directly
                 commaok = False
-            elif recv.__func__ is _pychan_recv_:
+            elif recv.__name__ == "recv_":      # XXX better check PyCFunction directly
                 commaok = True
             else:
                 pypanic("pyselect: recv expected: %r" % (recv,))

golang/_golang_test.pyx

@@ -30,8 +30,6 @@ from golang cimport go, panic, pypanic, topyexc
 from golang import chan, nilchan
 
 from golang import time
-from golang._golang import _pychan_recv, _pychan_send
-from golang._pycompat import im_class
 
 # pylen_{recv,send}q returns len(ch._{recv,send}q)
 def pylen_recvq(ch):
@@ -47,13 +45,13 @@ def pylen_sendq(ch):
 #
 # For example `pywaitBlocked(ch.send)` waits till sender blocks waiting on ch.
 def pywaitBlocked(chanop):
-    if im_class(chanop) is not chan:
-        pypanic("wait blocked: %r is method of a non-chan: %r" % (chanop, im_class(chanop)))
+    if chanop.__self__.__class__ is not chan:
+        pypanic("wait blocked: %r is method of a non-chan: %r" % (chanop, chanop.__self__.__class__))
     ch = chanop.__self__
     recv = send = False
-    if chanop.__func__ is _pychan_recv:
+    if chanop.__name__ == "recv":       # XXX better check PyCFunction directly
         recv = True
-    elif chanop.__func__ is _pychan_send:
+    elif chanop.__name__ == "send":     # XXX better check PyCFunction directly
         send = True
     else:
         pypanic("wait blocked: unexpected chan method: %r" % (chanop,))

golang/_pycompat.py

-# -*- coding: utf-8 -*-
-# Copyright (C) 2018-2019  Nexedi SA and Contributors.
-#                          Kirill Smelkov <kirr@nexedi.com>
-#
-# This program is free software: you can Use, Study, Modify and Redistribute
-# it under the terms of the GNU General Public License version 3, or (at your
-# option) any later version, as published by the Free Software Foundation.
-#
-# You can also Link and Combine this program with other software covered by
-# the terms of any of the Free Software licenses or any of the Open Source
-# Initiative approved licenses and Convey the resulting work. Corresponding
-# source of such a combination shall include the source code for all other
-# software used.
-#
-# This program is distributed WITHOUT ANY WARRANTY; without even the implied
-# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
-#
-# See COPYING file for full licensing terms.
-# See https://www.nexedi.com/licensing for rationale and options.
-"""compatibility in between py2/py3"""
-
-from __future__ import print_function, absolute_import
-
-from six import PY2
-
-if PY2:
-    def im_class(f):
-        return f.im_class
-
-# PY3
-else:
-    def im_class(f):
-        return f.__self__.__class__